Why is RLIMIT_STACK lost after fork or exec on linux?

On linux, it is said that rlimit of a process is kept intact after either fork or exec. But I lose my RLIMIT_STACK in the child either after fork or after exec. Would someone please give some explain?

Here is some descriptive output of my program.

//The parent has an RLIMIT_STACK like this

RLIMIT_STACK, soft - 10485760, hard - -1

//Right after fork, the child loses its RLIMIT_STACK

In child after fork, RLIMIT_STACK, soft - -1, hard - -1

//In the child, before exec, RLIMIT_STACK soft is set to 10485760 again

RLIMIT_STACK set OK.

In child after set, RLIMIT_STACK, soft - 10485760, hard - -1 Child pid = 3096

//After exec, the new process loses its RLIMIT_STACK again

RLIMIT_STACK got, soft - -1, hard - -1

Thanks in advance

Feng

This seems a problem(I am not sure if it is a bug) of linuxthread implementation of libpthread.
I wrote a simple program:

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <sys/resource.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

int main(int argc, char **argv){
struct rlimit resource_limit;
if(getrlimit(RLIMIT_STACK, &resource_limit) != 0){
    fprintf(stderr, "Failed to get rlimit: %s\n", strerror(errno));
    return 1;
}
else{
    fprintf(stderr, "In parent, RLIMIT_STACK, soft-%d, hard-%d\n",
            resource_limit.rlim_cur, resource_limit.rlim_max);
}

int child_status = 0;
pid_t pid = fork();
switch(pid){
    case 0://child
        if(getrlimit(RLIMIT_STACK, &resource_limit) != 0){
            fprintf(stderr, "Failed to get rlimit: %s\n", strerror(errno));
            return 1;
        }
        else{
            fprintf(stderr, "In child after fork, RLIMIT_STACK, soft-%d, hard-%d\n",
                    resource_limit.rlim_cur, resource_limit.rlim_max);
        }
        break;
    case -1:
        fprintf(stderr, "Fork error: %s\n", strerror(errno));
        break;
    default://parent
        waitpid(pid, &child_status, 0);
        break;
}
return 0;

}

If this program is compiled and linked without -lpthread option, it runs OK everywhere. But when it is linked with -lpthread option, wired things happen: if it is run on a machine where it is dynamically linked to a linuxthread version of libpthread, it gives:

In parent, RLIMIT_STACK, soft-10485760, hard--1
In child after fork, RLIMIT_STACK, soft--1, hard--1

But when run on a machine where it is dynamically linked to a NPTL version of libpthread, it gives the expected result:

In parent, RLIMIT_STACK, soft-10485760, hard--1
In child after fork, RLIMIT_STACK, soft-10485760, hard--1