pipe function returning random characters - c

Question

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
int main(void){
//Variables, p[2] for each end of the pipe. nbytes to read pipe return value SUCCESS or FAILURE. pid_t to hold pid of fork process.
// buffer to hold response from the child process.
int p[2], nbytes;
pid_t childpid;
char string[] = "Hello, World!\n";
char buffer[80];
//Declaration of pipe
pipe(p);
//Error handling.
if(((childpid = fork()) == -1) || (pipe(p) == -1))
{
perror("fork");
exit(1);
}
//Child process sends message to paprent.
if(childpid == 0)
{
/* Child process closes up input side of pipe */
close(p[0]);
/* Send "string" through the output side of pipe */
write(p[1], string, (strlen(string)+1));
exit(0);
}
else
{
/* Parent process closes up output side of pipe */
close(p[1]);
/* Read in a string from the pipe */
nbytes = read(p[0], buffer, sizeof(buffer));
printf("Received string: %s", buffer);
}
return(0);
}
Output > Received string: #�=zJ
The point of the exercise is to have a child process send a message through a pipe to the parent process and the parent returns the result. This exact code worked the first time I ran it, but then when I tried to run it a second time it started to return seemingly random characters each time. I tried to copy my buffer to another variable but then it was empty. Is the pipe actually not function the way I think it is? What am I doing wrong?

Answer 1

You first create a pipe with pipe(p); and then you create another with ... || (pipe(p) == -1)) Is that deliberate?
2nd Pipe was causing an issue.

Answer 2

You have:
pipe(p);
//Error handling.
if(((childpid = fork()) == -1) || (pipe(p) == -1))
{
perror("fork");
exit(1);
}
This creates two pipes — one in the line pipe(p); and the second in the condition if(((childpid = fork()) == -1) || (pipe(p) == -1)). This is wasteful at best. Moreover, the second pipe is after the fork(), so the parent and child processes don't access the same pipe any more — you overwrote the one created before the fork() which they do share. Test the result of pipe() before calling fork() and remove the extra condition in the if test:
if (pipe(p) != 0)
{
perror("pipe");
exit(1);
}
if ((childpid = fork()) < 0)
{
perror("fork");
exit(1);
}
Get used to testing for errors and writing appropriate code to handle them. It will be a major part of your life as a C programmer.
Later on in the code, you have:
{
/* Parent process closes up output side of pipe */
close(p[1]);
/* Read in a string from the pipe */
nbytes = read(p[0], buffer, sizeof(buffer));
printf("Received string: %s", buffer);
}
You need to heed the value of nbytes. Since it is an int, you could use:
printf("Received %d bytes: [%.*s]\n", nbytes, nbytes, buffer);
This limits the output to what was read, and reports 0 if that's what it gets. I suppose you should also check for -1 in nbytes before using it in the printf() statement:
if (nbytes < 0)
{
fprintf(stderr, "failed to read from pipe descriptor %d\n", p[0]);
// Or perror("read");
// Should you exit here with a non-zero status?
}
else
printf("Received %d bytes: [%.*s]\n", nbytes, nbytes, buffer);
Note: errors are reported on stderr; perror() does that automatically.

Answer 3

The problem is that you create two pipes when you really only need to check the first for errors:
// Declaration of pipe
if(pipe(p) == -1) { // check for error here
perror("pipe");
exit(1);
}
// Error handling.
if((childpid = fork()) == -1) { // and don't create another pipe here
perror("fork");
exit(1);
}
You should also check the return values from write and read. They may not write or read the full string in one go.