Segmentation fault: Stack allocation in a C program in Ubuntu when bufffer>4M

Question

Here\'s a small program to a college\'s task:

#include 

#ifndef BUFFERSIZE
#define BUFFERSIZE 1
#endif

main()
{
    char buffer[BUFFERSIZE]         


        

Answer 1

I think it is pretty simple: if you try to make the buffer too big, it overflows the stack.

If you use malloc() to allocate the buffer, I bet you will have no problem.

Note that there is a function called alloca() that explicitly allocates stack storage. Using alloca() is pretty much the same thing as declaring a stack variable, except that it happens while your program is running. Reading the man page for alloca() or discussions of it may help you to understand what is going wrong with your program. Here's a good discussion:

Why is the use of alloca() not considered good practice?

EDIT: In a comment, @jim mcnamara told us about ulimit, a command-line tool that can check on user limits. On this computer (running Linux Mint 14, so it should be the same limits as Ubuntu 12.10) the ulimit -s command shows a stack size limit of: 8192 K-bytes, which tracks nicely with the problem as you describe it.

EDIT: In case it wasn't completely clear, I recommend that you solve the problem by calling malloc(). Another acceptable solution would be to statically allocate the memory, which will work fine as long as your code is single-threaded.

You should only use stack allocation for small buffers, precisely because you don't want to blow up your stack. If your buffers are big, or your code will recursively call a function many times such that the little buffers will be allocated many many times, you will clobber your stack and your code will crash. The worst part is that you won't get any warning: it will either be fine, or your program already crashed.

The only disadvantage of malloc() is that it is relatively slow, so you don't want to call malloc() inside time-critical code. But it's fine for initial setup; once the malloc is done, the address of your allocated buffer is just an address in memory like any other memory address within your program's address space.

I specifically recommend against editing the system defaults to make the stack size bigger, because that makes your program much less portable. If you call standard C library functions like malloc() you could port your code to Windows, Mac, Android, etc. with little trouble; if you start calling system functions to change the default stack size, you would have much more problems porting. (And you may have no plans to port this now, but plans change!)