Passing data from one pipe to another - c

Question

I am new to pipes but how do I redirect the output from child_1 to the input for child_2?
I am trying to pass the value from the parent to child_1, adds 1 to the value, print the value, then use that output and pass it into child_2, add 1 again, and finally print the value.
The code below has the right output value for child_1, but not for child_2, how do I redirect the output from child_1 to the input for child_2?
Here is my code so far:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char * argv[]) {
int fd[2];
int PID;
pipe(fd); //fd1[0] = read | fd1[1] = write
PID = fork(); // spawn child_1
if (PID < 0){ // failed to fork
perror("Unable to fork child");
exit(1);
}
if (PID > 0) { // parent
int value = 100;
// since parent is only writing, close the reading end of pipe
close(fd[0]);
// write the data to the write end of the pipe
write(fd[1], &value, sizeof(int));
// then close the writing end of the pipe (parent)
close(fd[1]);
/**********************************************************/
} else { // child 1
int val = 0;
// read from the parent pipe
read(fd[0], &val, sizeof(int));
val += 1;
// is this how to redirect from one pipe to another?
dup2(fd[0], fd[1]);
// this prints the right value for val (val [101] = value [100] + 1)
printf("Child [%d] read value %d\n", getpid(), val);
// close the reading end of the pipe for child_1
close(fd[0]);
int PID2 = fork(); // make child 2
if(PID2 == 0) { // child 2
int val2 = 0;
close(0); // close stdin since we are trying to take the value from child_1
// read input from child_1 pipe (NOT WORKING?)
read(fd[0], &val2, sizeof(int));
val2 += 1;
printf("Child [%d] out %d\n", getpid(), val2);
close(fd[0]);
}
}
return EXIT_SUCCESS;
}

Answer 1

The way you have things set up, there's no need to use dup2() or any other I/O redirection.
Add #include <unistd.h> to the list of include files (and remove #include <string.h> — it seems to be unused)
Delete: dup2(fd[0], fd[1]);
Delete: close(fd[0]);
Delete: close(0);
Before the second fork(), add write(fd[1], &val, sizeof(val));
When you have close(fd[0]) in the first child, you effectively close fd[0] for the second child too.
You should check the status of the read and write operations before using the results.
Those changes lead to:
/* SO 7383-1815 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(void)
{
int fd[2];
int PID;
pipe(fd);
PID = fork();
if (PID < 0)
{
perror("Unable to fork child");
exit(EXIT_FAILURE);
}
if (PID > 0)
{
int value = 100;
close(fd[0]);
write(fd[1], &value, sizeof(int));
close(fd[1]);
}
else
{
int val = 0;
if (read(fd[0], &val, sizeof(val)) != sizeof(val))
{
perror("read() failed in child 1");
exit(EXIT_FAILURE);
}
val += 1;
printf("Child [%d] read value %d\n", getpid(), val);
if (write(fd[1], &val, sizeof(val)) != sizeof(val))
{
perror("write() failed in child 1");
exit(EXIT_FAILURE);
}
int PID2 = fork();
if (PID2 == 0)
{
int val2 = 0;
if (read(fd[0], &val2, sizeof(val2)) != sizeof(val2))
{
perror("read() failed in child 2");
exit(EXIT_FAILURE);
}
val2 += 1;
printf("Child [%d] out %d\n", getpid(), val2);
close(fd[0]);
close(fd[1]);
}
}
return EXIT_SUCCESS;
}
When compiled (cleanly with options set fussy), it produces output such as:
Child [34520] read value 101
Child [34521] out 102
I believe this is what was wanted.