Anatomy of a Program in Memory
Memory management is the heart of operating systems; it is crucial for both programming and system administration. In the next few posts I’ll cover memory with an eye towards practical aspects, but without shying away from internals. While the concepts are generic, examples are mostly from Linux and Windows on 32-bit x86. This first post describes how programs are laid out in memory.
Each process in a multi-tasking OS runs in its own memory sandbox. This sandbox is the virtual address space, which in 32-bit mode is always a 4GB block of memory addresses. These virtual addresses are mapped to physical memory by page tables, which are maintained by the operating system kernel and consulted by the processor. Each process has its own set of page tables, but there is a catch. Once virtual addresses are enabled, they apply to all software running in the machine, including the kernel itself. Thus a portion of the virtual address space must be reserved to the kernel:
This does not mean the kernel uses that much physical memory, only that it has that portion of address space available to map whatever physical memory it wishes. Kernel space is flagged in the page tables as exclusive to privileged code (ring 2 or lower), hence a page fault is triggered if user-mode programs try to touch it. In Linux, kernel space is constantly present and maps the same physical memory in all processes. Kernel code and data are always addressable, ready to handle interrupts or system calls at any time. By contrast, the mapping for the user-mode portion of the address space changes whenever a process switch happens:
Blue regions represent virtual addresses that are mapped to physical memory, whereas white regions are unmapped. In the example above, Firefox has used far more of its virtual address space due to its legendary memory hunger. The distinct bands in the address space correspond to memory segments like the heap, stack, and so on. Keep in mind these segments are simply a range of memory addresses and have nothing to do with Intel-style segments. Anyway, here is the standard segment layout in a Linux process:
When computing was happy and safe and cuddly, the starting virtual addresses for the segments shown above were exactly the same for nearly every process in a machine. This made it easy to exploit security vulnerabilities remotely. An exploit often needs to reference absolute memory locations: an address on the stack, the address for a library function, etc. Remote attackers must choose this location blindly, counting on the fact that address spaces are all the same. When they are, people get pwned. Thus address space randomization has become popular. Linux randomizes the stack, memory mapping segment, and heap by adding offsets to their starting addresses. Unfortunately the 32-bit address space is pretty tight, leaving little room for randomization and hampering its effectiveness.
The topmost segment in the process address space is the stack, which stores local variables and function parameters in most programming languages. Calling a method or function pushes a new stack frame onto the stack. The stack frame is destroyed when the function returns. This simple design, possible because the data obeys strict LIFO order, means that no complex data structure is needed to track stack contents - a simple pointer to the top of the stack will do. Pushing and popping are thus very fast and deterministic. Also, the constant reuse of stack regions tends to keep active stack memory in the cpu caches, speeding up access. Each thread in a process gets its own stack.
It is possible to exhaust the area mapping the stack by pushing more data than it can fit. This triggers a page fault that is handled in Linux by expand_stack(), which in turn calls acct_stack_growth() to check whether it’s appropriate to grow the stack. If the stack size is below RLIMIT_STACK (usually 8MB), then normally the stack grows and the program continues merrily, unaware of what just happened. This is the normal mechanism whereby stack size adjusts to demand. However, if the maximum stack size has been reached, we have a stack overflow and the program receives a Segmentation Fault. While the mapped stack area expands to meet demand, it does not shrink back when the stack gets smaller. Like the federal budget, it only expands.
Dynamic stack growth is the only situation in which access to an unmapped memory region, shown in white above, might be valid. Any other access to unmapped memory triggers a page fault that results in a Segmentation Fault. Some mapped areas are read-only, hence write attempts to these areas also lead to segfaults.
Below the stack, we have the memory mapping segment. Here the kernel maps contents of files directly to memory. Any application can ask for such a mapping via the Linux mmap() system call (implementation) or CreateFileMapping() / MapViewOfFile() in Windows. Memory mapping is a convenient and high-performance way to do file I/O, so it is used for loading dynamic libraries. It is also possible to create an anonymous memory mapping that does not correspond to any files, being used instead for program data. In Linux, if you request a large block of memory via malloc(), the C library will create such an anonymous mapping instead of using heap memory. ‘Large’ means larger than MMAP_THRESHOLD bytes, 128 kB by default and adjustable via mallopt().
Speaking of the heap, it comes next in our plunge into address space. The heap provides runtime memory allocation, like the stack, meant for data that must outlive the function doing the allocation, unlike the stack. Most languages provide heap management to programs. Satisfying memory requests is thus a joint affair between the language runtime and the kernel. In C, the interface to heap allocation is malloc() and friends, whereas in a garbage-collected language like C# the interface is the new keyword.
If there is enough space in the heap to satisfy a memory request, it can be handled by the language runtime without kernel involvement. Otherwise the heap is enlarged via the brk() system call (implementation) to make room for the requested block. Heap management is complex, requiring sophisticated algorithms that strive for speed and efficient memory usage in the face of our programs’ chaotic allocation patterns. The time needed to service a heap request can vary substantially. Real-time systems have special-purpose allocators to deal with this problem. Heaps also become fragmented, shown below:
Finally, we get to the lowest segments of memory: BSS, data, and program text. Both BSS and data store contents for static (global) variables in C. The difference is that BSS stores the contents of uninitialized static variables, whose values are not set by the programmer in source code. The BSS memory area is anonymous: it does not map any file. If you say static int cntActiveUsers, the contents of cntActiveUsers live in the BSS.
The data segment, on the other hand, holds the contents for static variables initialized in source code. This memory area is not anonymous. It maps the part of the program’s binary image that contains the initial static values given in source code. So if you say static int cntWorkerBees = 10, the contents of cntWorkerBees live in the data segment and start out as 10. Even though the data segment maps a file, it is a private memory mapping, which means that updates to memory are not reflected in the underlying file. This must be the case, otherwise assignments to global variables would change your on-disk binary image. Inconceivable!
The data example in the diagram is trickier because it uses a pointer. In that case, the contents of pointer gonzo - a 4-byte memory address - live in the data segment. The actual string it points to does not, however. The string lives in the text segment, which is read-only and stores all of your code in addition to tidbits like string literals. The text segment also maps your binary file in memory, but writes to this area earn your program a Segmentation Fault. This helps prevent pointer bugs, though not as effectively as avoiding C in the first place. Here’s a diagram showing these segments and our example variables:
You can examine the memory areas in a Linux process by reading the file /proc/pid_of_process/maps. Keep in mind that a segment may contain many areas. For example, each memory mapped file normally has its own area in the mmap segment, and dynamic libraries have extra areas similar to BSS and data. The next post will clarify what ‘area’ really means. Also, sometimes people say “data segment” meaning all of data + bss + heap.
You can examine binary images using the nm and objdump commands to display symbols, their addresses, segments, and so on. Finally, the virtual address layout described above is the “flexible” layout in Linux, which has been the default for a few years. It assumes that we have a value for RLIMIT_STACK. When that’s not the case, Linux reverts back to the “classic” layout shown below:
That’s it for virtual address space layout. The next post discusses how the kernel keeps track of these memory areas. Coming up we’ll look at memory mapping, how file reading and writing ties into all this and what memory usage figures mean.
