What is IPC?

Inter process communication is any way for one process to talk to another process. You've already seen one form of this virtual memory! A piece of virtual memory can be shared between parent and child, leading to communication. You may want to wrap that memory in pthread_mutexattr_setpshared(&attrmutex, PTHREAD_PROCESS_SHARED); mutex (or a process wide mutex) to prevent race conditions.

There are more standard ways of IPC, like pipes! Consider if you type the following into your terminal

$ ls -1 | cut -d'.' -f1 | uniq | sort | tee dir_contents

What does the following code do (It doesn't really matter so you can skip this if you want)? Well it ls's the current directory (the -1 means that it outputs one entry per line). The cut command then takes everything before the first period. Uniq makes sure all the lines are uniq, sort sorts them and tee outputs to a file.

The important part is that bash creates 5 separate processes and connects their standard outs/stdins with pipes the trail looks something like this.

(0) ls (1)------>(0) cut (1)------->(0) uniq (1)------>(0) sort (1)------>(0) tee (1)

The numbers in the pipes are the file descriptors for each process and the arrow represents the redirect or where the output of the pipe is going.

What is a pipe?

A POSIX pipe is almost like its real counterpart - you can stuff bytes down one end and they will appear at the other end in the same order. Unlike real pipes however, the flow is always in the same direction, one file descriptor is used for reading and the other for writing. The pipe system call is used to create a pipe.

int filedes[2];
pipe (filedes);
printf("read from %d, write to %d\n", filedes[0], filedes[1]);

These file descriptors can be used with read -

// To read...
char buffer[80];
int bytesread = read(filedes[0], buffer, sizeof(buffer));

And write -

write(filedes[1], "Go!", 4);

How can I use pipe to communicate with a child process?

A common method of using pipes is to create the pipe before forking.

int filedes[2];
pipe (filedes);
pid_t child = fork();
if (child > 0) { /* I must be the parent */
    char buffer[80];
    int bytesread = read(filedes[0], buffer, sizeof(buffer));
    // do something with the bytes read    
}

The child can then send a message back to the parent:

if (child == 0) {
   write(filedes[1], "done", 4);
}

Can I use pipes inside a single process?

Short answer: Yes, but I'm not sure why you would want to LOL!

Here's an example program that sends a message to itself:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main() {
    int fh[2];
    pipe(fh);
    FILE *reader = fdopen(fh[0], "r");
    FILE *writer = fdopen(fh[1], "w");
    // Hurrah now I can use printf rather than using low-level read() write()
    printf("Writing...\n");
    fprintf(writer,"%d %d %d\n", 10, 20, 30);
    fflush(writer);

    printf("Reading...\n");
    int results[3];
    int ok = fscanf(reader,"%d %d %d", results, results + 1, results + 2);
    printf("%d values parsed: %d %d %d\n", ok, results[0], results[1], results[2]);

    return 0;
}

The problem with using a pipe in this fashion is that writing to a pipe can block i.e. the pipe only has a limited buffering capacity. If the pipe is full the writing process will block! The maximum size of the buffer is system dependent; typical values from 4KB upto 128KB.

int main() {
    int fh[2];
    pipe(fh);
    int b = 0;
    #define MESG "..............................."
    while(1) {
        printf("%d\n",b);
        write(fh[1], MESG, sizeof(MESG))
        b+=sizeof(MESG);
    }
    return 0;
}

See [[Pipes, Part 2: Pipe programming secrets]]