asked on

fgets or pthread causing a segmentation fault

I get a segmentation fault when I run this program. "Producer created" appears on the screen. After I type in a character a "Segmentation fault occurs. If I don't enter data the "Consumer created" appears and I still get a segmentation fault. I'm just trying to get thread one to read the stdin data and thread 2 to write the data to stdout. I'm not that familiar with pthread_create, how it shares data, or all these pointers. I know something is overflowing in memory but I don't know how to fix the problem.
It may be that fgets and fputs that are causing problems too. Can someone please help me with this program? Thanks

#include <pthread.h>
#include <stdio.h>
void *Producer(void *data);
void *Consumer(void *data);

int main()
{
pthread_t t1, t2;

printf("main started\n");
pthread_create(&t1, NULL, Producer, NULL);
sleep(3);
pthread_create(&t2, NULL, Consumer, NULL);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
printf("main done \n");
}

void *Producer(void *data)
{ int i=0;

      printf("Producer created\n");
      for(i;i=1;i++)
{
                  sleep(3);
fgets(data,27,stdin);
                  printf("data read \n");
                  sleep(3);
}
}

void *Consumer(void *data)
{
int i=0;
printf("Consumer created\n");
for (i ; i= 1; i++)
fputs(data,stdout);

}

nixfreak

> for(i;i=1;i++)

for(i; i == 1; i++)

dwiseman3

ASKER

Thanks for your quick response...it's always those little things that cause problems. Still, after that fix, I don't think I'm passing anything from stdin to data. The Producer function doesn't printf("data read \n"). Consumer created is printed but no input goes to stdout. If I change the for loop in Producer to while(1) the segmentation fault returns.
Is fgets the problem?

nixfreak

To be honest I don't know much about threads :)
But when I corrected both the for loops for the error then the program seems to work fine. I am wondering where "data" is getting stored?

jkr

You are passing NULL as data and then use

fgets(data,27,stdin);

That for sure will cause a segfault, since it is equivalent to

fgets(NULL,27,stdin);

Try passing a valid buffer, i.e.

int main()
{
pthread_t t1, t2;
char in[28];
char out[28];

printf("main started\n");
pthread_create(&t1, NULL, Producer, (void*) out);
sleep(3);
pthread_create(&t2, NULL, Consumer, (void*) in);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
printf("main done \n");
}

dwiseman3

ASKER

That helped, but stdin and stdout are not getting my input. When I run this code:

#include <pthread.h>
#include <stdio.h>
void *Producer(void *data);
void *Consumer(void *data);

int main()
{
pthread_t t1, t2;
char in[28];
char out[28];
printf("main started\n");
pthread_create(&t1, NULL, Producer,(void*) in);
sleep(3);
pthread_create(&t2, NULL, Consumer, (void*)out);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
printf("main done \n");
}

void *Producer(void *data)
{ int i=0;

      printf("Producer created\n");
      for(i;i==1;i++)
{
                  sleep(3);
fgets(data,27,stdin);
                  printf("data read \n");
                  sleep(3);
}
}

void *Consumer(void *data)
{
int i=0;
printf("Consumer created\n");
for (i;i==1;i++)
{
       fputs(data,stdout);
       printf("data out \n");
}
return NULL;
}

The results are:

main started
Producer created
abc
Consumer created
main done
[dwiseman@06 ~/project]$ abc

jkr

The loop conditions are bogus. I is assigned a 0 the loop executed only when 'i==1' - IOW never. Make that

for(i = 0;i<1;i++)
{
sleep(3);
fgets(data,27,stdin);
printf("data read \n");
sleep(3);
}

nixfreak

Just a suggestion, shouldn't you use a single data pointer instead of "in" and "out"
....
char *data;

printf("main started\n");
pthread_create(&t1, NULL, Producer,(void*) data);
sleep(3);
pthread_create(&t2, NULL, Consumer,(void*) data);
.....

jkr

I'd take that as a next step when the whole thing works ;o)

nixfreak

I removed the loop as suggested by JKR and the program now works fine.

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
void *Producer(void *data);
void *Consumer(void *data);

int main()
{
pthread_t t1, t2;
char *data;

printf("main started\n");
pthread_create(&t1, NULL, Producer, data);
sleep(3);
pthread_create(&t2, NULL, Consumer, data);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
printf("main done \n");
}

void *Producer(void *data)
{
printf("Producer created\n");
fgets(data,27,stdin);
printf("data read \n");
}

void *Consumer(void *data)
{
printf("Consumer created\n");
fputs(data,stdout);
}

dwiseman3

ASKER

I fixed the for loops...the program flows like it should. Still, I got no stdin or stdout. So, I changed the function arguments from in/out to data....back to a Segmentation fault problem.

#include <pthread.h>
#include <stdio.h>
int main()
{
pthread_t t1, t2;
void *Producer(void *data);
void *Consumer(void *data);
char in[28];
char out[28];
char *data;
printf("main started\n");
pthread_create(&t1, NULL, Producer,(void*)data);
sleep(3);
pthread_create(&t2, NULL, Consumer, (void*)data);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
printf("main done \n");
}

void *Producer(void *data)
{ int i;
printf("Producer created\n");
      for(i=0;i<1;i++)
{
                  sleep(3);
fgets(data,27,stdin);
                  printf("data read \n");
                  sleep(3);
}
}
void *Consumer(void *data)
{
int i;
printf("Consumer created\n");
for (i=0;i<1;i++)
{
       fputs(data,stdout);
       printf("data out \n");
}
return NULL;
}

RESULTS:
main started
Producer created
abc
Segmentation fault

ASKER CERTIFIED SOLUTION

jkr

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dwiseman3

ASKER

by golly it worked!! I just have to put some locks on it or increase the sleep.
Thank you very much!

jkr

Yup, was just about to mention the locks. See also http://www.llnl.gov/computing/tutorials/pthreads/#Mutexes