/* atoms.c from Pegasus Epsilon's Desktop Environment
** Goal: Less than a megabyte resident
** (C)opyright 2019-2020 "Pegasus Epsilon" <pegasus@pimpninjas.org>
** Distribute Unmodified - https://pegasus.pimpninjas.org/license
#include <X11/Xlib.h> // XInternAtom()
#include <string.h> // strncmp(), strchr(), strncpy(), strncat()
#include <stdlib.h> // calloc(), exit(), free()
enum Atoms {
#define ATOM_EXPANDO(x) x,
#include "expandos.h"
Atom atom[ATOMS_LENGTH];
static char *atom_names[] = {
[WM_Sn] = NULL,
// Not needed, yet...
//#define ATOM_EXPANDO(x) [x] = #x,
//#include "expandos.h"
void shutdown_atom_cache (void) { free(atom_names[WM_Sn]); }
void initialize_atom_cache (Display *display, char **envp) {
// parse DISPLAY environment variable, and construct WM_S<screen number>
// atom name per ICCCM § Discriminated Names and § Manager Selections
for (unsigned i = 0; envp[i]; i++)
if (0 == strncmp(envp[i], "DISPLAY=:", 9)) {
char *end, *start = envp[i] + 9;
// This looks unsafe, but validity of the DISPLAY environment
// variable is already assured by XOpenDisplay before we ever
// get here. Maybe we won't understand the format of it, but
// we know it's *valid*, at least, so we can suck some sort of
// data out of it. Even if it's just a null string, which will
// result in WM_S. That's good enough, so long as we don't
// have to deal with another window manager. If the format of
// the DISPLAY environment variable ever changes, though, we
// *should* update this for the sake of continuing to operate
// according to the relevant standards, but we will at least
// continue to function (wrongly) without an update.
if (NULL == (end = strchr(start, '.'))) end = strchr(start, 0);
size_t len = (size_t)(end - start);
atom_names[WM_Sn] = calloc(1, len + 5);
// strcpy() returns a pointer to the beginning of the copied string
// strncpy() returns a pointer to the END of the copied string
// https://www.joelonsoftware.com/2001/12/11/back-to-basics/
strncat(strncpy(atom_names[WM_Sn], "WM_S", 5), start, len);
atom[WM_Sn] = XInternAtom(display, atom_names[WM_Sn], False);
#define ATOM_EXPANDO(x) atom[x] = XInternAtom(display, #x, False);
#include "expandos.h"