January 27, 2009 at 12:34 am | Filed Under Internals, Linux, Software Illustrated
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110 Responses to “Anatomy of a Program in Memory”
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Thank you!
Your posts are some of the most informative I’ve ever found on the internet.
I’m just researching this subject right now and your timing couldn’t have been better!
Thanks!
@JP: sweet, glad it came at a good time. You’re welcome!
First of all: great job! Your written are very good!
I have a question. I’m studying some code and I need documentation on Linux internals, specifically on memory management about processes. Can you suggest me any books or other documentation?
Thanks!
@michele: Take a look at the end of this post. It has a list of Linux kernel books.
My favorite book is still “Understanding the Linux Kernel” because it explains _everything_ in painstaking detail. It is dry, but the authors put monumental effort into going through everything.
The Intel manuals are free and also excellent.
These books are the best resource I know of. I hope to write more material for this blog and eventually maybe have a short ‘Intro to the Linux Kernel’ document online. However this is subject to my work schedule and so on.
@Gustavo:
thanks very much.
The project I’m studying needs to understand/manage memory stuff, like /proc//maps. I’ll read the doc you suggest me and keep reading your blog
thanks!
Thanks for a great post once again. I’ve been reading your blog for a while now and found a lot of your other posts really informative and easy to read. Big + goes for the diagrams, they help clarify on a lot of points. Keep up the good work!
Great article!
Quite shocked to know that windows takes double the kernel memory compared to Linux.
Excellent writeup!
I’m wondering, though, why does the kernel space consume 1gb? That seems like a lot..
And if you don’t mind divulging a trade secret, what do you use to draw your diagrams?
Great post. You left some stuff out though. In modern linux/windows OS’s the heap base is also randomized. And in Linux string literals such as char *blah = “hello there”; will be stored in an ELF section called .rodata. Rarely is constant data such as strings held in .text, but it does happen. Good post though, I like the graphics.
Great Post!
[...] Anatomy of a program in memory [...]
Gustavo:
Excellent article, I’ve been looking for a post like this for a long time. By the way, this made it to the front page of reddit, so brace for incoming traffic.
J.V.
Good post!
Only for completeness, can you include the program’s parameters? I think they go in the bottom of the stack, but I’m not sure. In any case, of course, they are put by the kernel when a execXX in called.
Saludos
thank you.
very informative and refreshing.
looking forward to the next post
[...] Anatomy of a Program in Memory [...]
Thank you for this wonderful post.
Thank you all for the feedback!
@web dev: The kernel is not really using that much memory physical though, it simply has that virtual range available to itself to map whatever physical memory it wishes. Thanks for the question though - I clarified this in the post.
Both the Linux and Windows kernel are extremely well built. It’s hard to find areas where one really has an edge, imho. Two outstanding pieces of software.
@numerodix: You know, this ‘tightness’ of the address space is a sort of recent phenomenon. When the kernels were designed, 2 or 3GB seemed like a lot
So partially it’s an evolutionary artifact, one that is fixed by 64-bit address spaces.
But also, it is good for performance to give the kernel an ample window into memory. I think the next couple posts should clarify why this is.
@Reader1: thanks for the corrections. I’ll add the heap randomization to the post. Regarding ELF sections, I thought about them, but I’m always balancing what to include in these blog posts. I try hard to keep it crisp, covering one area well, but without dumbing anything down. But there’s so much interconnected stuff, it’s not always clear where to put the line. I think I’m going to leave ELF sections out for now though.
@Jose: thanks for the heads up
@el_bot: This is one similar to ELF sections above. The tradeoff between conciseness and completeness. I’m planning a post covering the stack in detail, and talking about buffer overflows, and I think that’d go in well there.
Thanks for the post, it is very informative. Can you tell me what software do you use to create graphics?
I use Visio 2007 for the diagrams. Cheers.
As everyone has said, great post. I am looking forward to your follow up posts. Thanks.
I love your diagrams w/ the subtle gradients. What did you use to create them?
oops I didn’t see #19 reply. ok so it was visio
This blog includes very interesting articles. Continue your good work and never give up!
Excellent and informative post
I’ve been reading your posts for a while now, but I just wanted to take a moment to actually write a comment thanking you. These are some really informative posts you write. You should consider writing a book on the “internals” of systems level software.
Nice post. Very clear. I’ll keep reading you.
Nice Post. Well written and concise. Looked at your Physical with memory post and its good too.
You mentioned, “Each thread in a process gets its own stack.”, but I thought in linux, a thread is really just another process that happens to share certain things with other processes. Could you clear up my confusion?
[...] Anatomy of a Program in Memory : Gustavo Duarte (tags: windows reading linux programming kernel management memory) [...]
I will try to thank you in a simple way: for a long time doubts about where string literals stay in memory would linger in my mind but so far I was not able to find any easy explanation anywhere. This post makes it clear now.
Thank you very much. Can’t wait for the next articles in this series.
[...] his latest post, Anatomy of a Program in Memory, Gustavo Duarte explains beautifully the way in which programs are laid out in memory. He explains [...]
I just finished an Introduction to C Programming class and this beautifully written post is a godsend for helping me further my understanding of memory management.
Thank you so much!
First off, thank you all for the feedback!
It is great to hear that the post helped out a little bit. Contributing to the community is one of the major reasons I write this stuff, though it doesn’t hurt that it’s fun.
@vs: the idea of a book does surface in the comment threads from time to time. I see a few issues though: 1) I want to keep the content free, no matter what; 2) the color would be gone in a normal book; 3) the links would be gone.
Lately I’ve been thinking about maybe collecting all the stuff once there’s enough, and having an online book of sorts. Then maybe make color prints for a small fee if people wanted hard copies (I wouldn’t mind making money on these).
I really had no idea where this blog would go, though now it’s becoming a bit clearer. So yea, I’m munching on it.
@John: you are correct. Basically the set of threads in a thread group share all the memory regions except for the stack and thread-local-storage. Within the kernel, threads are represented with the same data structure used for processes, task_struct, so again you are correct.
Does this help clear it up? I could dig up the relevant links to kernel code if you’d like to see the stuff in action. Let me know.
[...] Anatomy of a Program in Memory Memory management is the heart of operating systems; it is crucial for both programming and system administration. In the next few posts I’ll cover memory with an eye towards practical aspects, but without shying away from internals. While the concepts are generic, examples are mostly from Linux and Windows on 32-bit x86. This first post describes how programs are laid out in memory. (tags: windows reference programming hardware) [...]
Right away I am book marking your site for further reading. Nice articles. This will be much helpful to newbie and those who wants to learn about computers and students. Keep it up!
@Gustavo, thank you so much. All is clear. I used to have a book on the linux kernel where the code was also annotated, but I unfortunately just don’t have the time, so your excellent posts and articles are greatly appreciated!
