How to print both execl in fork using pipe (two children)? - c

Question

I am trying using pipe() to execute ls | wc. The fork part successfully prints as I expected (first parent --> first child --> second parent --> second child), but it does not print out the second child's part (wc). I put the exactly same codes for both first child and second child, and I have no idea how I can successfully edit the codes. The entire codes are written below:
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<string.h>
#include<sys/wait.h>
/* NOTE: place a new child in parents. ALWAYS! */
int main()
{
// We use two pipes
// First pipe to send input string from parent
// Second pipe to send concatenated string from child
int fd1[2]; // Used to store two ends of first pipe
int fd2[2]; // Used to store two ends of second pipe
char fixed_str[] = "forgeeks.org";
char input_str[100];
pid_t child_a, child_b, child_c; //three children to run three commands
//first pipe
if (pipe(fd1) == -1)
{
fprintf(stderr, "Failed to pipe" );
return 1;
}
//second pipe
if (pipe(fd2) == -1)
{
fprintf(stderr, "Failed to pipe" );
return 1;
}
//scanf("%s", input_str);
child_a = fork(); //first fork
//first child
if (child_a == 0)
{
printf("first child\n");
close(fd1[1]); // Close writing end of first pipe
// Read a string using first pipe
char concat_str[100];
read(fd1[0], concat_str, 100);
execlp("ls", "ls", NULL); //ls | wc222 | wc //FIXME: Changed with concat_str somehow
// We only get here if exec() fails
perror("exec ls");
exit(1);
//concat_str[k] = '\0'; // string ends with '\0'
// Close both reading ends
close(fd1[0]);
close(fd2[0]);
// Write concatenated string and close writing end
write(fd2[1], concat_str, strlen(concat_str)+1);
close(fd2[1]);
exit(0);
}
//first parent
else if (child_a > 0)
{
printf("first parent\n");
child_b = fork(); //second fork
//second child
if (child_b == 0)
{
printf("second child\n");
close(fd1[1]); // Close writing end of first pipe
// Read a string using first pipe
char concat_str[100];
read(fd1[0], concat_str, 100);
execlp("wc", "wc", NULL); //ls | wc222 | wc //FIXME: Changed with concat_str somehow
// We only get here if exec() fails
perror("exec wc");
exit(1);
//concat_str[k] = '\0'; // string ends with '\0'
// Close both reading ends
close(fd1[0]);
close(fd2[0]);
// Write concatenated string and close writing end
write(fd2[1], concat_str, strlen(concat_str)+1);
close(fd2[1]);
exit(0);
}
//second parent
else if (child_b > 0)
{
printf("second parent\n");
char concat_str[100];
close(fd1[0]); // Close reading end of first pipe
// Write input string and close writing end of first
// pipe.
write(fd1[1], input_str, strlen(input_str)+1);
close(fd1[1]);
// Wait for child to send a string
wait(NULL);
close(fd2[1]); // Close writing end of second pipe
// Read string from child, print it and close
// reading end.
read(fd2[0], concat_str, 100);
printf("Concatenated string %s\n", concat_str);
close(fd2[0]);
}
//second error
else
{
fprintf(stderr, "Failed to fork" );
return 1;
}
char concat_str[100];
close(fd1[0]); // Close reading end of first pipe
// Write input string and close writing end of first
// pipe.
write(fd1[1], input_str, strlen(input_str)+1);
close(fd1[1]);
// Wait for child to send a string
wait(NULL);
close(fd2[1]); // Close writing end of second pipe
// Read string from child, print it and close
// reading end.
read(fd2[0], concat_str, 100);
printf("Concatenated string %s\n", concat_str);
close(fd2[0]);
}
//first error
else
{
fprintf(stderr, "Failed to fork" );
return 1;
}
}