how to write a C programme which creates two child processes

Question

how to write a C programme which creates two child processes. Accept a file name from the user. First child should read the contents of the file line by line and writes this content in to shared memory space. The second child should read the content and writes it in to a new file. These child processes should be controlled to make sure that the first child reads only one line at a time and it will write the content only when the second child reads the previously written content. Similarly, the second child should not read an already read content from shared memory.

Answer 1

TLDR: This answer assumes you're writing on a POSIX system. You may want to look into the following:

1. fork() to create the new child processes.
2. Semaphores for communication. Semaphores will probably fit this use case very well. You can also use shared mutexes, condition variables, or even pipes if you'd like.
3. open() and either getline() if on Linux or fgets() on general POSIX for file reading.

The class of problem that you're solving is producer-consumer.

Remember that once you fork(), each process gets a new private address space. They will no longer share any data, so you must make sure the synchronization primitives are in shared memory. Different operating systems do this differently, but POSIX semaphores works if you're on such a system.

A skeleton of your code will probably look something like this:

#include <fcntl.h>

#include <stdio.h>

#include <semaphore.h>

#include <unistd.h>

int child1(sem_t* empty_semaphore, sem_t* full_semaphore) {

// Child 1 logic here

}

int child2(sem_t* empty_semaphore, sem_t* full_semaphore) {

// Child 2 logic here

}

int main() {

sem_t empty_semaphore;

sem_t full_semaphore;

int sem_result = sem_init(&empty_semaphore, /* pshared */ 1, /* initial value here */);

if (sem_result != 0) {

return 1;

}

sem_result = sem_init(&full_semaphore, /* pshared */ 1, /* initial value here */);

if (sem_result != 0) {

return 1;

}

pid = fork();

if (pid == 0) {

return child1(empty_semaphore, full_semaphore);

}

pid = fork();

if (pid == 0) {

return child2(empty_semaphore, full_semaphore);

}

// This is the parent process

// Wait for the children to complete.

int status;

wait(&status);

wait(&status);

return 0;

}