Interesting arcticle, Very didactly and with figures!! xD … well being a litle bit serious, i think that is very clear and simply to explain the concepts, follow in this way !
pd: for more linux kernel understading, in “viewly” way it will be use kernel profiling, aka “/proc/profile & kerneltop ” very useful to see internal functions and the corresponding behavior of that.
cheers !
Great post. I want learn the art of writing great articles like this. I was looking for a point to get in to kernel level stuff. There won’t be any better source than this.
Subscribed to RSS. ( Looking for Twitter
)
Great article !
Drawing are neat ! What soft do you use for them ?
Once again, great article! I think this blog is one of the best website I’ve seen on introductory OS internals I’ve seen yet. Anything beyond that, I need to start reading my copy of Hennessey and Patterson
I thought I’d spotted a typo in one of the diagrams, but no — it turns out you’ve really shown attention to detail in these articles. Nice work.
[...] colors hold from the earliest post about memory to the latest. This convention is why the post about Intel CPU caches shows a blue index for the virtually [...]
Very clear, informative, nice post.
Good job.
If you decide to do a small online book with such great content on all aspects of OS management, I will gladly buy it!
wow thats informative,got lost a bit into it but have bookmarked to come back to.good work
Great article - Thanks for the effort of keeping things simple and informative!
BOOKMARKED..!!
awaiting your next posts…!!
Thanks…
Very clear explanation of memories either phyisical or virtual
Thanky you again!
Hi Gustavo,
The explanation was very clear and informative. Thanks.
One rquest. If you could explain in the same lucid way how a program wriiten in high level language , say C, gets compiled , what are symbols, how shared files gets linked , how addresses are determined when loaded into memor and such nitty gritty details, it would be great!
Thank you all for the feedback.
@Prabhu: that’s a great topic. There’s a good book about this called Linkers and Loaders. It’s from 2000, not sure how much has changed since. I’m going to add this to my write queue, though I have no idea when the post would actually come out
Great works, Thank you
[...] examining the virtual address layout of a process, we turn to the kernel and its mechanisms for managing user memory. Here is gonzo [...]
It’s a great post … Very helpfull. I’m really waiting for the next one as I’m not too clear for Heap system. Keep writing. Thanks a lot for sharing these helpfull contents.
[...] Anatomy of a Program in Memory – In-depth article by Gustavo Duarte about how a program is represented in memory [...]
Another excellent series on linkers is Ian Lance Taylor’s 20-article series starting near the bottom of and proceeding onwards for several pages.
Oh, curses. Fixed link to the linkers series start…
@Nix: great link, thanks!
[...] Anatomy of a Program in Memory VN:F [1.0.9_379]please wait…Rating: 0.0/10 (0 votes cast) [...]
Hi Gustavo, thank you for all your excellent articles.
I have a question, two actually. As you’ve said each thread has its own stack area. How are these stack areas located with respect to each other? e.g if there are two threads in a process T1 & T2 where would their stacks start and end in the memory. The second question is similar. Does each thread has two stacks - one for user mode and one for kernel mode?
Great jobs.keep going on and publishing more articles
@Raminder: you’re welcome! Sorry for the delay in an answer here, but I’ve been swamped with work these past few days. Can you drop me an email so that I can let you know when I’ve replied here?
@Raminder: Sorry for the delay, I’ve been working a bit lately. Per our email, I’ll talk about Windows only.
I don’t know where in the virtual space the thread stacks go. I googled briefly but didn’t see an answer, so I think the easiest thing to do is to write a short test program to spawn a few threads calling a function that prints the address of a local variable. If you do a loop of 10 or so the pattern should become clear. I found two relevant posts;
http://blogs.msdn.com/oldnewthing/archive/2005/07/29/444912.aspx
http://software.intel.com/en-us/articles/adjusting-thread-stack-address-to-improve-performance-on-intel-xeonr-processors/
Regarding the second question, YES, threads have two stacks: a large one for user mode (1MB by default it looks like) and a tiny one for kernel mode (just a few kilobytes, 12k for x86). The kernel stack is kept in kernel-mode data structures and can’t be touched from user mode.
Hope this helps.
I am from China. Although my English is not so good, but I like your articles. I am ready to reading all your articles on your blog.
Greate Job And Execllent Articles~~~
[...] examining the virtual address layout of a process, we turn to the kernel and its mechanisms for managing user memory. Here is gonzo [...]
[...] todo caso, interesante documento en Anatomy of a Program in Memory que describe la gestión de memoria alrededor de un [...]
Great post. Congratulations! Some questions, so what I understood is that a process can not “use” more than 3gb in a default running linux system, since 1gb is reserved for the kernel, is this true or not? I remember that I’ve seen processes that are using more than 3gbm as far as top is concerned, but I could be wrong (32bit system). Also for example, for top, why isn’t the 1gb, reserved for the kernel, added in the VIRT space?
[...] Anatomy of a program in memory. [...]
thank you very much, in fact it is very useful for me…
Very useful and comprehensiv , to point..
How do you know windows size limits?
@Nagareddy: which size limits? But regardless of which limits, they probably are either from the Windows Internals book, Windows header files, or Intel literature
@Alex,
Thanks! That’s right, a process can’t use more than 3GB of RAM. That’s why for example the memcached folks tell you to run multiple instances when your box has more than 3GB running in 32-bit mode with PAE. Regarding the numbers in top, that would be interesting to see. It could be a quirk with the numbers themselves, or it could be that there’s some exception going on - but in general your understanding is correct - processes can’t use more than 3GB.
Regarding the 1GB not being shown in VIRT, it’s because the kernel accounting ignores the kernel space. It’s just a design issue - why worry about it since it’s there for every process? People would be shocked to see /bin/ls running with 1GB of virt space
great post.. keep up the good work.. have a doubt regarding the memory mapped area for shared libraries..it starts 0×40000000. Does it grow upwards or downwards ? I remember it grows upwards. In your both figures, its drawn differently.
Hi ,
I ve doubt regarding malloc allocting memory. How does malloc stores information about the size of the pointer as free uses only pointer variable as argument and not the size. can u explain which part of address space it is stored..?
Thanks in advance…
Awesome post! Indeed, memory management cannot be overlooked.
@maverick: in x86 Linux it grows as shown in the diagrams, but this varies by CPU architecture and kernel.
@Narayanan: Malloc does its own house keeping to know how much was allocated to each pointer. The best place to check this out is reading the libc source code for malloc and free.
@Keith: thanks!
[...] colors hold from the earliest post about memory to the latest. This convention is why the post about Intel CPU caches shows a blue index for the virtually [...]
Thanks for this post Gustavo. I have a question, though.
I’m mainly a sysadmin, not a low-level developer, but I need to understand this stuff for low-level debugging at the system level. Near the top of this post, you mention “ring 2 or lower” as if we should all just know what that even means, and I’m sorry to say that I do not. Could you point me to a doc that’ll explain that, or could you expand on what this notion of “rings” relates to?
Thanks — all of your posts are top notch.
Hi Brian,
Here you go: http://duartes.org/gustavo/blog/post/cpu-rings-privilege-and-protection
cheers
Dear Gustavo,
Could you send the list of references you use to write Linux internals stuff ?
