using pipes

[Mr.Pytagoras]

Hi

I'm writing a program which use fork() and I need to read output from child process in parent process

That I did like this:


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

int main(void)
{
int pipefd1[2], pipefd2[2], pipefd3[2];
pid_t pid1, pid2, pid3, wait_pid;
int status;
int i;
int pipefd[2];

if(pipe(pipefd1) == -1){
fprintf(stderr, "Error: %s\n", strerror(errno));
exit (EXIT_FAILURE);
}


char buffer[1024];
switch(pid1 = fork()){
case -1: //fail
fprintf(stderr, "Error: fork 1\n");
exit(1);
case 0: //potomok
close(pipefd1[0]);
dup2(pipefd1[1], 1);
dup2(pipefd1[1], 2);

close(pipefd1[1]);
fprintf(stdout ,"1. from forked child number: 1\n");
fprintf(stdout ,"2. from forked child number: 1\n");
fprintf(stdout ,"3. from forked child number: 1\n");
exit(0);
break;
default:
close(pipefd1[1]); // close the write end of the pipe in the parent

while (read(pipefd1[0], buffer, sizeof(buffer)) > 0){
printf("%s", buffer);
}

}
wait_pid = wait(&status);
printf("process: %d ended with %d\n", wait_pid, status);
exit(0);
}
and the problem is that it prints some weird characters at the end.

I really don't know what's wrong with it.

The weir output:

1. from forked child number: 1
2. from forked child number: 1
3. from forked child number: 1
R��process: 6008 ended with 0

[karlson]

after


char buffer[1024]


do



memset(buffer, 0, sizeof(buffer));

[gmargo]

while (read(pipefd1[0], buffer, sizeof(buffer)) > 0){
printf("%s", buffer);
}


The problem is that the read() system call does not null-terminate the input buffer, which printf(%s) relies on. You must pay more attention to the return value from read().

After adding these extra includes (to make the warnings go away - always use gcc's "-Wall" option to find your undeclared functions):


#include <unistd.h>
#include <sys/wait.h>
And this extra variable (for read()'s return code):


ssize_t rc;
Then you could code the read loop like this:


while (1) {
/* use sizeof(buffer)-1 to leave space for terminating null */
rc = read(pipefd1[0], buffer, sizeof(buffer)-1);
if (rc == -1 && errno == EINTR) {
continue;
}
if (rc <= 0) {
break;
}
buffer[rc] = '\0';
printf("%s", buffer);
}