How is stack size of process on linux related to pthread, fork and exec

Question

Hi, guys. I have a question about the stack size of a process on Linux. Is this stack size determined at linkage time and is coded in the ELF file? I wrote a program which print its stack size by pthread_attr_getstacksize(&attr, &stacksize);. And if I run this program directly from shell, it gives a value of about 10MB. But when I exec it from a thread which belongs to a multi-thread program, it gives a value of about 2MB.
So I wanna know what factors there are that affects the stack size of a process which is fork and exec-ed from some parent process. And Is is possible to set the stack size of a process in its parent at run time before fork and exec the child?
Thanks in advance.

Answer 1

As the manpage for pthread_create(3) says:

"On Linux/x86-32, the default stack size for a new thread is 2 megabytes", Unless the RLIMIT_STACK resource limit (ulimit -s) is set: in that case, "it determines the default stack size of new threads".

You can check this fact by retrieving the current value of RLIMIT_STACK with getrlimit(2), as in the following program:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/resource.h>

int main()
{
    /* Warning: error checking removed to keep the example small */
    pthread_attr_t attr;
    size_t stacksize;
    struct rlimit rlim;

    pthread_attr_init(&attr);
    pthread_attr_getstacksize(&attr, &stacksize);
    getrlimit(RLIMIT_STACK, &rlim);
    /* Don't know the exact type of rlim_t, but surely it will
       fit into a size_t variable. */
    printf("%zd\n", (size_t) rlim.rlim_cur);
    printf("%zd\n", stacksize);
    pthread_attr_destroy(&attr);

    return 0;
}

These are the results when trying to run it (compiled to a.out) from the command line:

$ ulimit -s
8192
$ ./a.out 
8388608
8388608
$ ulimit -s unlimited
$ ./a.out 
-1
2097152
$ ulimit -s 4096
$ ./a.out 
4194304
4194304