// 
// PARENT PROCESS
// (child below)

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


int main (int argc, const char * argv[]) {
    int pipefd[2];
    pid_t pid;
    int writeEnd, readEnd;
    int i;
    
    char *path = "/path/to/ForkChild";
    char *arg0 = "ForkChild";
    char arg1[10];

    int rc;
    
    char buffer[256];
    int bufferSize = 256;
    
    // let's create some pipes for the instance
    if(pipe(pipefd) < 0) {
        perror("pipe failed");
        return -1;
    }

    writeEnd = pipefd[1];
    readEnd = pipefd[0];
        
    pid = fork();
    switch(pid) {
        case -1:
            perror("fork");
            return -1;

        case 0: // ******** child **********
            
            sprintf(arg1, "%d", writeEnd);
            
            if(execl(path, arg0, arg1, NULL) == -1) {
                perror("error spawning child");
            }
            
            break;

        default: // ******* parent **********
            
            // clear the buffer
            for(i = 0; i < bufferSize; i++) buffer[i] = 0;
            
            // Listen for messages coming on read end of pipe
            rc = read(readEnd, buffer, bufferSize);
            printf("buffer=%s\n", buffer);
            
            break;
    
    }
    
    return 0;
}






// ForkChild code

#include <stdio.h>

int main(int argc, const char * argv[]) {
    int fd;
    
    if(argc >= 2) {
        
        // translate first argument        
        fd = atoi(argv[1]);        
        
        FILE *outFP = fdopen(fd, "w");
        
        fprintf(outFP, "Test\n");
        
        fclose(outFP);
    
    }
    
    return 0;
}