how to pass integer value between 2 process in c

Question

How can I pass an integer value between 2 processes?

For example:
I have 2 processes: child1 and child2. Child1 sends an integer number to child2.

Answer 1

IPC (or Inter-process communication) is indeed a broad subject.
You can use shared files, shared memory or signals to name a few.
Which one to use is really up to you and determined by the design of your application.

Since You wrote that You are using windows, here's a working example using pipes:

Note that I'm treating the buffer as null-terminated string. You can treat it as numbers.

Server:

// Server
#include <stdio.h>
#include <Windows.h>

#define BUFSIZE     (512)
#define PIPENAME    "\\\\.\\pipe\\popeye"

int main(int argc, char **argv)
{
    char msg[] = "You too!";
    char buffer[BUFSIZE];
    DWORD dwNumberOfBytes;
    BOOL bRet = FALSE;
    HANDLE hPipe = INVALID_HANDLE_VALUE;

    hPipe = CreateNamedPipeA(PIPENAME,
        PIPE_ACCESS_DUPLEX,
        PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
        PIPE_UNLIMITED_INSTANCES,
        BUFSIZE,
        BUFSIZE,
        0,
        NULL);

    bRet = ConnectNamedPipe(hPipe, NULL);

    bRet = ReadFile(hPipe, buffer, BUFSIZE, &dwNumberOfBytes, NULL);
    printf("receiving: %s\n", buffer);

    bRet = WriteFile(hPipe, msg, strlen(msg)+1, &dwNumberOfBytes, NULL);
    printf("sending: %s\n", msg);

    CloseHandle(hPipe);

    return 0;
}

Client:

// client
#include <stdio.h>
#include <Windows.h>

#define BUFSIZE     (512)
#define PIPENAME    "\\\\.\\pipe\\popeye"

int main(int argc, char **argv)
{
    char msg[] = "You're awesome!";
    char buffer[BUFSIZE];
    DWORD dwNumberOfBytes;

    printf("sending: %s\n", msg);
    CallNamedPipeA(PIPENAME, msg, strlen(msg)+1, buffer, BUFSIZE, &dwNumberOfBytes, NMPWAIT_WAIT_FOREVER);
    printf("receiving: %s\n", buffer);

    return 0;
}

Hope that helps!