diff --git a/Makefile.am b/Makefile.am
index 002982d9..f5ba9689 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,9 +19,10 @@ bin_PROGRAMS = xsecurelock
 xsecurelock_SOURCES = \
 	auth_child.c auth_child.h \
 	env_settings.c env_settings.h \
-	saver_child.c saver_child.h \
+	logging.c logging.h \
 	mlock_page.h \
 	main.c \
+	saver_child.c saver_child.h \
 	wm_properties.c wm_properties.h \
 	xscreensaver_api.c xscreensaver_api.h
 xsecurelock_CPPFLAGS = \
@@ -69,9 +70,10 @@ endif
 helpers_PROGRAMS = \
 	saver_multiplex
 saver_multiplex_SOURCES = \
-	helpers/saver_multiplex.c \
 	env_settings.c env_settings.h \
 	helpers/monitors.c helpers/monitors.h \
+	helpers/saver_multiplex.c \
+	logging.c logging.h \
 	saver_child.c saver_child.h \
 	wm_properties.c wm_properties.h \
 	xscreensaver_api.c xscreensaver_api.h
@@ -85,9 +87,10 @@ if HAVE_PAM
 helpers_PROGRAMS += \
 	auth_pam_x11
 auth_pam_x11_SOURCES = \
-	helpers/auth_pam_x11.c \
 	env_settings.c env_settings.h \
+	helpers/auth_pam_x11.c \
 	helpers/monitors.c helpers/monitors.h \
+	logging.c logging.h \
 	mlock_page.h \
 	xscreensaver_api.c xscreensaver_api.h
 auth_pam_x11_CPPFLAGS = \
diff --git a/auth_child.c b/auth_child.c
index 426af388..e06b0b19 100644
--- a/auth_child.c
+++ b/auth_child.c
@@ -18,14 +18,14 @@ limitations under the License.
 
 #include <errno.h>     // for ECHILD, EINTR, errno
 #include <signal.h>    // for kill, SIGTERM
-#include <stdio.h>     // for perror, fprintf, stderr
-#include <stdlib.h>    // for NULL, exit, EXIT_FAILURE, etc
+#include <stdlib.h>    // for NULL, exit, EXIT_FAILURE, EXIT_SUCCESS
 #include <string.h>    // for strlen
-#include <sys/wait.h>  // for waitpid, WEXITSTATUS, etc
-#include <unistd.h>    // for close, pid_t, ssize_t, dup2, etc
+#include <sys/wait.h>  // for WIFEXITED, WIFSIGNALED, waitpid, WEXIT...
+#include <unistd.h>    // for close, pid_t, ssize_t, dup2, execl, fork
 
-#include "env_settings.h"
-#include "xscreensaver_api.h"
+#include "env_settings.h"      // for GetIntSetting
+#include "logging.h"           // for LogErrno, Log
+#include "xscreensaver_api.h"  // for ExportWindowID
 
 //! The PID of a currently running saver child, or 0 if none is running.
 static pid_t auth_child_pid = 0;
@@ -106,7 +106,7 @@ int WatchAuthChild(Display *dpy, Window w, const char *executable,
           break;
         default:
           // Assume the child still lives. Shouldn't ever happen.
-          perror("waitpid");
+          LogErrno("waitpid");
           break;
       }
     } else if (pid == auth_child_pid) {
@@ -129,13 +129,12 @@ int WatchAuthChild(Display *dpy, Window w, const char *executable,
         // Only report signals; "normal" exit is not worth logging as it usually
         // means authentication failure anyway.
         if (WIFSIGNALED(status)) {
-          fprintf(stderr, "Auth child killed by signal %d.\n",
-                  WTERMSIG(status));
+          Log("Auth child killed by signal %d", WTERMSIG(status));
         }
       }
       // Otherwise, it was suspended or whatever. We need to keep waiting.
     } else if (pid != 0) {
-      fprintf(stderr, "Unexpectedly woke up for PID %d.\n", (int)pid);
+      Log("Unexpectedly woke up for PID %d", (int)pid);
     }
     // Otherwise, we're still alive.
   }
@@ -144,11 +143,11 @@ int WatchAuthChild(Display *dpy, Window w, const char *executable,
     // Start auth child.
     int pc[2];
     if (pipe(pc)) {
-      perror("pipe");
+      LogErrno("pipe");
     } else {
       pid_t pid = fork();
       if (pid == -1) {
-        perror("fork");
+        LogErrno("fork");
       } else if (pid == 0) {
         // Child process.
         setsid();
@@ -157,7 +156,7 @@ int WatchAuthChild(Display *dpy, Window w, const char *executable,
         close(pc[0]);
         close(pc[1]);
         execl(executable, executable, NULL);  // Flawfinder: ignore
-        perror("execl");
+        LogErrno("execl");
         exit(EXIT_FAILURE);
       } else {
         // Parent process after successful fork.
@@ -184,12 +183,12 @@ int WatchAuthChild(Display *dpy, Window w, const char *executable,
       ssize_t to_write = (ssize_t)strlen(stdinbuf);  // Flawfinder: ignore
       ssize_t written = write(auth_child_fd, stdinbuf, to_write);
       if (written < 0) {
-        perror("Failed to send all data to the auth child");
+        LogErrno("Failed to send all data to the auth child");
       } else if (written != to_write) {
-        fprintf(stderr, "Failed to send all data to the auth child.\n");
+        Log("Failed to send all data to the auth child");
       }
     } else {
-      fprintf(stderr, "No auth child. Can't send key events...\n");
+      Log("No auth child. Can't send key events");
     }
   }
 
diff --git a/env_settings.c b/env_settings.c
index fabdb7e9..5f4df4ee 100644
--- a/env_settings.c
+++ b/env_settings.c
@@ -20,6 +20,8 @@ limitations under the License.
 #include <stdio.h>   // for fprintf, NULL, stderr
 #include <stdlib.h>  // for getenv, strtol, strtoull
 
