segmentation fault in create_pthread

[ken18]

Trying to learn about pthread and am getting a segmentation fault using the following program.

Any idea why the segmentation fault?


#include <stdio.h>
#include <sys/types.h>
#include <sys/syscall.h>
#include <pthread.h>
#include <unistd.h>
#include <math.h>


void * thread(void *arg) {
int i;
double tmp;

for (i=0; i<100000; i++) {
tmp = cos( (double) i);
tmp *= tmp;
}
return 0;
}

int main(void) {
int i;
int n = 40000; // result is segmentation fault. n = 30000 is ok...
// note: cat /proc/sys/kernel/threads-max returns 90245
// compile string: gcc pthread7.c -Wall -lm -lpthread
pthread_t tid[n];

for (i=0; i<n; i++ ) { pthread_create( &tid[i], NULL, thread, NULL ); }
for (i=0; i<n; i++ ) { pthread_join( tid[i], NULL ); }
return 0;
}

[spjackson]

Your tid array isn't initialized, so if pthread_create fails, which it might, then you will be passing uninitialized data to pthread_join, resulting perhaps in your segmentation fault.

Instead of




for (i=0; i<n; i++ ) { pthread_create( &tid[i], NULL, thread, NULL ); }


Try


for (i=0; i<n; i++ ) {
if (pthread_create( &tid[i], NULL, thread, NULL ) != 0) {
fprintf(stderr, "pthread_create failed with i = %d. errno = %d, %s\n",
i, errno, strerror(errno));
n = i; /* Don't call join with uninitialised data */
break;
}
}

On my system the output is: pthread_create failed with i = 32752. errno = 12, Cannot allocate memory

[ken18]

That was it, thanks!


pthread_create failed with i = 32752. errno = 12, Cannot allocate memory

Odd that it's the same exact number as yours. Also, I have 6GB of ram, but only 0.26GB was used (via getrusage) when the error occurred. Hmmm, looks like I'll have some fun chasing that one. I doubt I'll ever need any where close to that many threads, but it bugs me to get errors that don't make sense to me.

Ken