Threads passing arguments

Question

#include<stdio.h>
#include<string.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>

pthread_t id1,id2;
struct arg{int a[2];}*p;

void *sum(void *args)
{
    struct arg *b=((struct arg*)args);
    printf("hello:%d%d",b->a[0],b->a[1]);
}

void *mul(void *args)
{
    struct arg *c=((struct arg*)args);
    printf("hi:%d%d",c->a[0],c->a[1]);
}

main()
{
    int err1,err2;
    p->a[0]=2;
    p->a[1]=3;
    err1=pthread_create(&id1,NULL,&sum,(void*)p);
    err2=pthread_create(&id2,NULL,&mul,(void*)p);sleep(5);
}

I am trying to pass data to threads using structure .....but i always gets a segmentation fault error....can anyone tell me what's wrong with my code..

Before assuming you're setting up threading incorrectly, consider that if you threw out the threading, and just invoked either sum or mul with p setup as you have it (or lack thereof as the case may be), you would likely get the same result (crash in a ball of flames). In short, you forgot (or never learned) how pointers work in C. — WhozCraig
– WhozCraig, Commented Oct 5, 2014 at 6:37

alk · Accepted Answer · 2014-10-05 07:47:32Z

2

You are getting segmentation fault because you have not allocated memory for p; it tries to assign values to memory address 0 which leads to a segfault.

Try allocating memory with malloc:

main()
{
int err1,err2;
struct arg *p=(struct arg *)malloc(sizeof(struct arg));
p->a[0]=2;
p->a[1]=3;
err1=pthread_create(&id1,NULL,&sum,(void*)p);
err2=pthread_create(&id2,NULL,&mul,(void*)p);sleep(5);
}

edited Oct 5, 2014 at 7:47

alk

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answered Oct 5, 2014 at 6:37

alireza_fn

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1 Comment

IllusiveBrian Over a year ago

You should not cast the result of malloc, but otherwise a good answer.

Cory Nelson · Accepted Answer · 2014-10-05 06:27:20Z

1

You're getting a segfault because p is initialized to 0. You aren't assigning anything to it.

answered Oct 5, 2014 at 6:27

Cory Nelson

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1 Comment

Cory Nelson Over a year ago

@alk being a global it is initialized to a null pointer as per C standard.

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