+#include "logging.h"
+
 unsigned long long GetUnsignedLongLongSetting(const char* name,
                                               unsigned long long def) {
   const char* value = getenv(name);  // Flawfinder: ignore
@@ -30,11 +32,11 @@ unsigned long long GetUnsignedLongLongSetting(const char* name,
   errno = 0;
   unsigned long long number = strtoull(value, &endptr, 0);
   if (errno == ERANGE) {
-    fprintf(stderr, "Ignoring out-of-range value of %s: %s.", name, value);
+    Log("Ignoring out-of-range value of %s: %s", name, value);
     return def;
   }
   if ((endptr != NULL && *endptr != 0)) {
-    fprintf(stderr, "Ignoring non-numeric value of %s: %s.", name, value);
+    Log("Ignoring non-numeric value of %s: %s", name, value);
     return def;
   }
   return number;
@@ -49,11 +51,11 @@ long GetLongSetting(const char* name, long def) {
   errno = 0;
   long number = strtol(value, &endptr, 0);
   if (errno == ERANGE) {
-    fprintf(stderr, "Ignoring out-of-range value of %s: %s.", name, value);
+    Log("Ignoring out-of-range value of %s: %s", name, value);
     return def;
   }
   if ((endptr != NULL && *endptr != 0)) {
-    fprintf(stderr, "Ignoring non-numeric value of %s: %s.", name, value);
+    Log("Ignoring non-numeric value of %s: %s", name, value);
     return def;
   }
   return number;
@@ -63,7 +65,7 @@ int GetIntSetting(const char* name, int def) {
   long lnumber = GetLongSetting(name, def);
   int number = (int)lnumber;
   if (lnumber != (long)number) {
-    fprintf(stderr, "Ignoring out-of-range value of %s: %d.", name, number);
+    Log("Ignoring out-of-range value of %s: %d", name, number);
     return def;
   }
   return number;
diff --git a/helpers/auth_pam_x11.c b/helpers/auth_pam_x11.c
index fbc158ef..6696b8cd 100644
--- a/helpers/auth_pam_x11.c
+++ b/helpers/auth_pam_x11.c
@@ -22,7 +22,7 @@ limitations under the License.
 #include <pwd.h>                    // for getpwuid, passwd
 #include <security/_pam_types.h>    // for PAM_SUCCESS, pam_strerror, pam_re...
 #include <security/pam_appl.h>      // for pam_end, pam_start, pam_acct_mgmt
-#include <stdio.h>                  // for fprintf, stderr, perror, NULL
+#include <stdio.h>                  // for fprintf, stderr, LogErrno, NULL
 #include <stdlib.h>                 // for mblen, exit, free, calloc, getenv
 #include <string.h>                 // for memcpy, strlen, memset
 #include <sys/select.h>             // for timeval, select, FD_SET, FD_ZERO
@@ -34,6 +34,7 @@ limitations under the License.
 #endif
 
 #include "../env_settings.h"      // for GetStringSetting
+#include "../logging.h"           // for Log, LogErrno
 #include "../mlock_page.h"        // for MLOCK_PAGE
 #include "../xscreensaver_api.h"  // for ReadWindowID
 #include "monitors.h"             // for Monitor, GetMonitors, IsMonitorCh...
@@ -94,19 +95,19 @@ const char *get_indicators() {
   xkb = XkbGetMap(display, 0, XkbUseCoreKbd);
   if (XkbGetNames(display, XkbIndicatorNamesMask | XkbGroupNamesMask, xkb) !=
       Success) {
-    fprintf(stderr, "XkbGetNames failed\n");
+    Log("XkbGetNames failed");
     XkbFreeClientMap(xkb, 0, True);
     return "";
   }
   XkbStateRec state;
   if (XkbGetState(display, XkbUseCoreKbd, &state) != Success) {
-    fprintf(stderr, "XkbGetState failed\n");
+    Log("XkbGetState failed");
     XkbFreeClientMap(xkb, 0, True);
     return "";
   }
   unsigned int istate;
   if (XkbGetIndicatorState(display, XkbUseCoreKbd, &istate) != Success) {
-    fprintf(stderr, "XkbGetIndicatorState failed\n");
+    Log("XkbGetIndicatorState failed");
     XkbFreeClientMap(xkb, 0, True);
     return "";
   }
@@ -116,7 +117,7 @@ const char *get_indicators() {
   const char *word = "Keyboard: ";
   size_t n = strlen(word);  // Flawfinder: ignore
   if (n >= sizeof(buf) - (p - buf)) {
-    fprintf(stderr, "Not enough space to store intro '%s'.\n", word);
+    Log("Not enough space to store intro '%s'", word);
     return "";
   }
   memcpy(p, word, n);  // Flawfinder: ignore
@@ -125,7 +126,7 @@ const char *get_indicators() {
   word = XGetAtomName(display, xkb->names->groups[state.group]);
   n = strlen(word);  // Flawfinder: ignore
   if (n >= sizeof(buf) - (p - buf)) {
-    fprintf(stderr, "Not enough space to store group name '%s'.\n", word);
+    Log("Not enough space to store group name '%s'", word);
     return "";
   }
   memcpy(p, word, n);  // Flawfinder: ignore
@@ -143,10 +144,10 @@ const char *get_indicators() {
     const char *word = XGetAtomName(display, namea);
     size_t n = strlen(word);  // Flawfinder: ignore
     if (n + 2 >= sizeof(buf) - (p - buf)) {
-      fprintf(stderr, "Not enough space to store modifier name '%s'.\n", word);
+      Log("Not enough space to store modifier name '%s'", word);
       continue;
     }
-    memcpy(p, ", ", 2);  // Flawfinder: ignore
+    memcpy(p, ", ", 2);      // Flawfinder: ignore
     memcpy(p + 2, word, n);  // Flawfinder: ignore
     p += n + 2;
   }
@@ -318,7 +319,7 @@ int prompt(const char *msg, char **response, int echo) {
   int blinks = 0;
 
   if (!echo && MLOCK_PAGE(&priv, sizeof(priv)) < 0) {
-    perror("mlock");
+    LogErrno("mlock");
     // We continue anyway, as the user being unable to unlock the screen is
     // worse. But let's alert the user.
     alert("Password will not be stored securely.", 1);
@@ -391,7 +392,7 @@ int prompt(const char *msg, char **response, int echo) {
       FD_SET(0, &set);
       int nfds = select(1, &set, NULL, NULL, &timeout);
       if (nfds < 0) {
-        perror("select");
+        LogErrno("select");
         done = 1;
         break;
       }
@@ -409,7 +410,7 @@ int prompt(const char *msg, char **response, int echo) {
 
       ssize_t nread = read(0, &priv.inputbuf, 1);  // Flawfinder: ignore
       if (nread <= 0) {
-        fprintf(stderr, "EOF on password input - bailing out.\n");
+        Log("EOF on password input - bailing out");
         done = 1;
         break;
       }
@@ -450,7 +451,7 @@ int prompt(const char *msg, char **response, int echo) {
         case '\n':
           *response = malloc(priv.pwlen + 1);
           if (!echo && MLOCK_PAGE(*response, priv.pwlen + 1) < 0) {
-            perror("mlock");
+            LogErrno("mlock");
             // We continue anyway, as the user being unable to unlock the screen
             // is worse. But let's alert the user of this.
             alert("Password has not been stored securely.", 1);
@@ -474,7 +475,7 @@ int prompt(const char *msg, char **response, int echo) {
                 PARANOID_PASSWORD_LENGTH;
 #endif
           } else {
-            fprintf(stderr, "Password entered is too long - bailing out.\n");
+            Log("Password entered is too long - bailing out");
             done = 1;
             break;
           }
@@ -495,7 +496,7 @@ int prompt(const char *msg, char **response, int echo) {
   memset(&priv, 0, sizeof(priv));
 
