How to Use FIFOs to Communicate Between Two Programs in C?

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When working with inter-process communication (IPC) in C, using FIFOs (named pipes) is a widely accepted approach. In this article, we will address a common issue: how to call "Program 2" from the child process of "Program 1," allowing "Program 1" to receive input from the user and pass it through a FIFO queue. The goal is to ensure that the parent process can retrieve data written to the queue by the child program and display it accordingly.

Understanding the Problem


The requirement is to establish a communication channel between two C programs using FIFO. When you run "Program 1," it forks a child process that executes "Program 2," which takes input from the user and writes it to a FIFO. "Program 1" needs to read the data back after "Program 2" has completed its execution.

Why the Issue Occurs


The problem typically arises when the FIFO is created in the child process ("Program 2") while the parent process ("Program 1") waits for the child to finish. If the FIFO is not opened correctly, or if there are synchronization issues, this can lead to deadlocks where the programs appear to halt without terminating.

Solution Steps


To resolve these issues, follow the structured approach below:

Step 1: Create FIFO Properly


Ensure the FIFO is created only if it doesn't exist and is opened correctly. Modify your Program 2 like this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>

int main() {
const char *fifo_name = "f";
char input[100];

// Create FIFO if it doesn't exist
if (mkfifo(fifo_name, 0777) == -1 && errno != EEXIST) {
perror("mkfifo");
return 1;
}

printf("FIFO Created \n");

// Take input from user
fgets(input, sizeof(input), stdin);
input[strcspn(input, "\n")] = '\0';
printf("User input accepted \n");

int fd = open(fifo_name, O_WRONLY);
if (fd == -1) {
perror("Error opening FIFO");
return 1;
}
write(fd, input, sizeof(input));
printf("FIFO content written successfully \n");
close(fd);
return 0;
}

Step 2: Modify Program 1 to Open FIFO Correctly


In Program 1, make sure to create the FIFO only if it does not already exist (this can be done in Program 2). Ensure the parent process is waiting appropriately:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/wait.h>

int main() {
const char *fifo_name = "f";
char output[100];

// Fork a child process
pid_t forkCall = fork();
if (forkCall == 0) {
// Child process executes Program 2
execl("./Program2", "Program2", NULL);
perror("execl failed");
exit(1);
} else if (forkCall == -1) {
perror("Error creating child process");
exit(0);
} else {
// Parent process
int fd = open(fifo_name, O_RDONLY);
if (fd == -1) {
perror("Error opening FIFO for reading");
return 1;
}
wait(NULL); // Wait for child to finish
read(fd, output, sizeof(output));
close(fd);
printf("Reader process read: %s\n", output);
unlink(fifo_name); // Remove FIFO after use
}
return 0;
}

Step 3: Compile and Run Your Programs


Compile both programs:

gcc -o Program1 Program1.c
gcc -o Program2 Program2.c


To run them, execute Program 1 in one terminal and input data when prompted. Make sure your terminal is set to the correct paths.

Troubleshooting Common Issues

1. FIFO Not Accessible: Ensure you have the correct permissions and that the FIFO is created before trying to open it.
2. Blocking Issues: When opening the FIFO for reading or writing, ensure the other program is prepared to read or write to avoid blocking.
3. Data Handling: Always ensure that the input buffer is managed correctly, including null-termination.

Frequently Asked Questions (FAQ)

Why am I seeing stale data in the output?


It's possible that the FIFO has not been cleared or is holding previous data. Make sure to unlink (remove) the FIFO before trying again to ensure no old data is present.

What if the programs still hang?


Check the order of operations and ensure that the FIFO is correctly synchronized, with proper wait commands and input/output management. Debugging with print statements can help identify where the hang occurs.

Conclusion


Using FIFOs for IPC in C can be tricky due to the synchronous nature of the operations. By properly managing FIFO creation, reading/writing processes, and ensuring that order of execution is maintained, both programs can communicate effectively. Ensure that you follow best practices in error handling to catch any potential issues during execution.

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