If you already posted it, please, let me know the url of them.
Best,
Peter.
Very good article. Thanks!
Thanks for the great article thats so simple and crisp.
Great post! Solve tons of doubts of mine.
Though I still have a few questions hope you can clarify for me.
If I understand them right:
1)The kernel stuff of kernel space (1GB) is in the physical memory (1GB) all the time, unless a user process is trying to access the physical memory mapped to kernel space. If that is the case, swapping will happen. Right?
2)So if I only have enough physical memory for kernel mapping, all my user processes will need to use the memory mapped to kernel. So I will have a lot swapping going on. Right?
3)Why 1GB? Is it based on the size of resident processes and other necessary structures? Or is it hardware related?
Thanks in advance!
Superbly explained. Love the diagrams. Great work!
AWESOME articles, extremely helpful! MANY THANKS for all your postings! Keep them coming…I have you book-marked! As a professional technician, I admire your efforts!
This is excellent, probably the best technical documentation I’ve ever found on a blog. Thank you!
Nice article. It should be noted that there is a special case: When clone(2) is used instead of fork(2) to create a process, the address mappings are replicated (stack excepted).
It’s not clear which kernel/libc version above information applies to.
For instance, on CentOS 5.3 running a 2.6.18 kernel with Redhat’s patches 2.6.18-128.7.1.el5PAE and GNU libc 2.5, some shared libraries are located beneath the code segment - which contradicts the figure shown above.
> tac /proc/21779/maps
bfeba000-bff0f000 rw-p bffaa000 00:00 0 [stack]
b7f98000-b7f9f000 r–s 00000000 fd:00 36079468 /usr/lib/gconv/gconv-modules.cache
b7f93000-b7f95000 rw-p b7f93000 00:00 0
b7d93000-b7f93000 r–p 00000000 fd:00 36001563 /usr/lib/locale/locale-archive
08ae9000-08b4c000 rw-p 08ae9000 00:00 0 [heap]
0809c000-080f9000 rw-p 0809c000 00:00 0
08098000-0809c000 rw-p 00050000 fd:00 90800216 /bin/tcsh
08047000-08098000 r-xp 00000000 fd:00 90800216 /bin/tcsh
02b4d000-02b4e000 rwxp 00002000 fd:00 95783275 /lib/libtermcap.so.2.0.8
02b4a000-02b4d000 r-xp 00000000 fd:00 95783275 /lib/libtermcap.so.2.0.8
028ac000-028d3000 rwxp 028ac000 00:00 0
028ab000-028ac000 rwxp 00009000 fd:00 95783323 /lib/libcrypt-2.5.so
028aa000-028ab000 r-xp 00008000 fd:00 95783323 /lib/libcrypt-2.5.so
028a1000-028aa000 r-xp 00000000 fd:00 95783323 /lib/libcrypt-2.5.so
0090a000-0090b000 rwxp 00009000 fd:00 95780903 /lib/libnss_files-2.5.so
00909000-0090a000 r-xp 00008000 fd:00 95780903 /lib/libnss_files-2.5.so
00900000-00909000 r-xp 00000000 fd:00 95780903 /lib/libnss_files-2.5.so
008da000-008dc000 rwxp 008da000 00:00 0
008d9000-008da000 rwxp 0000f000 fd:00 95783322 /lib/libresolv-2.5.so
008d8000-008d9000 r-xp 0000e000 fd:00 95783322 /lib/libresolv-2.5.so
008c9000-008d8000 r-xp 00000000 fd:00 95783322 /lib/libresolv-2.5.so
006d4000-006d7000 rwxp 006d4000 00:00 0
006d3000-006d4000 rwxp 00140000 fd:00 95781738 /lib/libc-2.5.so
006d1000-006d3000 r-xp 0013e000 fd:00 95781738 /lib/libc-2.5.so
00593000-006d1000 r-xp 00000000 fd:00 95781738 /lib/libc-2.5.so
00480000-00481000 rwxp 0001a000 fd:00 95780954 /lib/ld-2.5.so
0047f000-00480000 r-xp 00019000 fd:00 95780954 /lib/ld-2.5.so
00465000-0047f000 r-xp 00000000 fd:00 95780954 /lib/ld-2.5.so
00451000-00452000 r-xp 00451000 00:00 0 [vdso]
00115000-00116000 rwxp 00004000 fd:00 95780901 /lib/libnss_dns-2.5.so
00114000-00115000 r-xp 00003000 fd:00 95780901 /lib/libnss_dns-2.5.so
00110000-00114000 r-xp 00000000 fd:00 95780901 /lib/libnss_dns-2.5.so
Thanks for the info… I was looking for a graph like that. Studying kernel and memory functions in class at the moment, so it’s helpful to have a visual.
Good work
very well explained.
The article explains the memory layout for 32-bit architecture. How it will be different in 64-bit architecture? Please give brief explaination or provide any link for the same.
Thanks,
-Mukesh Chauhan
Useful post.
How do you explain this segment:
08049000-0804a000 r–p 00000000 08:05 276412 /home/Linux/MemoryMgmt/printMem
where the permission on this segment is read-only and printMem is the c code ? Text are read-execute right ? So, only read perm ?
I just happened to view your post when I was searching for the structure of a c program in memory. You have provided a lot of information which is quite rare on the Internet. Thanks!
Great article…no other links I googled explains fundamentals better than this ! Not sure how come Google did not rank it higher
[...] Anatomy of a Program in Memory [...]
Hi
This is very nice blog I ever came across !!!
The contents are very clear and written in very simple terms.
I want to ask you, how can I change the address space layout from ‘classic’ to flexible layout
Hi, great post overall but I don’t see the addresses of all segments shown by examining the maps i.e /proc/pid/maps. I see the stack, heap and mapped .so’s but not the other segments. The maps for my firefox is shown below. Mine is a debian system with 2.6.32.2 SMP kernel.
Is there any other way to see the complete layout of a process in memory?