   if (!done) {
-    fprintf(stderr, "Unreachable code - the loop above must set done.\n");
+    Log("Unreachable code - the loop above must set done");
   }
   return status;
 }
@@ -539,11 +540,10 @@ int converse(int num_msg, const struct pam_message **msg,
   (void)appdata_ptr;
 
   if (conv_error) {
-    fprintf(stderr,
-            "converse() got called again with %d messages (first: %s) after "
-            "having failed before - this is very likely a bug in the PAM "
-            "module having made the call. Bailing out.\n",
-            num_msg, num_msg <= 0 ? "(none)" : msg[0]->msg);
+    Log("converse() got called again with %d messages (first: %s) after "
+        "having failed before - this is very likely a bug in the PAM "
+        "module having made the call. Bailing out",
+        num_msg, num_msg <= 0 ? "(none)" : msg[0]->msg);
     exit(1);
   }
 
@@ -610,32 +610,32 @@ int authenticate(const char *username, const char *hostname,
       GetStringSetting("XSECURELOCK_PAM_SERVICE", PAM_SERVICE_NAME);
   int status = pam_start(service_name, username, conv, pam);
   if (status != PAM_SUCCESS) {
-    fprintf(stderr, "pam_start: %d\n",
-            status);  // Or can one call pam_strerror on a NULL handle?
+    Log("pam_start: %d",
+        status);  // Or can one call pam_strerror on a NULL handle?
     return status;
   }
 
   status = pam_set_item(*pam, PAM_RHOST, hostname);
   if (status != PAM_SUCCESS) {
-    fprintf(stderr, "pam_set_item: %s\n", pam_strerror(*pam, status));
+    Log("pam_set_item: %s", pam_strerror(*pam, status));
     return status;
   }
   status = pam_set_item(*pam, PAM_RUSER, username);
   if (status != PAM_SUCCESS) {
-    fprintf(stderr, "pam_set_item: %s\n", pam_strerror(*pam, status));
+    Log("pam_set_item: %s", pam_strerror(*pam, status));
     return status;
   }
   const char *display = getenv("DISPLAY");  // Flawfinder: ignore
   status = pam_set_item(*pam, PAM_TTY, display);
   if (status != PAM_SUCCESS) {
-    fprintf(stderr, "pam_set_item: %s\n", pam_strerror(*pam, status));
+    Log("pam_set_item: %s", pam_strerror(*pam, status));
     return status;
   }
 
   status = call_pam_with_retries(pam_authenticate, *pam, 0);
   if (status != PAM_SUCCESS) {
     if (!conv_error) {
-      fprintf(stderr, "pam_authenticate: %s\n", pam_strerror(*pam, status));
+      Log("pam_authenticate: %s", pam_strerror(*pam, status));
     }
     return status;
   }
@@ -647,7 +647,7 @@ int authenticate(const char *username, const char *hostname,
 #ifdef PAM_CHECK_ACCOUNT_TYPE
     if (status2 != PAM_SUCCESS) {
       if (!conv_error) {
-        fprintf(stderr, "pam_chauthtok: %s\n", pam_strerror(*pam, status2));
+        Log("pam_chauthtok: %s", pam_strerror(*pam, status2));
       }
       return status2;
     }
@@ -661,7 +661,7 @@ int authenticate(const char *username, const char *hostname,
     // If this one is true, it must be coming from pam_acct_mgmt, as
     // pam_chauthtok's result already has been checked against PAM_SUCCESS.
     if (!conv_error) {
-      fprintf(stderr, "pam_acct_mgmt: %s\n", pam_strerror(*pam, status2));
+      Log("pam_acct_mgmt: %s", pam_strerror(*pam, status2));
     }
     return status2;
   }
@@ -685,13 +685,13 @@ int main() {
 #endif
 
   if ((display = XOpenDisplay(NULL)) == NULL) {
-    fprintf(stderr, "could not connect to $DISPLAY\n");
+    Log("Could not connect to $DISPLAY");
     return 1;
   }
 
   char hostname[256];  // Flawfinder: ignore
   if (gethostname(hostname, sizeof(hostname))) {
-    perror("gethostname");
+    LogErrno("gethostname");
     return 1;
   }
   hostname[sizeof(hostname) - 1] = 0;
@@ -705,19 +705,19 @@ int main() {
   }
   pwd_buf = malloc((size_t)pwd_bufsize);
   if (!pwd_buf) {
-    perror("malloc(pwd_bufsize)");
+    LogErrno("malloc(pwd_bufsize)");
     return 1;
   }
   getpwuid_r(getuid(), &pwd_storage, pwd_buf, (size_t)pwd_bufsize, &pwd);
   if (!pwd) {
-    perror("getpwuid_r");
+    LogErrno("getpwuid_r");
     free(pwd_buf);
     return 1;
   }
 
   window = ReadWindowID();
   if (window == None) {
-    fprintf(stderr, "Invalid/no window ID in XSCREENSAVER_WINDOW.\n");
+    Log("Invalid/no window ID in XSCREENSAVER_WINDOW");
     free(pwd_buf);
     return 1;
   }
@@ -730,17 +730,16 @@ int main() {
   if (font_name[0] != 0) {
     font = XLoadQueryFont(display, font_name);
     if (font == NULL) {
-      fprintf(stderr,
-              "could not load the specified font %s - trying to fall back to "
-              "fixed\n",
-              font_name);
+      Log("Could not load the specified font %s - trying to fall back to "
+          "fixed",
+          font_name);
     }
   }
   if (font == NULL) {
     font = XLoadQueryFont(display, "fixed");
   }
   if (font == NULL) {
-    fprintf(stderr, "could not load a mind-bogglingly stupid font\n");
+    Log("Could not load a mind-bogglingly stupid font");
     exit(1);
   }
 
@@ -772,7 +771,7 @@ int main() {
     return 1;
   }
   if (status2 != PAM_SUCCESS) {
-    fprintf(stderr, "pam_end: %s\n", pam_strerror(pam, status2));
+    Log("pam_end: %s", pam_strerror(pam, status2));
     return 1;
   }
 
diff --git a/helpers/auth_pamtester.in b/helpers/auth_pamtester.in
index 65733415..41bd1284 100644
--- a/helpers/auth_pamtester.in
+++ b/helpers/auth_pamtester.in
@@ -18,4 +18,4 @@ echo "Enter password:" |\
   display -window "$XSCREENSAVER_WINDOW" TEXT:-
 
 exec >/dev/null 2>&1
-exec pamtester @pam_service_name@ "$USER" authenticate
+exec @with_pamtester@ @pam_service_name@ "$USER" authenticate
diff --git a/helpers/monitors.c b/helpers/monitors.c
index ae6cf02b..aa8a0ca9 100644
--- a/helpers/monitors.c
+++ b/helpers/monitors.c
@@ -17,19 +17,19 @@ limitations under the License.
 #include "monitors.h"
 
