Implement -U.

If this argument is provided Conky won't start if another Conky process
is already running.

doc/man.md.j2

@@ -160,6 +160,10 @@ file.
 
 :   Update interval.
 
+**-U \| \--unique**
+
+:   Conky won't start if another Conky process is already running.
+
 **-v \| -V \| \--version** 
 
 :   Prints version, build information and general info. Exits after

src/conky.cc

@@ -2106,7 +2106,7 @@ void set_current_config() {
 
 /* : means that character before that takes an argument */
 const char *getopt_string =
-    "vVqdDSs:t:u:i:hc:p:"
+    "vVqdDSs:t:u:i:hc:p:U"
 #ifdef BUILD_X11
     "x:y:w:a:X:m:f:"
 #ifdef OWN_WINDOW
@@ -2136,7 +2136,8 @@ const struct option longopts[] = {
     {"double-buffer", 0, nullptr, 'b'}, {"window-id", 1, nullptr, 'w'},
 #endif /* BUILD_X11 */
     {"text", 1, nullptr, 't'},          {"interval", 1, nullptr, 'u'},
-    {"pause", 1, nullptr, 'p'},         {nullptr, 0, nullptr, 0}};
+    {"pause", 1, nullptr, 'p'},         {"unique", 0, nullptr, 'U'},
+    {nullptr, 0, nullptr, 0}};
 
 void setup_inotify() {
 #ifdef HAVE_SYS_INOTIFY_H

src/main.cc

@@ -36,6 +36,8 @@
 #include "display-output.hh"
 #include "lua-config.hh"
 
+#include <dirent.h>
+
 #ifdef BUILD_X11
 #include "x11.h"
 #endif /* BUILD_X11 */
@@ -268,10 +270,54 @@ static void print_help(const char *prog_name) {
          "   -i COUNT                  number of times to update " PACKAGE_NAME
          " (and quit)\n"
          "   -p, --pause=SECS          pause for SECS seconds at startup "
-         "before doing anything\n",
+         "before doing anything\n"
+         "   -U, --unique              only one conky process can be created\n",
          prog_name);
 }
 
+static bool is_conky_already_running() {
+  DIR* dir;
+  struct dirent* ent;
+  char* endptr;
+  char buf[512];
+
+  const size_t len_conky = 5; // "conky"
+  int instances = 0;
+
+  if (!(dir = opendir("/proc"))) {
+      NORM_ERR("can't open /proc: %s\n", strerror(errno));
+      return false;
+  }
+
+  while ((ent = readdir(dir)) != NULL) {
+    long lpid = strtol(ent->d_name, &endptr, 10);
+    if (*endptr != '\0') {
+        continue;
+    }
+
+    snprintf(buf, sizeof(buf), "/proc/%ld/cmdline", lpid);
+    FILE* fp = fopen(buf, "r");
+    if (fp) {
+      if (fgets(buf, sizeof(buf), fp) != NULL) {
+        const char* name = strtok(buf, " ");
+        const size_t len = strlen(name);
+
+        if (len >= len_conky) {
+          name = name + len - len_conky;
+        }
+
+        if (!strcmp(name, "conky")) {
+          instances++;
+        }
+      }
+      fclose(fp);
+    }
+  }
+
+  closedir(dir);
+  return instances > 1;
+}
+
 inline void reset_optind() {
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || \
     defined(__OpenBSD__) || defined(__NetBSD__) || defined(__DragonFly__)
@@ -293,6 +339,8 @@ int main(int argc, char **argv) {
   g_sighup_pending = 0;
   g_sigusr2_pending = 0;
 
+  bool unique_process = false;
+
 #ifdef BUILD_CURL
   struct curl_global_initializer {
     curl_global_initializer() {
@@ -349,12 +397,20 @@ int main(int argc, char **argv) {
         window.window = strtol(optarg, nullptr, 0);
         break;
 #endif /* BUILD_X11 */
+      case 'U':
+        unique_process = true;
+        break;
 
       case '?':
         return EXIT_FAILURE;
     }
   }
 
+  if (unique_process && is_conky_already_running()) {
+    NORM_ERR("already running");
+    return 0;
+  }
+
   try {
     set_current_config();