I think the next couple posts should clarify why this is. Reader1: thanks for the corrections. I try hard to keep it crisp, covering one area well, but without dumbing anything down. Jose: thanks for the heads up. The tradeoff between conciseness and completeness. Thanks for the post, it is very informative.
Can you tell me what software do you use to create graphics? Chaitanya Gupta on January 27th, am. Excellent and informative post. These are some really informative posts you write. Could you clear up my confusion? 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. He explains […]. Raam Dev on January 27th, pm. 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. Gustavo Duarte on January 28th, am. It is great to hear that the post helped out a little bit. 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.
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. Does this help clear it up? Let me know. While the concepts are generic, examples are mostly from Linux and Windows on bit x This first post describes how programs are laid out in memory. 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. Ulver on January 28th, am. Interesting arcticle, Very didactly and with figures!! Chanux on January 28th, am. I want learn the art of writing great articles like this. I was looking for a point to get in to kernel level stuff. Subscribed to RSS. Looking for Twitter. Fab on January 28th, am. Ben Fowler on January 28th, pm.
Once again, great article! Anything beyond that, I need to start reading my copy of Hennessey and Patterson. Nice work. This convention is why the post about Intel CPU caches shows a blue index for the virtually […]. If you decide to do a small online book with such great content on all aspects of OS management, I will gladly buy it!
The explanation was very clear and informative. 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!
Gustavo Duarte on February 2nd, am. Here is gonzo […]. Keep writing. Thanks a lot for sharing these helpfull contents. Software Quality Digest — No bug left behind on February 4th, pm.
Oh, curses. Fixed link to the linkers series start…. Gustavo Duarte on February 5th, am. Hi Gustavo, thank you for all your excellent articles. I have a question, two actually. How are these stack areas located with respect to each other? The second question is similar. Does each thread has two stacks — one for user mode and one for kernel mode? Gustavo Duarte on February 10th, am. Gustavo Duarte on February 13th, am.
If you do a loop of 10 or so the pattern should become clear. I found two relevant posts;. 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 x 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.
Justin Blanton Anatomy of a program in memory on March 12th, pm. Gustavo Duarte on March 23rd, pm. Nagareddy: which size limits? But regardless of which limits, they probably are either from the Windows Internals book, Windows header files, or Intel literature. Regarding the numbers in top, that would be interesting to see. Does it grow upwards or downwards? I remember it grows upwards. In your both figures, its drawn differently.
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. Keith Johnson on April 30th, am. Gustavo Duarte on May 3rd, pm. 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.
Brian on June 4th, am. Gustavo Duarte on June 8th, am. 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.
So I will have a lot swapping going on. Is it based on the size of resident processes and other necessary structures? Or is it hardware related? Dean on August 4th, pm. Keep them coming…I have you book-marked! As a professional technician, I admire your efforts! 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.
For instance, on CentOS 5. Travis on September 25th, am. Thanks for the info… I was looking for a graph like that. Good work. The article explains the memory layout for bit architecture. How it will be different in bit architecture? Please give brief explaination or provide any link for the same. All the other addresses are given to the program at startup. It is unusual for Linux binaries to believe that they will be loaded above 2GiB; the usual start address is 0x It's pretty arbitrary really.
It's rare to need more than 2 gigs of memory in a single user process. On the other hand, the more address space you have in the kernel the easier it is to use spare memory for IO buffer caching. From the way I read those comments, I gathered that the comments implied that Windows "consumed" two gigs of ram, I'm afraid I didn't make myself clear before. I also think that commenters were wrong. I just tried to explain why. The article says that's the default arrangement, but if non-kernel memory grows bigger than 2 GB, I assume that it'll be allocated in the virtual space that's unused by kernel.
New people are being born all the time! What about all the old people being born? Address space is not much bigger than physical memory now for 32 bits.
I've read that my laptop could in fact have 4GB instead of the nominal 3GB. Hoff on Jan 27, root parent next [—]. Given you don't reference what operating system is in use, you're probably using Microsoft Windows here. The implementation of virtual memory has common traits, but details vary by OS and even OS version. Physical address space and virtual address space are two different aspects of system design, and it is completely feasible to have these values be the same, or to have either of the two be larger than the other.
Yes, you can have a bit virtual memory system and 34 or more bits of physical memory, and there can be good reasons to do that, too. It's also feasible to have address "holes" in virtual address space, and "holes" in physical address space.
Address ranges or gaps that are simply not implemented by the particular processor, or that are not instantiated by the memory controller. But I digress. Save for low-end boxes, all current gear is bit virtual addressing variously with "holes" , and most current systems can have 48 bits x or 50 bits IA or other implementation-specific ranges for physical addressing. Details vary by processor and by implementation. Available memory varies by budget - 50 bits of memory is expensive - and target market for the box.
And consuming address space is different than consuming memory. On various systems, address space is an inexpensive resource. And on a virtual memory system, memory is limited by physical memory and by backing storage and by your willingness to wait for the transitions between these two resources; for memory paging to occur. And what your application thinks is in memory might be on disk.
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 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 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.
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