Check if a macOS network interface is a "direct link" interface (what is used for iOS device debugging over USB)

is-direct-link-interface.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <sys/errno.h>

// adopted from https://developer.apple.com/documentation/technotes/tn3158-resolving-xcode-15-device-connection-issues#Update-software-using-packet-filtering-rules

/**
 Tests whether an interface is a direct-link interface.
 - Parameter name: The BSD interface name, for example, `en0`.
 - Returns 1 if it is a direct-link interface, 0 if it’s not, and -1 if an error occurred.
    In the last case, see `errno` for the error code.
 */
int isDirectLinkInterface(const char * name) {
    int fd = socket(PF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        return -1;
    }


    struct ifreq ifr = { 0 };
    strlcpy(ifr.ifr_name, name, sizeof(ifr.ifr_name));
    // also displayed as DIRECTLINK under eflags with `ifconfig -v`
    int success = ioctl(fd, SIOCGIFDIRECTLINK, &ifr) >= 0;
    if (!success) {
        int error = errno;
        (void) close(fd);
        errno = error;
        return -1;
    }
    
    (void) close(fd);
    return ifr.ifr_ifru.ifru_is_directlink != 0;
}

int main(int argc, const char * argv[]) {
    if (argc <= 1) {
        printf("Usage: %s ifname\n", argv[0]);
        return EXIT_FAILURE;
    }
    int isDirectLinkInterfaceVal = isDirectLinkInterface(argv[1]);
    if (isDirectLinkInterfaceVal >= 0) {
        printf("isDirectLinkInterface: %d\n", isDirectLinkInterfaceVal);
        return EXIT_SUCCESS;
    } else {
        printf("isDirectLinkInterface error, errno: %d\n", errno);
        return EXIT_FAILURE;
    }
}