08048000-0804f000 r-xp 00000000 08:01 1893676 /usr/lib/xulrunner-1.9/xulrunner-stub
0804f000-08050000 rw-p 00006000 08:01 1893676 /usr/lib/xulrunner-1.9/xulrunner-stub
0810c000-0c599000 rw-p 00000000 00:00 0 [heap]
ad6ff000-ad700000 —p 00000000 00:00 0
ad700000-adf00000 rw-p 00000000 00:00 0
adf00000-ae000000 rw-p 00000000 00:00 0
ae84a000-ae84b000 —p 00000000 00:00 0
ae84b000-af04b000 rw-p 00000000 00:00 0
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b553e000-b5547000 r-xp 00000000 08:01 2213355 /usr/lib/iceweasel/components/libbrowserdirprovider.so
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b55c9000-b55cb000 r-xp 00000000 08:01 2958671 /lib/i686/cmov/libutil-2.7.so
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b55df000-b55e1000 rw-p 00000000 00:00 0
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b55fd000-b5694000 r-xp 00000000 08:01 1749770 /usr/lib/libgnutls.so.26.4.6
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b569a000-b56d0000 r-xp 00000000 08:01 1749771 /usr/lib/libdbus-1.so.3.4.0
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b56d2000-b56ed000 r-xp 00000000 08:01 1749988 /usr/lib/libdbus-glib-1.so.2.1.0
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b56ee000-b570e000 r-xp 00000000 08:01 5038143 /usr/lib/libaudiofile.so.0.0.2
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b5711000-b5719000 r-xp 00000000 08:01 5038145 /usr/lib/libesd.so.0.2.36
b5719000-b571a000 rw-p 00007000 08:01 5038145 /usr/lib/libesd.so.0.2.36
b571a000-b5720000 r-xp 00000000 08:01 1751390 /usr/lib/libgailutil.so.18.0.1
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b5721000-b5729000 r-xp 00000000 08:01 2949412 /lib/libpopt.so.0.0.0
b5729000-b572a000 rw-p 00007000 08:01 2949412 /lib/libpopt.so.0.0.0
b572a000-b585d000 r-xp 00000000 08:01 1748162 /usr/lib/libxml2.so.2.6.32
b585d000-b5862000 rw-p 00132000 08:01 1748162 /usr/lib/libxml2.so.2.6.32
b5862000-b5863000 rw-p 00000000 00:00 0
b5863000-b58ac000 r-xp 00000000 08:01 5038133 /usr/lib/libORBit-2.so.0.1.0
b58ac000-b58b5000 rw-p 00049000 08:01 5038133 /usr/lib/libORBit-2.so.0.1.0
b58b5000-b58b6000 rw-p 00000000 00:00 0
b58b6000-b58c8000 r-xp 00000000 08:01 5038137 /usr/lib/libbonobo-activation.so.4.0.0
b58c8000-b58ca000 rw-p 00012000 08:01 5038137 /usr/lib/libbonobo-activation.so.4.0.0
b58ca000-b591c000 r-xp 00000000 08:01 5038138 /usr/lib/libbonobo-2.so.0.0.0
b591c000-b5926000 rw-p 00051000 08:01 5038138 /usr/lib/libbonobo-2.so.0.0.0
b5926000-b5935000 r-xp 00000000 08:01 5038163 /usr/lib/libgnome-keyring.so.0.1.1
b5935000-b5936000 rw-p 0000f000 08:01 5038163 /usr/lib/libgnome-keyring.so.0.1.1
b5936000-b5965000 r-xp 00000000 08:01 1751370 /usr/lib/libgconf-2.so.4.1.5
b5965000-b5968000 rw-p 0002e000 08:01 1751370 /usr/lib/libgconf-2.so.4.1.5
b5968000-b59c0000 r-xp 00000000 08:01 5038155 /usr/lib/libgnomevfs-2.so.0.2200.0
b59c0000-b59c3000 rw-p 00057000 08:01 5038155 /usr/lib/libgnomevfs-2.so.0.2200.0
b59c3000-b59d8000 r-xp 00000000 08:01 5038127 /usr/lib/libart_lgpl_2.so.2.3.20
b59d8000-b59d9000 rw-p 00014000 08:01 5038127 /usr/lib/libart_lgpl_2.so.2.3.20
b59d9000-b59ed000 r-xp 00000000 08:01 5038157 /usr/lib/libgnome-2.so.0.1999.2
b59ed000-b59ee000 rw-p 00013000 08:01 5038157 /usr/lib/libgnome-2.so.0.1999.2
b59ee000-b5a1d000 r-xp 00000000 08:01 1751440 /usr/lib/libgnomecanvas-2.so.0.2001.0
b5a1d000-b5a1e000 rw-p 0002f000 08:01 1751440 /usr/lib/libgnomecanvas-2.so.0.2001.0
b5a1e000-b5a79000 r-xp 00000000 08:01 5038335 /usr/lib/libbonoboui-2.so.0.0.0
b5a79000-b5a7c000 rw-p 0005a000 08:01 5038335 /usr/lib/libbonoboui-2.so.0.0.0
b5a7c000-b5b05000 r-xp 00000000 08:01 5038337 /usr/lib/libgnomeui-2.so.0.2000.1
b5b05000-b5b09000 rw-p 00088000 08:01 5038337 /usr/lib/libgnomeui-2.so.0.2000.1
b5b09000-b5b13000 r-xp 00000000 08:01 2958661 /lib/i686/cmov/libnss_files-2.7.so
b5b13000-b5b15000 rw-p 00009000 08:01 2958661 /lib/i686/cmov/libnss_files-2.7.so
b5b15000-b5b1e000 r-xp 00000000 08:01 2958663 /lib/i686/cmov/libnss_nis-2.7.so
b5b1e000-b5b20000 rw-p 00008000 08:01 2958663 /lib/i686/cmov/libnss_nis-2.7.so
b5b20000-b5b35000 r-xp 00000000 08:01 2958658 /lib/i686/cmov/libnsl-2.7.so
b5b35000-b5b37000 rw-p 00014000 08:01 2958658 /lib/i686/cmov/libnsl-2.7.so
b5b37000-b5b39000 rw-p 00000000 00:00 0
b5b3a000-b5b40000 r-xp 00000000 08:01 1893798 /usr/lib/xulrunner-1.9/components/libdbusservice.so
b5b40000-b5b41000 rw-p 00005000 08:01 1893798 /usr/lib/xulrunner-1.9/components/libdbusservice.so
b5b41000-b5b43000 r-xp 00000000 08:01 1746335 /usr/lib/gconv/UTF-16.so
b5b43000-b5b45000 rw-p 00001000 08:01 1746335 /usr/lib/gconv/UTF-16.so
b5b45000-b5b4c000 r–s 00000000 08:01 1747425 /usr/lib/gconv/gconv-modules.cache
b5b4c000-b5c86000 r–p 00000000 08:01 1762716 /usr/lib/locale/locale-archive
b5c86000-b5c8a000 r-xp 00000000 08:01 1750470 /usr/lib/libXdmcp.so.6.0.0
b5c8a000-b5c8b000 rw-p 00003000 08:01 1750470 /usr/lib/libXdmcp.so.6.0.0
b5c8b000-b5c8d000 r-xp 00000000 08:01 1750468 /usr/lib/libXau.so.6.0.0
b5c8d000-b5c8e000 rw-p 00001000 08:01 1750468 /usr/lib/libXau.so.6.0.0
