Wayland client get compositor name - c

Question

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 1

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: %s\n", proc_buf, strerror(errno));
exit(-1);
}
if (fgets(name, PROCESS_NAME_MAX_LENGTH, fp) == NULL) {
fprintf(stderr, "reading '%s' failed\n", 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 display\n");
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: %s\n", pid, process_name);
free(process_name);
wl_display_disconnect(display);
return 0;
}