 #include <X11/Xlib.h>  // for XWindowAttributes, Display, XGetW...
-#include <stdio.h>     // for fprintf, stderr
 #include <stdlib.h>    // for qsort
 #include <string.h>    // for memcmp, memset
 
 #ifdef HAVE_XRANDR
-#include <X11/extensions/Xrandr.h>  // for XRRCrtcInfo, XRROutputInfo, XRRSc...
-#include <X11/extensions/randr.h>   // for RRNotify, RRCrtcChangeNotifyMask
+#include <X11/extensions/Xrandr.h>  // for XRRMonitorInfo, XRRCrtcInfo, XRRO...
+#include <X11/extensions/randr.h>   // for RANDR_MAJOR, RRNotify, RANDR_MINOR
 #if RANDR_MAJOR > 1 || (RANDR_MAJOR == 1 && RANDR_MINOR >= 5)
 #define HAVE_XRANDR15
 #endif
 #endif
 
 #include "../env_settings.h"  // for GetIntSetting
+#include "../logging.h"       // for Log
 
 #ifdef HAVE_XRANDR
 static Display* initialized_for = NULL;
@@ -94,19 +94,19 @@ static int IntervalsOverlap(int astart, int asize, int bstart, int bsize) {
 static void AddMonitor(Monitor* out_monitors, size_t* num_monitors,
                        size_t max_monitors, int x, int y, int w, int h) {
 #ifdef DEBUG_EVENTS
-  fprintf(stderr, "try %d %d %d %d\n", x, y, w, h);
+  Log("AddMonitor %d %d %d %d", x, y, w, h);
 #endif
   // Too many monitors? Stop collecting them.
   if (*num_monitors >= max_monitors) {
 #ifdef DEBUG_EVENTS
-    fprintf(stderr, "skip (too many)\n");
+    Log("Skip (too many)");
 #endif
     return;
   }
   // Skip empty "monitors".
   if (w <= 0 || h <= 0) {
 #ifdef DEBUG_EVENTS
-    fprintf(stderr, "skip (zero)\n");
+    Log("Skip (zero)");
 #endif
     return;
   }
@@ -116,13 +116,13 @@ static void AddMonitor(Monitor* out_monitors, size_t* num_monitors,
     if (IntervalsOverlap(x, w, out_monitors[i].x, out_monitors[i].width) &&
         IntervalsOverlap(y, h, out_monitors[i].y, out_monitors[i].height)) {
 #ifdef DEBUG_EVENTS
-      fprintf(stderr, "skip (overlap with %d)\n", (int)i);
+      Log("Skip (overlap with %d)", (int)i);
 #endif
       return;
     }
   }
 #ifdef DEBUG_EVENTS
-  fprintf(stderr, "monitor %d = %d %d %d %d\n", (int)*num_monitors, x, y, w, h);
+  Log("Monitor %d = %d %d %d %d", (int)*num_monitors, x, y, w, h);
 #endif
   out_monitors[*num_monitors].x = x;
   out_monitors[*num_monitors].y = y;
@@ -209,7 +209,7 @@ static int GetMonitorsXRandR(Display* dpy, Window w,
   Window child;
   if (!XTranslateCoordinates(dpy, w, DefaultRootWindow(dpy), xwa->x, xwa->y,
                              &wx, &wy, &child)) {
-    fprintf(stderr, "XTranslateCoordinates failed.\n");
+    Log("XTranslateCoordinates failed");
     wx = xwa->x;
     wy = xwa->y;
   }
@@ -264,7 +264,7 @@ size_t GetMonitors(Display* dpy, Window w, Monitor* out_monitors,
     }
 #endif
     GetMonitorsGuess(&xwa, out_monitors, &num_monitors, max_monitors);
-  } while(0);
+  } while (0);
 
   // Sort the monitors in some deterministic order.
   qsort(out_monitors, num_monitors, sizeof(*out_monitors), CompareMonitors);
diff --git a/helpers/saver_multiplex.c b/helpers/saver_multiplex.c
index fada249c..be52a4d4 100644
--- a/helpers/saver_multiplex.c
+++ b/helpers/saver_multiplex.c
@@ -24,6 +24,7 @@ limitations under the License.
 #include <unistd.h>      // for sleep
 
 #include "../env_settings.h"      // for GetStringSetting
+#include "../logging.h"           // for Log, LogErrno
 #include "../saver_child.h"       // for MAX_SAVERS
 #include "../wm_properties.h"     // for SetWMProperties
 #include "../xscreensaver_api.h"  // for ReadWindowID
@@ -92,7 +93,7 @@ static void KillSavers(void) {
  */
 int main(int argc, char** argv) {
   if (GetIntSetting("XSECURELOCK_INSIDE_SAVER_MULTIPLEX", 0)) {
-    fprintf(stderr, "starting saver_multiplex inside saver_multiplex?!?\n");
+    Log("Starting saver_multiplex inside saver_multiplex?!?");
     // If we die, the parent process will revive us, so let's sleep a while to
     // conserve battery and avoid log spam in this case.
     sleep(60);
@@ -101,14 +102,14 @@ int main(int argc, char** argv) {
   setenv("XSECURELOCK_INSIDE_SAVER_MULTIPLEX", "1", 1);
 
   if ((display = XOpenDisplay(NULL)) == NULL) {
-    fprintf(stderr, "could not connect to $DISPLAY\n");
+    Log("Could not connect to $DISPLAY");
     return 1;
   }
   int x11_fd = ConnectionNumber(display);
 
   Window parent = ReadWindowID();
   if (parent == None) {
-    fprintf(stderr, "Invalid/no parent ID in XSCREENSAVER_WINDOW.\n");
+    Log("Invalid/no parent ID in XSCREENSAVER_WINDOW");
     return 1;
   }
 