b5c8e000-b5c95000 r-xp 00000000 08:01 2958668 /lib/i686/cmov/librt-2.7.so
b5c95000-b5c97000 rw-p 00006000 08:01 2958668 /lib/i686/cmov/librt-2.7.so
b5c97000-b5cab000 r-xp 00000000 08:01 1750480 /usr/lib/libICE.so.6.3.0
b5cab000-b5cac000 rw-p 00014000 08:01 1750480 /usr/lib/libICE.so.6.3.0
b5cac000-b5cae000 rw-p 00000000 00:00 0
b5cae000-b5cb5000 r-xp 00000000 08:01 1750482 /usr/lib/libSM.so.6.0.0
b5cb5000-b5cb6000 rw-p 00006000 08:01 1750482 /usr/lib/libSM.so.6.0.0
b5cb6000-b5cbe000 r-xp 00000000 08:01 1750606 /usr/lib/libXcursor.so.1.0.2
b5cbe000-b5cbf000 rw-p 00007000 08:01 1750606 /usr/lib/libXcursor.so.1.0.2
b5cbf000-b5cc4000 r-xp 00000000 08:01 1750760 /usr/lib/libXrandr.so.2.1.0
b5cc4000-b5cc5000 rw-p 00005000 08:01 1750760 /usr/lib/libXrandr.so.2.1.0
b5cc5000-b5ccc000 r-xp 00000000 08:01 1750692 /usr/lib/libXi.so.6.0.0
b5ccc000-b5ccd000 rw-p 00007000 08:01 1750692 /usr/lib/libXi.so.6.0.0
b5ccd000-b5ccf000 r-xp 00000000 08:01 1750694 /usr/lib/libXinerama.so.1.0.0
b5ccf000-b5cd0000 rw-p 00001000 08:01 1750694 /usr/lib/libXinerama.so.1.0.0
b5cd0000-b5cdd000 r-xp 00000000 08:01 1750592 /usr/lib/libXext.so.6.4.0
b5cdd000-b5cde000 rw-p 0000c000 08:01 1750592 /usr/lib/libXext.so.6.4.0
b5cde000-b5ce2000 r-xp 00000000 08:01 1750478 /usr/lib/libXfixes.so.3.1.0
b5ce2000-b5ce3000 rw-p 00003000 08:01 1750478 /usr/lib/libXfixes.so.3.1.0
b5ce3000-b5ce5000 r-xp 00000000 08:01 1750686 /usr/lib/libXdamage.so.1.1.0
b5ce5000-b5ce6000 rw-p 00001000 08:01 1750686 /usr/lib/libXdamage.so.1.1.0
b5ce6000-b5ce8000 r-xp 00000000 08:01 5038113 /usr/lib/libXcomposite.so.1.0.0
b5ce8000-b5ce9000 rw-p 00001000 08:01 5038113 /usr/lib/libXcomposite.so.1.0.0
b5ce9000-b5cea000 r-xp 00000000 08:01 1750474 /usr/lib/libxcb-xlib.so.0.0.0
b5cea000-b5ceb000 rw-p 00000000 08:01 1750474 /usr/lib/libxcb-xlib.so.0.0.0
b5ceb000-b5d13000 r-xp 00000000 08:01 1749972 /usr/lib/libpcre.so.3.12.1
b5d13000-b5d14000 rw-p 00027000 08:01 1749972 /usr/lib/libpcre.so.3.12.1
b5d14000-b5d38000 r-xp 00000000 08:01 1747333 /usr/lib/libexpat.so.1.5.2
b5d38000-b5d3a000 rw-p 00023000 08:01 1747333 /usr/lib/libexpat.so.1.5.2
b5d3a000-b5d62000 r-xp 00000000 08:01 5038087 /usr/lib/libpixman-1.so.0.10.0
b5d62000-b5d63000 rw-p 00027000 08:01 5038087 /usr/lib/libpixman-1.so.0.10.0
b5d63000-b5d7a000 r-xp 00000000 08:01 1750472 /usr/lib/libxcb.so.1.0.0
b5d7a000-b5d7b000 rw-p 00017000 08:01 1750472 /usr/lib/libxcb.so.1.0.0
b5d7b000-b5d81000 r-xp 00000000 08:01 5038089 /usr/lib/libxcb-render.so.0.0.0
b5d81000-b5d82000 rw-p 00005000 08:01 5038089 /usr/lib/libxcb-render.so.0.0.0
b5d82000-b5d85000 r-xp 00000000 08:01 5038091 /usr/lib/libxcb-render-util.so.0.0.0
b5d85000-b5d86000 rw-p 00002000 08:01 5038091 /usr/lib/libxcb-render-util.so.0.0.0
b5d86000-b5d99000 r-xp 00000000 08:01 1750908 /usr/lib/libdirect-1.0.so.0.1.0
b5d99000-b5d9a000 rw-p 00012000 08:01 1750908 /usr/lib/libdirect-1.0.so.0.1.0
b5d9a000-b5da1000 r-xp 00000000 08:01 5038081 /usr/lib/libfusion-1.0.so.0.1.0
b5da1000-b5da2000 rw-p 00006000 08:01 5038081 /usr/lib/libfusion-1.0.so.0.1.0
b5da2000-b5e07000 r-xp 00000000 08:01 1750909 /usr/lib/libdirectfb-1.0.so.0.1.0
b5e07000-b5e09000 rw-p 00065000 08:01 1750909 /usr/lib/libdirectfb-1.0.so.0.1.0
b5e09000-b5e0c000 r-xp 00000000 08:01 1749981 /usr/lib/libgmodule-2.0.so.0.1600.6
b5e0c000-b5e0d000 rw-p 00002000 08:01 1749981 /usr/lib/libgmodule-2.0.so.0.1600.6
b5e0d000-b5e14000 r-xp 00000000 08:01 5038314 /usr/lib/libstartup-notification-1.so.0.0.0
b5e14000-b5e15000 rw-p 00007000 08:01 5038314 /usr/lib/libstartup-notification-1.so.0.0.0
b5e15000-b5e19000 r-xp 00000000 08:01 1749978 /usr/lib/libgthread-2.0.so.0.1600.6
b5e19000-b5e1a000 rw-p 00003000 08:01 1749978 /usr/lib/libgthread-2.0.so.0.1600.6
b5e1a000-b5e66000 r-xp 00000000 08:01 1750484 /usr/lib/libXt.so.6.0.0
b5e66000-b5e69000 rw-p 0004c000 08:01 1750484 /usr/lib/libXt.so.6.0.0
b5e69000-b5e6a000 rw-p 00000000 00:00 0
b5e6a000-b5e81000 r-xp 00000000 08:01 1751381 /usr/lib/libgdk_pixbuf-2.0.so.0.1200.12
b5e81000-b5e82000 rw-p 00017000 08:01 1751381 /usr/lib/libgdk_pixbuf-2.0.so.0.1200.12
b5e82000-b5f05000 r-xp 00000000 08:01 1751382 /usr/lib/libgdk-x11-2.0.so.0.1200.12
b5f05000-b5f08000 rw-p 00083000 08:01 1751382 /usr/lib/libgdk-x11-2.0.so.0.1200.12
b5f08000-b5f21000 r-xp 00000000 08:01 5038097 /usr/lib/libatk-1.0.so.0.2209.1
b5f21000-b5f23000 rw-p 00018000 08:01 5038097 /usr/lib/libatk-1.0.so.0.2209.1
b5f23000-b62a7000 r-xp 00000000 08:01 1751380 /usr/lib/libgtk-x11-2.0.so.0.1200.12
b62a7000-b62ad000 rw-p 00383000 08:01 1751380 /usr/lib/libgtk-x11-2.0.so.0.1200.12
b62ad000-b62ae000 rw-p 00000000 00:00 0
b62ae000-b62c3000 r-xp 00000000 08:01 2958666 /lib/i686/cmov/libpthread-2.7.so
b62c3000-b62c5000 rw-p 00014000 08:01 2958666 /lib/i686/cmov/libpthread-2.7.so
b62c5000-b62c7000 rw-p 00000000 00:00 0
b62c7000-b62f8000 r-xp 00000000 08:01 5038202 /usr/lib/libnspr4.so.0d
b62f8000-b62f9000 rw-p 00031000 08:01 5038202 /usr/lib/libnspr4.so.0d
b62f9000-b62fb000 rw-p 00000000 00:00 0
b62fb000-b62fe000 r-xp 00000000 08:01 5038203 /usr/lib/libplc4.so.0d
