What is the method for obtaining the pid of a socket's peer using programming in MacOS?

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In MacOS, you can obtain the process ID (pid) of a socket's peer using the getsockopt() function with the SO_PEERCRED option. Here's an example code snippet in C:

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ucred.h>

// ...

int sockfd; // socket descriptor

ucred ucred;
socklen_t len = sizeof(ucred);

if (getsockopt(sockfd, SOL_SOCKET, SO_PEERCRED, &ucred, &len) == 0) {
    pid_t pid = ucred.pid;
    // do something with the pid
} else {
    // error handling
}

Note that this method requires that the socket is connected, and that the peer process sets up the socket with its credentials before sending data.