Linux: Using pipe to send the information in between the processes - c

Question

The pipe is created with fd[] and fd1[]. I am writing with the pipes. Taking two pipes gave me error as i was referencing to STDIN_FILENO, so i deleted it. I just used the pipe to read and write. Now it is still not reading from the pipe. Pipe with file descriptor fd[] works. but the fd1[] does not.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv)
{
int fd[2];
pid_t childpid;
pipe(fd);
int fd2[2];
pipe(fd2);
int array[5] = {1,2,3,4,5};
int array2[5] = {11,12,13,14,15};
//fprintf(stderr,"size%ld",sizeof(int));
//char buffer[111];
int *subarr1;
int buffer_num;
int status;
if ((childpid = fork()) == 0) {
fprintf(stderr,"\nProcess:%d Parent:%d",getpid(),getppid());
//dup2(fd[1], STDOUT_FILENO);
close(fd[0]); //read end
write(fd[1],array,100);
close(fd[1]);
//execl("/bin/ls", "ls", "-l", NULL);
perror("The exec of ls failed\n");
if ((childpid = fork()) == 0)
{
fprintf(stderr,"\nProcess:%d Parent:%d",getpid(),getppid());
//dup2(fd2[1], STDOUT_FILENO);
close(fd2[0]); //read end
write(fd2[1],array2,sizeof(array2));
close(fd2[1]);
//execl("/bin/ls", "ls", "-l", NULL);
perror("The exec of ls failed\n");
}
else
{// parent
wait(NULL);
}
}
else {
wait(&status);
fprintf(stderr,"\nTHis is parent");
subarr1 = (int*)malloc(sizeof(int)*(5));
fprintf(stderr,"\nProcess:%d Parent:%d",getpid(),getppid());
//dup2(fd[0], STDIN_FILENO);
close(fd[1]);
int j;
for(j=0;j<5;j++)
{
read(fd[0],&buffer_num,sizeof(int));
subarr1[j] = buffer_num;
//printf("%s",buffer);
}
for(j=0;j<5;j++)
{
fprintf(stderr,"\n%d\n",subarr1[j]);
}
close(fd[0]);//read end
//dup2(fd2[0], STDIN_FILENO);
buffer_num = 0;
close(fd2[1]);
//read(fd2[0],&buffer_num,100);
for(j=0;j<5;j++)
{
read(fd2[0],&buffer_num,sizeof(int));
subarr1[j] = buffer_num;
//printf("%s",buffer);
}
for(j=0;j<5;j++)
{
fprintf(stderr,"\n%d\n",subarr1[j]);
}
//fprintf(stderr, "\nThis is second pipe%s", buffer);
close(fd2[0]);
/*//printf("\nAAAAAAAA%s\n",buffer);
dup2(fd2[0], STDIN_FILENO);
close(fd2[1]);
printf("\nAAAAAAAA%s\n",buffer);
read(fd2[0],buffer,100);
printf("%s",buffer);
close(fd2[0]);//read end
printf("\nAAAAAAAA%s\n",buffer);
*/
//execl("/usr/bin/sort", "sort", "-n", NULL);
perror("The exec of sort failed\n");
}
return 0;
}

Answer 1

This call:
read(fd2[0],&buffer_num,100);
tries to read 100 bytes into memory starting at the local (int) variable buffer, so overwrites whatever else is on the stack after it, smashing the stack and causing undefined behavior (probably crashing at some point).
You have similar problems with your write calls, writing 100 bytes from objects that are much smaller, but those likely only write garbage you then ignore, rather than causing a crash (though that is likewise undefined behavior).