b62fe000-b62ff000 rw-p 00002000 08:01 5038203 /usr/lib/libplc4.so.0d
b62ff000-b6301000 r-xp 00000000 08:01 5038204 /usr/lib/libplds4.so.0d
b6301000-b6302000 rw-p 00001000 08:01 5038204 /usr/lib/libplds4.so.0d
b6302000-b63ed000 r-xp 00000000 08:01 1750476 /usr/lib/libX11.so.6.2.0
b63ed000-b63f1000 rw-p 000ea000 08:01 1750476 /usr/lib/libX11.so.6.2.0
b63f1000-b63f9000 r-xp 00000000 08:01 1750604 /usr/lib/libXrender.so.1.3.0
b63f9000-b63fa000 rw-p 00007000 08:01 1750604 /usr/lib/libXrender.so.1.3.0
b63fa000-b64ae000 r-xp 00000000 08:01 1749982 /usr/lib/libglib-2.0.so.0.1600.6
b64ae000-b64af000 rw-p 000b4000 08:01 1749982 /usr/lib/libglib-2.0.so.0.1600.6
b64af000-b64ea000 r-xp 00000000 08:01 1749979 /usr/lib/libgobject-2.0.so.0.1600.6
b64ea000-b64eb000 rw-p 0003b000 08:01 1749979 /usr/lib/libgobject-2.0.so.0.1600.6
b64eb000-b6515000 r-xp 00000000 08:01 1750090 /usr/lib/libfontconfig.so.1.3.0
b6515000-b6516000 rw-p 0002a000 08:01 1750090 /usr/lib/libfontconfig.so.1.3.0
b6516000-b6587000 r-xp 00000000 08:01 1749996 /usr/lib/libfreetype.so.6.3.18
b6587000-b658b000 rw-p 00070000 08:01 1749996 /usr/lib/libfreetype.so.6.3.18
b658b000-b65c9000 r-xp 00000000 08:01 5038105 /usr/lib/libpango-1.0.so.0.2002.3
b65c9000-b65cb000 rw-p 0003d000 08:01 5038105 /usr/lib/libpango-1.0.so.0.2002.3
b65cb000-b65f1000 r-xp 00000000 08:01 5038107 /usr/lib/libpangoft2-1.0.so.0.2002.3
b65f1000-b65f2000 rw-p 00026000 08:01 5038107 /usr/lib/libpangoft2-1.0.so.0.2002.3
b65f2000-b665c000 r-xp 00000000 08:01 5038093 /usr/lib/libcairo.so.2.17.5
b665c000-b665e000 rw-p 0006a000 08:01 5038093 /usr/lib/libcairo.so.2.17.5
b665e000-b6667000 r-xp 00000000 08:01 5038106 /usr/lib/libpangocairo-1.0.so.0.2002.3
b6667000-b6668000 rw-p 00008000 08:01 5038106 /usr/lib/libpangocairo-1.0.so.0.2002.3
b6668000-b66a7000 r-xp 00000000 08:01 5038374 /usr/lib/libhunspell-1.2.so.0.0.0
b66a7000-b66ab000 rw-p 0003e000 08:01 5038374 /usr/lib/libhunspell-1.2.so.0.0.0
b66ab000-b66bf000 r-xp 00000000 08:01 1747421 /usr/lib/libz.so.1.2.3.3
b66bf000-b66c0000 rw-p 00013000 08:01 1747421 /usr/lib/libz.so.1.2.3.3
b66c0000-b66d1000 r-xp 00000000 08:01 5038211 /usr/lib/libnssutil3.so.1d
b66d1000-b66d4000 rw-p 00011000 08:01 5038211 /usr/lib/libnssutil3.so.1d
b66d4000-b67a3000 r-xp 00000000 08:01 5038210 /usr/lib/libnss3.so.1d
b67a3000-b67a7000 rw-p 000cf000 08:01 5038210 /usr/lib/libnss3.so.1d
b67a7000-b67c3000 r-xp 00000000 08:01 5038212 /usr/lib/libsmime3.so.1d
b67c3000-b67c5000 rw-p 0001c000 08:01 5038212 /usr/lib/libsmime3.so.1d
b67c5000-b67ea000 r-xp 00000000 08:01 5038213 /usr/lib/libssl3.so.1d
b67ea000-b67ec000 rw-p 00025000 08:01 5038213 /usr/lib/libssl3.so.1d
b67ec000-b687c000 r-xp 00000000 08:01 1747488 /usr/lib/libmozjs.so.1d
b687c000-b6881000 rw-p 0008f000 08:01 1747488 /usr/lib/libmozjs.so.1d
b6881000-b68b1000 r-xp 00000000 08:01 5038370 /usr/lib/liblcms.so.1.0.16
b68b1000-b68b2000 rw-p 00030000 08:01 5038370 /usr/lib/liblcms.so.1.0.16
b68b2000-b68b5000 rw-p 00000000 00:00 0
b68b5000-b68d8000 r-xp 00000000 08:01 1750030 /usr/lib/libpng12.so.0.27.0
b68d8000-b68d9000 rw-p 00023000 08:01 1750030 /usr/lib/libpng12.so.0.27.0
b68d9000-b68f7000 r-xp 00000000 08:01 1749812 /usr/lib/libjpeg.so.62.0.0
b68f7000-b68f8000 rw-p 0001e000 08:01 1749812 /usr/lib/libjpeg.so.62.0.0
b68f8000-b6965000 r-xp 00000000 08:01 1749005 /usr/lib/libsqlite3.so.0.8.6
b6965000-b6967000 rw-p 0006c000 08:01 1749005 /usr/lib/libsqlite3.so.0.8.6
b6967000-b73d9000 r-xp 00000000 08:01 1893684 /usr/lib/xulrunner-1.9/libxul.so
b73d9000-b74b6000 rw-p 00a72000 08:01 1893684 /usr/lib/xulrunner-1.9/libxul.so
b74b6000-b74c6000 rw-p 00000000 00:00 0
b74c6000-b74d2000 r-xp 00000000 08:01 2949164 /lib/libgcc_s.so.1
b74d2000-b74d3000 rw-p 0000b000 08:01 2949164 /lib/libgcc_s.so.1
b74d3000-b74f7000 r-xp 00000000 08:01 2958656 /lib/i686/cmov/libm-2.7.so
b74f7000-b74f9000 rw-p 00023000 08:01 2958656 /lib/i686/cmov/libm-2.7.so
b74f9000-b764e000 r-xp 00000000 08:01 2958652 /lib/i686/cmov/libc-2.7.so
b764e000-b764f000 r–p 00155000 08:01 2958652 /lib/i686/cmov/libc-2.7.so
b764f000-b7651000 rw-p 00156000 08:01 2958652 /lib/i686/cmov/libc-2.7.so
b7651000-b7655000 rw-p 00000000 00:00 0
b7655000-b7738000 r-xp 00000000 08:01 1745058 /usr/lib/libstdc++.so.6.0.10
b7738000-b773b000 r–p 000e2000 08:01 1745058 /usr/lib/libstdc++.so.6.0.10
b773b000-b773d000 rw-p 000e5000 08:01 1745058 /usr/lib/libstdc++.so.6.0.10
b773d000-b7743000 rw-p 00000000 00:00 0
b7743000-b7745000 r-xp 00000000 08:01 2958655 /lib/i686/cmov/libdl-2.7.so
b7745000-b7747000 rw-p 00001000 08:01 2958655 /lib/i686/cmov/libdl-2.7.so
b7747000-b7749000 r-xp 00000000 08:01 1804257 /usr/lib/pango/1.6.0/modules/pango-basic-fc.so
b7749000-b774a000 rw-p 00001000 08:01 1804257 /usr/lib/pango/1.6.0/modules/pango-basic-fc.so
b774a000-b774c000 r-xp 00000000 08:01 5038147 /usr/lib/libavahi-glib.so.1.0.1
b774c000-b774d000 rw-p 00002000 08:01 5038147 /usr/lib/libavahi-glib.so.1.0.1
b774d000-b7754000 r-xp 00000000 08:01 2958659 /lib/i686/cmov/libnss_compat-2.7.so
b7754000-b7756000 rw-p 00006000 08:01 2958659 /lib/i686/cmov/libnss_compat-2.7.so
