Share a file descriptor between parent and child after fork and exec

Question

I have two processes on Linux, A & B. I want to share the file descriptor from process A with process B, now I just serialize it to a char* and pass it to the execl parameters, but that doesn't work.

A.c looks like this:

union descriptor{
    char c[sizeof(int)];
    int i;
} fd;
pid_t pid;

fd.i = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
// Perform other socket functions

pid = fork();
if(pid == 0){
    // Read data from socket
    if(execl("./B", fd.c, NULL) < 0){
        exit(EXIT_FAILURE);
    }else(
        exit(EXIT_SUCCESS);
    }
}else if(pid < 0){
    exit(EXIT_FAILURE);
}else{
    waitpid(pid, NULL, 0);
}

B.c looks like this:

union descriptor{
    char c[sizeof(int)];
    int i;
} fd;

memcpy(&fd.c[0], argv[0], sizeof(int));

write(fd.i, "TEST", 4);
close(fd.i);

But this doesn't work, and I don't really understand why not. How can I make this work? And if it works, is it the best solution to share a file descriptor between a parent and a child after a fork and a exec?

Update

The problem is unrelated to the question I asked, it is caused by a wrong way of passing an integer as pointed out by @OliCharlesworth. Please close this question.

Answer 1

File descriptors are always passed between a parent and child process

When you fork a process, the file descriptors that are open in the parent(at the time of fork()) are implicitly passed on to the child. There is no need to send them explicitly.

For example:

The pseudo-code looks as follows:

In process A:

fd = open_socket_or_file;
char str_fd[3];
str_fd[0]=fd;
str_fd[1]=fd;
str_fd[2]=0;
if(fork()==0)
{
     execl("./B",str_fd,NULL);
}

In the child process B you can do:

int fd = argv[1][0];
/* now do whatever you want with the fd...*/

EDIT:

In case the processes are different, you need to pass the file descriptor explicitly. This is generally done using UNIX-Domain Sockets(If you are using Linux Flavors). For code related to this, you can see this answer