@@ -135,14 +136,14 @@ int main(int argc, char** argv) {
     sa.sa_flags = 0;
     sa.sa_handler = handle_sigterm;
     if (sigaction(SIGTERM, &sa, NULL) != 0) {
-      perror("sigaction(SIGTERM)");
+      LogErrno("sigaction(SIGTERM)");
     }
     sa.sa_handler = handle_sigchld;
     if (sigaction(SIGCHLD, &sa, NULL) != 0) {
-      perror("sigaction(SIGCHLD)");
+      LogErrno("sigaction(SIGCHLD)");
     }
   } else {
-    perror("sigprocmask failed; not installing signal handlers");
+    LogErrno("sigprocmask failed; not installing signal handlers");
   }
   for (;;) {
     fd_set in_fds;
diff --git a/helpers/saver_xscreensaver.in b/helpers/saver_xscreensaver.in
index ab072b0a..503a47e6 100755
--- a/helpers/saver_xscreensaver.in
+++ b/helpers/saver_xscreensaver.in
@@ -92,7 +92,7 @@ while :; do
     $saver
   fi
   sleep 2
-  echo >&2 "Screen saver failed: $saver. Trying another one..."
+  echo >&2 "xsecurelock: Screen saver failed: $saver. Trying another one..."
   selected=$((selected + 1))
   mode=random
 done &
diff --git a/logging.c b/logging.c
new file mode 100644
index 00000000..097e5d57
--- /dev/null
+++ b/logging.c
@@ -0,0 +1,27 @@
+#include "logging.h"
+
+#include <errno.h>   // for errno
+#include <stdarg.h>  // for va_end, va_list, va_start
+#include <stdio.h>   // for fputs, stderr, vfprintf, perror, NULL
+
+void Log(const char *format, ...) {
+  va_list args;
+  va_start(args, format);
+  fputs("xsecurelock: ", stderr);
+  vfprintf(stderr, format, args);  // Flawfinder: ignore
+  fputs(".\n", stderr);
+  va_end(args);
+}
+
+void LogErrno(const char *format, ...) {
+  int errno_save = errno;
+  va_list args;
+  va_start(args, format);
+  fputs("xsecurelock: ", stderr);
+  vfprintf(stderr, format, args);  // Flawfinder: ignore
+  fputs(": ", stderr);
+  errno = errno_save;
+  perror(NULL);
+  va_end(args);
+  errno = errno_save;
+}
diff --git a/logging.h b/logging.h
new file mode 100644
index 00000000..06515dae
--- /dev/null
+++ b/logging.h
@@ -0,0 +1,39 @@
+/*
+Copyright 2018 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#ifndef LOGGING_H
+#define LOGGING_H
+
+/*! \brief Prints the given string to the error log (stderr).
+ *
+ * For a format expanding to "Foo", this will log "xsecurelock: Foo.".
+ *
+ * \param format A printf format string, followed by its arguments.
+ */
+void Log(const char *format, ...)
+    __attribute__((format(printf, 1, 2)));  // Flawfinder: ignore
+
+/*! \brief Prints the given string to the error log (stderr).
+ *
+ * For a format expanding to "Foo", this may log "xsecurelock: Foo: No such
+ * file or directory". The value of errno is preserved by this function.
+ *
+ * \param format A printf format string, followed by its arguments.
+ */
+void LogErrno(const char *format, ...)
+    __attribute__((format(printf, 1, 2)));  // Flawfinder: ignore
+
+#endif
diff --git a/main.c b/main.c
index 6f3a0016..d5de8dc4 100644
--- a/main.c
+++ b/main.c
@@ -22,16 +22,16 @@ limitations under the License.
  */
 
 #include <X11/X.h>       // for Window, GrabModeAsync, Curren...
-#include <X11/Xatom.h>   // for XA_ATOM
+#include <X11/Xatom.h>   // for XA_CARDINAL, XA_ATOM
 #include <X11/Xlib.h>    // for XEvent, False, XSelectInput
 #include <X11/Xutil.h>   // for XLookupString
 #include <errno.h>       // for ECHILD, EINTR, errno
 #include <locale.h>      // for NULL, setlocale, LC_CTYPE
 #include <signal.h>      // for sigaction, sigemptyset, SIGPIPE
-#include <stdio.h>       // for fprintf, stderr, perror, printf
+#include <stdio.h>       // for printf, size_t
 #include <stdlib.h>      // for EXIT_SUCCESS, exit, EXIT_FAILURE
 #include <string.h>      // for __s1_len, __s2_len, memset
-#include <sys/select.h>  // for timeval, select, FD_SET, FD_ZERO
+#include <sys/select.h>  // for timeval, fd_set, select, FD_SET
 #include <sys/wait.h>    // for WEXITSTATUS, waitpid, WIFEXITED
 #include <time.h>        // for nanosleep, timespec
 #include <unistd.h>      // for access, pid_t, X_OK, chdir
@@ -49,6 +49,7 @@ limitations under the License.
 
 #include "auth_child.h"     // for WantAuthChild, WatchAuthChild
 #include "env_settings.h"   // for GetIntSetting, GetStringSetting
+#include "logging.h"        // for Log, LogErrno
 #include "mlock_page.h"     // for MLOCK_PAGE
 #include "saver_child.h"    // for WatchSaverChild
 #include "wm_properties.h"  // for SetWMProperties
@@ -178,7 +179,7 @@ int IgnoreErrorsHandler(Display *display, XErrorEvent *error) {
   char buf[128];  // Flawfinder: ignore
   XGetErrorText(display, error->error_code, buf, sizeof(buf));
   buf[sizeof(buf) - 1] = 0;
-  fprintf(stderr, "Got non-fatal X11 error: %s.\n", buf);
+  Log("Got non-fatal X11 error: %s", buf);
   return 0;
 }
 
@@ -245,16 +246,14 @@ int parse_arguments(int argc, char **argv) {
   int i;
   for (i = 1; i < argc; ++i) {
     if (!strncmp(argv[i], "auth_", 5)) {
-      fprintf(stderr,
-              "Setting auth child name from command line is DEPRECATED.\nUse "
-              "the XSECURELOCK_AUTH environment variable instead.\n");
+      Log("Setting auth child name from command line is DEPRECATED. Use "
+          "the XSECURELOCK_AUTH environment variable instead");
       auth_executable = argv[i];
       continue;
     }
     if (!strncmp(argv[i], "saver_", 6)) {
-      fprintf(stderr,
-              "Setting saver child name from command line is DEPRECATED.\nUse "
-              "the XSECURELOCK_SAVER environment variable instead.\n");
+      Log("Setting saver child name from command line is DEPRECATED. Use "
+          "the XSECURELOCK_SAVER environment variable instead");
       saver_executable = argv[i];
       continue;
     }
@@ -263,7 +262,7 @@ int parse_arguments(int argc, char **argv) {
       break;
     }
     // If we get here, the argument is unrecognized. Exit, then.
-    fprintf(stderr, "Unrecognized argument: %s.\n", argv[i]);
+    Log("Unrecognized argument: %s", argv[i]);
     return 0;
   }
   return 1;
@@ -285,36 +284,36 @@ int check_settings() {
   // that could make the system un-unlockable.
 
