Pipes, Forks and Polls in parent-child process - c

Question

I am working on an assignment where I have to count the number of chars from the command line arguments. The parent is to pass the child one char at a time and the child is to count the number of chars and return the count to the parent so it can print the number of chars. When I run my program it just sits and does nothing. I think my problem is when I get to the stage of passing the count back to the parent and reaping the child. I think my code is fairly solid up until that point and then that is were I get a little fuzzy. Any help would be greatly appreciated.
// Characters from command line arguments are sent to child process
// from parent process one at a time through pipe.
//
// Child process counts number of characters sent through pipe.
//
// Child process returns number of characters counted to parent process.
//
// Parent process prints number of characters counted by child process.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> //for fork and pip
#include <sys/wait.h>
#include <string.h>
int main(int argc, char **argv)
{
pid_t pid;
int comm[2];
int status;
char src;
// set up pipe
if (pipe(comm))
{
printf("Pipe Error!\n");
return -1;
}
// call fork()
pid = fork();
//check if fork failed
if (pid < 0)
{
printf("Fork Error! %d\n", pid);
return -1;
}
if (pid == 0)
{
// -- running in child process --
//close output side of pipe
close(comm[1]);
int nChars = 0;
printf("in child\n");
// Receive characters from parent process via pipe
// one at a time, and count them.
while (read(comm[0], &src, 1))
{
++nChars;
printf("testing child loop = %d\n", nChars);
}
//close input side of pipe
close(comm[0]);
// Return number of characters counted to parent process.
return nChars;
}
else
{
// -- running in parent process --
int nChars = 0;
//close input side of pipe
close(comm[0]);
printf("Assignment 3\n");
// Send characters from command line arguments starting with
// argv[1] one at a time through pipe to child process.
int i;
for (i = 1; i < argc; ++i) //loop through each argument
{
int j;
for (j = 0; j < strlen(argv[i]); ++j) //loop through each character in argument
write(comm[1], &argv[i][j], 1);
}
//closing the write end of the pipe
close(comm[1]);
// Wait for child process to return. Reap child process.
waitpid(pid, &status, 0);
// Receive number of characters counted via the value
// returned when the child process is reaped
printf("child counted %d chars\n", nChars);
return 0;
}
}

Answer 1

Here is basically how you should have done it - unless you were absolutely forced to go the return code route.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
#define c2p 0
#define p2c 1
#define READEND 0
#define WRITEEND 1
int main(int argc, char **argv)
{
pid_t pid;
int comm[2][2];
char src;
for (int i = 0; i < 2; ++i)
if (pipe(comm[i]))
errExit("pipe");
if ((pid = fork()) == -1)
errExit("fork");
if (! pid)
{
close(comm[p2c][WRITEEND]);
close(comm[c2p][READEND]);
int nChars = 0;
while (read(comm[p2c][READEND], &src, 1))
++nChars;
write(comm[c2p][WRITEEND], &nChars, sizeof(nChars));
close(comm[c2p][WRITEEND]); //sends eof to parent
printf("child counted %d chars\n", nChars);
return 0;
}
int nChars = 0;
close(comm[p2c][READEND]);
close(comm[c2p][WRITEEND]);
printf("Assignment 3\n");
for (int i = 1; i < argc; ++i) //loop through each argument
{
int len = strlen(argv[i]);
for (int j = 0; j < len; ++j)
write(comm[p2c][WRITEEND], &argv[i][j], 1);
}
close(comm[p2c][WRITEEND]); //sends eof to child
read(comm[c2p][READEND], &nChars, sizeof(nChars)); //should really be in a loop - your task
close(comm[c2p][READEND]);
wait(0);
printf("parent reports %d chars\n", nChars);
return 0;
}