Wayland client get compositor name

Is it possible for a c application using libwayland-client.so to get the name of the compositor / display server it opened a connection to (e.g. KWin, Sway, …)? I fail to find it in the docs.

For reference, in X11 this is possible using XProps specified by EWMH: _NET_SUPPORTING_WM_CHECK to get the window id of the window manager and then using _NET_WM_NAME.

Im fine with anything giving me a way to identify it, for example a pretty name, the process name, the pid or similar.

Current solution is to detect which socket file wayland will be using (${XDG_RUNTIME_DIR}/${WAYLAND_DISPLAY:-wayland-0}), detecting which process are listening on it and picking the one which is most probably the compositor (similar to what neofetch does in bash). But since i need to open a connection anyway, and this method is very bug prone, i think you can see why i want to have a cleaner solution.

Answer

Requirements:

• determine the PID of the peer compositor process for a display connection on the client side
• must run under Linux
• optionally determines the process name

Since this is not directly supported by the API, you can

• get the file descriptor of the display context (wl_display_get_fd)
• use the file descriptor to read the associated PID of the peer process (getsockopt with the SO_PEERCRED option, see e.g. this nice SO answer)
• finally, you can get the process name by reading /proc/<pid>/comm.

You could also retrieve the process command line if you need more information.

However, the output of the following test program would look like this under Ubuntu 22.04 LTS:

pid: 1733, process name: gnome-shell

Self-contained Example in C

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wayland-client.h>
#include <sys/socket.h>
#include <errno.h>

#define PROCESS_NAME_MAX_LENGTH 1024

static pid_t pid_from_fd(int fd) {
    struct ucred ucred;
    socklen_t len = sizeof(struct ucred);
    if (getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &ucred, &len) == -1) {
        perror("getsockopt failed");
        exit(-1);
    }
    return ucred.pid;
}

static char *process_name_from_pid(const pid_t pid) {
    char *name = malloc(PROCESS_NAME_MAX_LENGTH);
    if (!name) {
        perror("malloc failed");
        exit(-1);
    }
    char proc_buf[64];
    sprintf(proc_buf, "/proc/%d/comm", pid);
    FILE *fp;
    if ((fp = fopen(proc_buf, "r")) == NULL) {
        fprintf(stderr, "opening '%s' failed: %sn", proc_buf, strerror(errno));
        exit(-1);
    }
    if (fgets(name, PROCESS_NAME_MAX_LENGTH, fp) == NULL) {
        fprintf(stderr, "reading '%s' failedn", proc_buf);
        exit(-1);
    }
    name[strcspn(name, "n")] = 0;
    fclose(fp);
    return name;
}

int main(void) {
    struct wl_display *display = wl_display_connect(NULL);
    if (display == NULL) {
        fprintf(stderr, "can't connect to displayn");
        exit(-1);
    }
    int fd = wl_display_get_fd(display);
    pid_t pid = pid_from_fd(fd);
    char *process_name = process_name_from_pid(pid);
    printf("pid: %d, process name: %sn", pid, process_name);
    free(process_name);
    wl_display_disconnect(display);
    return 0;
}