   if (auth_executable == NULL) {
-    fprintf(stderr, "Auth module has not been specified in any way.\n");
+    Log("Auth module has not been specified in any way");
     return 0;
   }
   if (strncmp(auth_executable, "auth_", 5)) {
-    fprintf(stderr, "Auth module name must start with auth_.\n");
+    Log("Auth module name must start with auth_");
     return 0;
   }
   if (strchr(auth_executable, '.')) {
-    fprintf(stderr, "Auth module name may not contain a dot.\n");
+    Log("Auth module name may not contain a dot");
     return 0;
   }
   if (access(auth_executable, X_OK)) {  // Flawfinder: ignore
-    fprintf(stderr, "Auth module must be executable.\n");
+    Log("Auth module must be executable");
     return 0;
   }
 
   if (saver_executable == NULL) {
-    fprintf(stderr, "Saver module has not been specified in any way.\n");
+    Log("Saver module has not been specified in any way");
     return 0;
   }
   if (strncmp(saver_executable, "saver_", 6)) {
-    fprintf(stderr, "Saver module name must start with saver_.\n");
+    Log("Saver module name must start with saver_");
     return 0;
   }
   if (strchr(saver_executable, '.')) {
-    fprintf(stderr, "Saver module name may not contain a dot.\n");
+    Log("Saver module name may not contain a dot");
     return 0;
   }
   if (access(saver_executable, X_OK)) {  // Flawfinder: ignore
-    fprintf(stderr, "Saver module must be executable.\n");
+    Log("Saver module must be executable");
     return 0;
   }
 
@@ -330,17 +329,17 @@ void MaybeRaiseWindow(Display *display, Window w) {
   Window *children, *siblings;
   unsigned int nchildren, nsiblings;
   if (!XQueryTree(display, w, &root, &parent, &children, &nchildren)) {
-    fprintf(stderr, "XQueryTree failed.\n");
+    Log("XQueryTree failed");
     return;
   }
   XFree(children);
   if (!XQueryTree(display, parent, &root, &grandparent, &siblings,
                   &nsiblings)) {
-    fprintf(stderr, "XQueryTree failed.\n");
+    Log("XQueryTree failed");
     return;
   }
   if (nsiblings == 0) {
-    fprintf(stderr, "My parent window has no children.\n");
+    Log("My parent window has no children");
     XFree(siblings);
     return;
   }
@@ -361,22 +360,21 @@ void MaybeRaiseWindow(Display *display, Window w) {
 void NotifyOfLock(int x11_fd) {
   int fd = GetIntSetting("XSS_SLEEP_LOCK_FD", -1);
   if (fd == x11_fd) {
-    fprintf(stderr,
-            "XSS_SLEEP_LOCK_FD matches DISPLAY - what?!? We're probably "
-            "inhibiting sleep now.\n");
+    Log("XSS_SLEEP_LOCK_FD matches DISPLAY - what?!? We're probably "
+        "inhibiting sleep now");
   } else if (fd != -1) {
     if (close(fd) != 0) {
-      perror("close(XSS_SLEEP_LOCK_FD)");
+      LogErrno("close(XSS_SLEEP_LOCK_FD)");
     }
   }
   if (notify_command != NULL && *notify_command != NULL) {
     pid_t pid = fork();
     if (pid == -1) {
-      perror("fork");
+      LogErrno("fork");
     } else if (pid == 0) {
       // Child process.
       execvp(notify_command[0], notify_command);  // Flawfinder: ignore
-      perror("execvp");
+      LogErrno("execvp");
       exit(EXIT_FAILURE);
     } else {
       // Parent process after successful fork.
@@ -395,7 +393,7 @@ int main(int argc, char **argv) {
   // Change the current directory to HELPER_PATH so we don't need to process
   // path names.
   if (chdir(HELPER_PATH)) {
-    fprintf(stderr, "Could not switch to directory %s.\n", HELPER_PATH);
+    Log("Could not switch to directory %s", HELPER_PATH);
     return 1;
   }
 
@@ -413,15 +411,14 @@ int main(int argc, char **argv) {
   // Connect to X11.
   Display *display = XOpenDisplay(NULL);
   if (display == NULL) {
-    fprintf(stderr, "Could not connect to $DISPLAY.\n");
+    Log("Could not connect to $DISPLAY");
     return 1;
   }
 
   // TODO(divVerent): Support that?
   if (ScreenCount(display) != 1) {
-    fprintf(stderr,
-            "Warning: 'Zaphod' configurations are not supported at this point. "
-            "Only locking the default screen.\n");
+    Log("Warning: 'Zaphod' configurations are not supported at this point. "
+        "Only locking the default screen.\n");
   }
 
   // Who's the root?
@@ -432,7 +429,7 @@ int main(int argc, char **argv) {
   int w = DisplayWidth(display, DefaultScreen(display));
   int h = DisplayHeight(display, DefaultScreen(display));
 #ifdef DEBUG_EVENTS
-  fprintf(stderr, "DisplayWidthHeight %d %d\n", w, h);
+  Log("DisplayWidthHeight %d %d", w, h);
 #endif
 
   // Prepare some nice window attributes for a screen saver window.
@@ -461,10 +458,10 @@ int main(int argc, char **argv) {
                              &composite_minor_version) &&
       (composite_major_version >= 1 || composite_minor_version >= 3);
   if (!have_composite) {
-    fprintf(stderr, "XComposite extension not detected.\n");
+    Log("XComposite extension not detected");
   }
   if (have_composite && no_composite) {
-    fprintf(stderr, "XComposite extension detected but disabled by user.\n");
+    Log("XComposite extension detected but disabled by user");
     have_composite = 0;
   }
   Window composite_window;
@@ -533,7 +530,7 @@ int main(int argc, char **argv) {
   // Initialize XInput so we can get multibyte key events.
   XIM xim = XOpenIM(display, NULL, NULL, NULL);
   if (xim == NULL) {
-    fprintf(stderr, "XOpenIM failed. Assuming Latin-1 encoding.\n");
+    Log("XOpenIM failed. Assuming Latin-1 encoding");
   }
   XIC xic = NULL;
   if (xim != NULL) {
@@ -557,7 +554,7 @@ int main(int argc, char **argv) {
       }
     }
     if (xic == NULL) {
-      fprintf(stderr, "XCreateIC failed. Assuming Latin-1 encoding.\n");
+      Log("XCreateIC failed. Assuming Latin-1 encoding");
     }
   }
 