b7756000-b7759000 r-xp 00000000 08:01 1893677 /usr/lib/xulrunner-1.9/libxpcom.so
b7759000-b775a000 rw-p 00002000 08:01 1893677 /usr/lib/xulrunner-1.9/libxpcom.so
b775a000-b775c000 rw-p 00000000 00:00 0
b775c000-b775d000 r-xp 00000000 00:00 0 [vdso]
b775d000-b7777000 r-xp 00000000 08:01 2949122 /lib/ld-2.7.so
b7777000-b7779000 rw-p 0001a000 08:01 2949122 /lib/ld-2.7.so
bfce1000-bfd17000 rw-p 00000000 00:00 0 [stack]
[...] area is a read only area of memory.You can learn more details about the anatomy of program memory, here.Technorati Tags: virtual file system, paging, layout of program memory Related Posts:Modular [...]
Very Nice post… good effort, thanks a lot for such a valuable information. I have a query.
If a program define 5GB of a static variable, where it will go since process have only 4GB block of virtual memory addresses in 32-bit mode.
[...] Anatomy of a program in memory – “Memory management is the heart of operating systems; it is crucial for both programming and system administration. In the next few posts I’ll cover memory with an eye towards practical aspects, but without shying away from internals. While the concepts are generic, examples are mostly from Linux and Windows on 32-bit x86. This first post describes how programs are laid out in memory….” [...]
good writing and thanks for your sharing.
Please forgive my ignorance or immaturity on this subject, Ive been reading on it for the past 2 weeks and I get confusing information about the elf.
I did a hexdump “>>hexdump -C ” in my linux machine on my elf(executable) and I was able to trace out the different sections and segments with the hexdump “>>readelf -x .text “. Now the hexdump from the readelf utility has addresses associated with them like this:
0×08048540 ffc70424 74870408 e8e3feff ffe8e300 …$t………..
0×08048550 0000b8a4 8704088d 54242f89 54240489 ……..T$/.T$..
0×08048560 0424e8b9 feffff0f b644242f 0fbec88b .$…….D$/….
0×08048570 5424248b 44242889 4c240889 54240489 T$$.D$(.L$..T$..
I want to know exactly how the kernel associates each byte in the elf file to an address like 0×08048540. Please help clarify things, I don’t believe in magic. I also read that section .symtab is not included in the elf executable file, why is it that the readelf utility can perform a hexdump on it like this “>>readelf -x .symtab ”
0×00000000 00000000 00000000 00000000 00000000 …………….
0×00000010 00000000 34810408 00000000 03000100 ….4………..
0×00000020 00000000 48810408 00000000 03000200 ….H………..
0×00000030 00000000 68810408 00000000 03000300 ….h………..
Salut Gustavo,
Thanks for your pages, they are great.
Being knowledgeable in the topic, there is however a tough one I was not able to solve myself. Maybe you or someone reading this page can cast some light on it?
I recently installed openSUSE 11.2 32 bits and Kubuntu 9.10, both running Linux kernel 2.6.31.
I don’t understand why cat /proc//maps (related to the top command here for example) returns 4 lines per shared libraries (for some, not all - the usual 3 lines then).
3 lines, like in older kernel (2.6.28, …), makes sense: bss, data and text segments. In some kernels I noticed also two entries only per shared library (bss merged with data, or data only? That makes sense, at least)
However, 4 entries (!), one being a page of rights “—p” (cannot be read, written or executed) seems very odd to me. I get the same results on my SUSE and Kubtuntu, so - hence no bug. But what then? There is nothing much in the ELF format that give me any hint.
An exemple for libncurses.so.5.6 (related to top):
b76ab000-b76e1000 r-xp 00000000 08:02 127570 /lib/libncurses.so.5.6
b76e1000-b76e2000 —p 00036000 08:02 127570 /lib/libncurses.so.5.6
b76e2000-b76e4000 r–p 00036000 08:02 127570 /lib/libncurses.so.5.6
b76e4000-b76e8000 rw-p 00038000 08:02 127570 /lib/libncurses.so.5.6
– Stéphane
Hi Guru,
A basic question.
Is there any difference in generation of logical addresses for a program in linux OS and windows OS?
When i was studying my diploma there was a subject called COA which has this topic at that time i didn’t get it and this time also. (Don’t mind author bcoz my knowledge in hardware functionality is very poor) But definitely this post will useful for tech savvy guys
Wow, what a great article.
I was searching for exactly this kind of (easy to understand) detailed information on Process Address Space.
I couldn’t help not thank you for this article.
Thanks a lot,
Shobhit
Excellent Work …
Pretty cool, thank you for your great job! It really helps me a lot.
Thanks for the in-detailed article
Pretty Cool, THX very much
Really very informative articles on Linux memory internals.
If you write one more article relating Virtual memory,Kernel memory with pysical memory(RAM), covering how the kernel gets loaded in RAM and stored, how the remaining RAM space is used for this memory mapping, really this will give great insight for beginners,
Thanks,
Nandu
I think you mean “always a 4KB block of memory addresses” instead if “always a 4GB block of memory addresses”.