How to pipe stdin to a child and execl cat in C - c

Question

In the code below, I am simply trying to send a file via stdin to a child process which will exec the cat OS command. The code compiles fine. Here is how I call it from the command line:
$ ./uniquify < words.txt
However, when I run it I get a seg fault error. I am really having a hard time understanding how the flow if information is supposed to work through pipes to children. I am trying to make the code as simple as possible, so I can understand it, but it is not yet making sense. Any help would be appreciated.
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#define NUM_CHILDREN 2
int main(int argc, char *argv[])
{
pid_t catPid;
int writeFds[NUM_CHILDREN];
int catFds[2];
int c = 0;
FILE *writeToChildren[NUM_CHILDREN];
//create a pipe
(void) pipe(catFds);
if ((catPid = fork()) < 0) {
perror("cat fork failed");
exit(1);
}
//this is the child case
if (catPid == 0) {
//close the write end of the pipe
close(catFds[1]);
//close stdin?
close(0);
//duplicate the read side of the pipe
dup(catFds[0]);
//exec cat
execl("/bin/cat", "cat", (char *) 0);
perror("***** exec of cat failed");
exit(20);
}
else { //this is the parent case
//close the read end of the pipe
close(catFds[0]);
int p[2];
//create a pipe
pipe(p);
writeToChildren[c] = fdopen(p[1], "w");
} //only the the parent continues from here
//close file descriptor so the cat child can exit
close(catFds[1]);
char words[NUM_CHILDREN][50];
//read through the input file two words at a time
while (fscanf(stdin, "%s %s", words[0], words[1]) != EOF) {
//loop twice passing one of the words to each rev child
for (c = 0; c < NUM_CHILDREN; c++) {
fprintf(writeToChildren[c], "%s\n", words[c]);
}
}
//close all FILEs and fds by sending and EOF
for (c = 0; c < NUM_CHILDREN; c++) {
fclose(writeToChildren[c]);
close(writeFds[c]);
}
int status = 0;
//wait on all children
for (c = 0; c < (NUM_CHILDREN + 1); c++) {
wait(&status);
}
return 0;
}

Answer 1

Since your question seems to be about understanding how pipes and forks work, I hope below programs can help you. Please notice that this is for illustration only. It wouldn't qualify for commercial implementation, but I wanted to keep it short!
You can compile the two programs as follows:
cc pipechild.c -o pipechild
cc pipeparent.c -o pipeparent
Then execute with ./pipeparent
pipeparent.c source
/* pipeparent.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define MESSAGE "HELLO!\n"
#define INBUFSIZE 80
#define RD 0 // Read end of pipe
#define WR 1 // Write end of pipe
int main(void)
{
int ptocpipe[2]; // Parent-to-child pipe
int ctoppipe[2]; // Chile-to-parent pipe
pid_t childpid; // Process ID of child
char inbuf[80]; // Input from child
int rd; // read() return
int rdup; // dup():ed stdin for child
int wdup; // dup():ed stdout for child
char *eol; // End of line
// Create pipe for writing to child
if (pipe(ptocpipe) < 0) {
fprintf(stderr, "pipe(ptocpipe) failed!\n");
return 2;
}
// Create pipe for writing back to parent
if (pipe(ctoppipe) < 0) {
fprintf(stderr, "pipe(ctoppipe) failed!\n");
return 2;
}
// Verify that one of the pipes are working by filling it first
// in one end and then reading it from the other. The OS will
// buffer the contents for us. Note, this is not at all necessary,
// it's just to illustrate how it works!
write(ptocpipe[WR], MESSAGE, strlen(MESSAGE));
read(ptocpipe[RD], inbuf, INBUFSIZE);
if (strlen(inbuf) != strlen(MESSAGE)) {
fprintf(stderr, "Failed to flush the toilet!\n");
return 6;
} else {
printf("Wrote to myself: %s", inbuf);
}
// Next, we want to launch some interactive program which
// replies with exactly one line to each line we send to it,
// until it gets tired and returns EOF to us.
// First, we must clone ourselves by using fork(). Then the
// child process must be replaced by the interactive program.
// Problem is: How do we cheat the program to read its stdin
// from us, and send its stdout back to us?
switch (childpid = fork()) {
case -1: // Error
fprintf(stderr, "Parent: fork() failed!\n");
return 3;
case 0: // Child process
// Close the ends we don't need. If not, we might
// write back to ourselves!
close(ptocpipe[WR]);
close(ctoppipe[RD]);
// Close stdin
close(0);
// Create a "new stdin", which WILL be 0 (zero)
if ((rdup = dup(ptocpipe[RD])) < 0) {
fprintf(stderr, "Failed dup(stdin)\n");
return 4;
}
// Close stdout
close(1);
// Create a "new stdout", which WILL be 1 (one)
if ((wdup = dup(ctoppipe[WR])) < 0) {
fprintf(stderr, "Failed dup(stdout)\n");
return 5;
}
// For debugging, verify stdin and stdout
fprintf(stderr, "rdup: %d, wdup %d\n", rdup, wdup);
// Overload current process by the interactive
// child process which we want to execute.
execlp("./pipechild", "pipechild", (char *) NULL);
// Getting here means we failed to launch the child
fprintf(stderr, "Parent: execl() failed!\n");
return 4;
}
// This code is executed by the parent only!
// Close the ends we don't need, to avoid writing back to ourself
close(ptocpipe[RD]);
close(ctoppipe[WR]);
// Write one line to the child and expect a reply, or EOF.
do {
write(ptocpipe[WR], MESSAGE, strlen(MESSAGE));
if ((rd = read(ctoppipe[RD], inbuf, INBUFSIZE)) > 0) {
// Chop off ending EOL
if ((eol = rindex(inbuf, '\n')) != NULL)
*eol = '\0';
printf("Parent: Read \"%s\" from child.\n", inbuf);
}
} while (rd > 0);
fprintf(stderr, "Parent: Child done!\n");
return 0;
}
pipechild.c source
/* pipechild.c
* Note - This is only for illustration purpose!
* To be stable, we should catch/ignore signals,
* and use select() to read.
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <strings.h>
#include <string.h>
#define MAXCOUNT 5 // Maximum input lines toread
#define INBUFSIZE 80 // Buffer size
int main(void)
{
char buff[INBUFSIZE];
int remains = MAXCOUNT;
pid_t mypid;
char *eol;
mypid = getpid(); // Process-ID
fprintf(stderr, "Child %d: Started!\n", mypid);
// For each line read, write one tostdout.
while (fgets(buff, INBUFSIZE, stdin) && remains--) {
// Chop off ending EOL
if ((eol = rindex(buff, '\n')) != NULL)
*eol = '\0';
// Debug to console
fprintf(stderr, "Child %d: I got %s. %d remains.\n",
mypid, buff, 1 + remains);
// Reply to parent
sprintf(buff, "Child %d: %d remains\n", mypid, 1 + remains);
write(1, buff, strlen(buff));
}
fprintf(stderr, "Child %d: I'm done!\n", mypid);
return 0;
}