@@ -576,7 +573,7 @@ int main(int argc, char **argv) {
   // In case keys to disable grabs are available, turn them off for the duration
   // of the lock.
   if (XF86MiscSetGrabKeysState(display, False) != MiscExtGrabStateSuccess) {
-    fprintf(stderr, "Could not set grab keys state.\n");
+    Log("Could not set grab keys state");
     return 1;
   }
 #endif
@@ -593,7 +590,7 @@ int main(int argc, char **argv) {
     nanosleep(&(const struct timespec){0, 100000000L}, NULL);
   }
   if (retries < 0) {
-    fprintf(stderr, "Could not grab pointer.\n");
+    Log("Could not grab pointer");
     return 1;
   }
   for (retries = 10; retries >= 0; --retries) {
@@ -604,7 +601,7 @@ int main(int argc, char **argv) {
     nanosleep(&(const struct timespec){0, 100000000L}, NULL);
   }
   if (retries < 0) {
-    fprintf(stderr, "Could not grab keyboard.\n");
+    Log("Could not grab keyboard");
     return 1;
   }
 
@@ -625,7 +622,7 @@ int main(int argc, char **argv) {
     int len;
   } priv;
   if (MLOCK_PAGE(&priv, sizeof(priv)) < 0) {
-    perror("mlock");
+    LogErrno("mlock");
     return 1;
   }
 
@@ -669,11 +666,11 @@ int main(int argc, char **argv) {
     if (XGrabPointer(display, parent_window, False, ALL_POINTER_EVENTS,
                      GrabModeAsync, GrabModeAsync, None, coverattrs.cursor,
                      CurrentTime) != GrabSuccess) {
-      fprintf(stderr, "Critical: cannot re-grab pointer.\n");
+      Log("Critical: cannot re-grab pointer");
     }
     if (XGrabKeyboard(display, parent_window, False, GrabModeAsync,
                       GrabModeAsync, CurrentTime) != GrabSuccess) {
-      fprintf(stderr, "Critical: cannot re-grab keyboard.\n");
+      Log("Critical: cannot re-grab keyboard");
     }
 #endif
 
@@ -699,21 +696,21 @@ int main(int argc, char **argv) {
             break;
           default:
             // Assume the child still lives. Shouldn't ever happen.
-            perror("waitpid");
+            LogErrno("waitpid");
             break;
         }
       } else if (pid == notify_command_pid) {
         if (WIFEXITED(status)) {
           // Done notifying.
           if (WEXITSTATUS(status) != EXIT_SUCCESS) {
-            fprintf(stderr, "Notification command failed with status %d.\n",
-                    WEXITSTATUS(status));
+            Log("Notification command failed with status %d",
+                WEXITSTATUS(status));
           }
           notify_command_pid = 0;
         }
         // Otherwise it was suspended or whatever. We need to keep waiting.
       } else if (pid != 0) {
-        fprintf(stderr, "Unexpectedly woke up for PID %d.\n", (int)pid);
+        Log("Unexpectedly woke up for PID %d", (int)pid);
       }
       // Otherwise, we're still alive. Re-check next time.
     }
@@ -727,16 +724,16 @@ int main(int argc, char **argv) {
       switch (priv.ev.type) {
         case ConfigureNotify:
 #ifdef DEBUG_EVENTS
-          fprintf(stderr, "ConfigureNotify %lu %d %d\n",
-                  (unsigned long)priv.ev.xconfigure.window,
-                  priv.ev.xconfigure.width, priv.ev.xconfigure.height);
+          Log("ConfigureNotify %lu %d %d",
+              (unsigned long)priv.ev.xconfigure.window,
+              priv.ev.xconfigure.width, priv.ev.xconfigure.height);
 #endif
           if (priv.ev.xconfigure.window == root_window) {
             // Root window size changed. Adjust the saver_window window too!
             w = priv.ev.xconfigure.width;
             h = priv.ev.xconfigure.height;
 #ifdef DEBUG_EVENTS
-            fprintf(stderr, "DisplayWidthHeight %d %d\n", w, h);
+            Log("DisplayWidthHeight %d %d", w, h);
 #endif
             XMoveResizeWindow(display, background_window, 0, 0, w, h);
             XMoveResizeWindow(display, saver_window, 0, 0, w, h);
@@ -752,9 +749,9 @@ int main(int argc, char **argv) {
           break;
         case VisibilityNotify:
 #ifdef DEBUG_EVENTS
-          fprintf(stderr, "VisibilityNotify %lu %d\n",
-                  (unsigned long)priv.ev.xvisibility.window,
-                  priv.ev.xvisibility.state);
+          Log("VisibilityNotify %lu %d",
+              (unsigned long)priv.ev.xvisibility.window,
+              priv.ev.xvisibility.state);
 #endif
           if (priv.ev.xvisibility.state != VisibilityUnobscured) {
             // If something else shows an OverrideRedirect window, we want to
@@ -764,9 +761,8 @@ int main(int argc, char **argv) {
             } else if (priv.ev.xvisibility.window == background_window) {
               XRaiseWindow(display, background_window);
             } else {
-              fprintf(stderr,
-                      "Received unexpected VisibilityNotify for window %d.\n",
-                      (int)priv.ev.xvisibility.window);
+              Log("Received unexpected VisibilityNotify for window %d",
+                  (int)priv.ev.xvisibility.window);
             }
           }
           break;
@@ -803,8 +799,7 @@ int main(int argc, char **argv) {
           }
           if (have_key && (size_t)priv.len >= sizeof(priv.buf)) {
             // Detect possible overruns. This should be unreachable.
-            fprintf(stderr, "Received invalid length from XLookupString: %d\n",
-                    priv.len);
+            Log("Received invalid length from XLookupString: %d", priv.len);
             have_key = 0;
           }
           if (have_key) {
@@ -833,8 +828,7 @@ int main(int argc, char **argv) {
           break;
         case MapNotify:
 #ifdef DEBUG_EVENTS
-          fprintf(stderr, "MapNotify %lu\n",
-                  (unsigned long)priv.ev.xmap.window);
+          Log("MapNotify %lu", (unsigned long)priv.ev.xmap.window);
 #endif
           if (priv.ev.xmap.window == background_window) {
             background_window_mapped = 1;
@@ -849,8 +843,7 @@ int main(int argc, char **argv) {
           break;
         case UnmapNotify:
 #ifdef DEBUG_EVENTS
-          fprintf(stderr, "UnmapNotify %lu\n",
-                  (unsigned long)priv.ev.xmap.window);
+          Log("UnmapNotify %lu", (unsigned long)priv.ev.xmap.window);
 #endif
           if (priv.ev.xmap.window == background_window) {
             background_window_mapped = 0;
@@ -860,36 +853,33 @@ int main(int argc, char **argv) {
             // This should never happen, but let's handle it anyway.
             // Compton might do this when --unredir-if-possible is set and a
             // fullscreen game launches while the screen is locked.
-            fprintf(stderr, "Someone unmapped our parent. Undoing that.\n");
+            Log("Someone unmapped our parent. Undoing that");
             XMapRaised(display, parent_window);
           }
           break;
         case FocusIn:
         case FocusOut:
 #ifdef DEBUG_EVENTS
-          fprintf(stderr, "Focus%d %lu\n", priv.ev.xfocus.mode,
-                  (unsigned long)priv.ev.xfocus.window);
+          Log("Focus%d %lu", priv.ev.xfocus.mode,
+              (unsigned long)priv.ev.xfocus.window);
 #endif
           if (priv.ev.xfocus.window == parent_window &&
               priv.ev.xfocus.mode == NotifyUngrab) {
-            fprintf(stderr, "WARNING: lost grab, trying to grab again.\n");
+            Log("WARNING: lost grab, trying to grab again");
             if (XGrabKeyboard(display, parent_window, False, GrabModeAsync,
                               GrabModeAsync, CurrentTime) != GrabSuccess) {
-              fprintf(stderr,
-                      "Critical: lost grab but cannot re-grab keyboard.\n");
+              Log("Critical: lost grab but cannot re-grab keyboard");
             }
             if (XGrabPointer(display, parent_window, False, ALL_POINTER_EVENTS,
                              GrabModeAsync, GrabModeAsync, None, None,
                              CurrentTime) != GrabSuccess) {
-              fprintf(stderr,
-                      "Critical: lost grab but cannot re-grab pointer.\n");
+              Log("Critical: lost grab but cannot re-grab pointer");
             }
           }
           break;
         default:
 #ifdef DEBUG_EVENTS
-          fprintf(stderr, "Event%d %lu\n", priv.ev.type,
-                  (unsigned long)priv.ev.xany.window);
+          Log("Event%d %lu", priv.ev.type, (unsigned long)priv.ev.xany.window);
 #endif
 #ifdef HAVE_SCRNSAVER
           // Handle screen saver notifications. If the screen is blanked anyway,
@@ -904,7 +894,7 @@ int main(int argc, char **argv) {
             break;
           }
 #endif
-          fprintf(stderr, "Received unexpected event %d.\n", priv.ev.type);
+          Log("Received unexpected event %d", priv.ev.type);
           break;
       }
     }
diff --git a/mlock_page.h b/mlock_page.h
index 663114d7..377b4857 100644
--- a/mlock_page.h
+++ b/mlock_page.h
@@ -33,11 +33,11 @@ limitations under the License.
  *
  * The area is expanded to fill whole memory pages.
  */
-#define MLOCK_PAGE(ptr, size)                                                  \
-  mlock((void *)((((uintptr_t)(ptr)) / (uintptr_t)PAGESIZE) *                  \
-                 (uintptr_t)PAGESIZE),                                         \
-        (((uintptr_t)(ptr) + (size)-1) / (uintptr_t)PAGESIZE -                 \
-         ((uintptr_t)(ptr)) / (uintptr_t)PAGESIZE + 1) *                       \
+#define MLOCK_PAGE(ptr, size)                                  \
+  mlock((void *)((((uintptr_t)(ptr)) / (uintptr_t)PAGESIZE) *  \
+                 (uintptr_t)PAGESIZE),                         \
+        (((uintptr_t)(ptr) + (size)-1) / (uintptr_t)PAGESIZE - \
+         ((uintptr_t)(ptr)) / (uintptr_t)PAGESIZE + 1) *       \
             (uintptr_t)PAGESIZE)
 #elif _POSIX_MEMLOCK > 0
 #warning mlock() is unavailable. More memory will be locked than necessary.
diff --git a/saver_child.c b/saver_child.c
index 1fdffca2..ffb3c000 100644
--- a/saver_child.c
+++ b/saver_child.c
@@ -17,13 +17,13 @@ limitations under the License.
 #include "saver_child.h"
 
