Sometimes cannot get stack range from /proc/self/maps - c++

Question

I have one problem. I am trying to get stack offset range from /proc/self/maps pseudofile. But I have sometime weird things.
Here is my code
fp = fopen("/proc/self/maps", "r");
if (fp == NULL) {
perror("Error opening file");
return NULL;
}
while (fgets(line, 2048, fp) != NULL) {
if (strstr(line, "stack") != NULL) {
printf("%s", line);
}
}
If you start the programm with one or multiple thread you can view this pseudo file and get something like this
7f20423a6000-7f2042ba6000 rw-p 00000000 00:00 0 [stack:3936]
7fffbe95e000-7fffbe97f000 rw-p 00000000 00:00 0 [stack]
the first line here is stack of thread , the second line is the stack of the process.
But the problem is that sometimes I cannot get stack of thread. It can be from the first time or appear on some next execution so it is not determined. On some distos it doesn't show stack of thread at all, I don't think the problem is in different implementation of pseudo file in distros but in something other.
Please help to solve this problem
EDIT
I actually call this function inside thread , so I create thread through pthread_create(&tid, NULL, proc_stack, NULL);I have been also thinking about this. Maybe it needs some time to update this pseudofile after thread start, this is only one reason I see here.
EDIT2
I've tried to call sleep forcely , but this didn't help, but the most weird is that on one distro it shows thread stack, on another doesn't.

Answer 1

On my system Your program also dosn't show second stack segment, but it seems 5 segments are allocated above [heap] and one of them is used for thread stack (local variables of proc_stack() are stored in this segment).
Code I used to check for it (test.c):
#include <stdio.h>
#include <string.h>
#include <pthread.h>
void* proc_stack(void* p){
FILE *fp;
char line[2048];
fp = fopen("/proc/self/maps", "r");
if (fp == NULL) {
perror("Error opening file");
return NULL;
}
while (fgets(line, 2048, fp) != NULL) {
// if (strstr(line, "stack") != NULL) {
printf("%s", line);
// }
}
printf("addr = %p %p\n", &fp, &line);
return NULL;
}
int main(){
pthread_t tid;
void *rv;
proc_stack( NULL );
puts("main");
pthread_create( &tid, NULL, proc_stack, NULL );
pthread_join( tid, &rv );
return 0;
}
Compiled with gcc -Wall test.c -pthread. Results (some lines removed):
...
01933000-01954000 rw-p 00000000 00:00 0 [heap]
...
7fff75be3000-7fff75c04000 rw-p 00000000 00:00 0 [stack]
7fff75d97000-7fff75d98000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
addr = 0x7fff75c00e68 0x7fff75c00e70
main
...
01933000-01954000 rw-p 00000000 00:00 0 [heap]
7f3be0000000-7f3be0021000 rw-p 00000000 00:00 0
7f3be0021000-7f3be4000000 ---p 00000000 00:00 0
7f3be6b79000-7f3be6b7a000 rw-p 00000000 00:00 0
7f3be6b7a000-7f3be6b7b000 ---p 00000000 00:00 0
7f3be6b7b000-7f3be737b000 rw-p 00000000 00:00 0
...
7fff75be3000-7fff75c04000 rw-p 00000000 00:00 0 [stack]
7fff75d97000-7fff75d98000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
addr = 0x7f3be73796c8 0x7f3be73796d0