 #include <errno.h>     // for ECHILD, EINTR, errno
-#include <signal.h>    // for kill, SIGTERM
-#include <stdio.h>     // for perror, fprintf, NULL, etc
-#include <stdlib.h>    // for exit, EXIT_FAILURE, etc
-#include <sys/wait.h>  // for WEXITSTATUS, waitpid, etc
+#include <signal.h>    // for kill, SIGTERM, sigemptyset, sigprocmask
+#include <stdlib.h>    // for NULL, exit, EXIT_FAILURE, EXIT_SUCCESS
+#include <sys/wait.h>  // for WEXITSTATUS, WIFEXITED, WIFSIGNALED
 #include <unistd.h>    // for pid_t, execl, fork, setsid
 
-#include "xscreensaver_api.h"
+#include "logging.h"           // for Log, LogErrno
+#include "xscreensaver_api.h"  // for ExportWindowID
 
 //! The PIDs of currently running saver children, or 0 if not running.
 static pid_t saver_child_pid[MAX_SAVERS] = {0};
@@ -31,8 +31,7 @@ static pid_t saver_child_pid[MAX_SAVERS] = {0};
 void WatchSaverChild(Display* dpy, Window w, int index, const char* executable,
                      int should_be_running) {
   if (index < 0 || index >= MAX_SAVERS) {
-    fprintf(stderr, "Saver index out of range: !(0 <= %d < %d).", index,
-            MAX_SAVERS);
+    Log("Saver index out of range: !(0 <= %d < %d)", index, MAX_SAVERS);
     return;
   }
 
@@ -61,7 +60,7 @@ void WatchSaverChild(Display* dpy, Window w, int index, const char* executable,
             break;
           default:
             // Assume the child still lives. Shouldn't ever happen.
-            perror("waitpid");
+            LogErrno("waitpid");
             break;
         }
       } else if (pid == saver_child_pid[index]) {
@@ -75,21 +74,19 @@ void WatchSaverChild(Display* dpy, Window w, int index, const char* executable,
           saver_child_pid[index] = 0;
           if (WIFSIGNALED(status) &&
               (should_be_running || WTERMSIG(status) != SIGTERM)) {
-            fprintf(stderr, "Saver child killed by signal %d.\n",
-                    WTERMSIG(status));
+            Log("Saver child killed by signal %d", WTERMSIG(status));
           }
           if (WIFEXITED(status) && WEXITSTATUS(status) != EXIT_SUCCESS) {
-            fprintf(stderr, "Saver child failed with status %d.\n",
-                    WEXITSTATUS(status));
+            Log("Saver child failed with status %d", WEXITSTATUS(status));
           }
           // Now is the time to remove anything the child may have displayed.
           XClearWindow(dpy, w);
         }
         // Otherwise it was suspended or whatever. We need to keep waiting.
       } else if (pid != 0) {
-        fprintf(stderr, "Unexpectedly woke up for PID %d.\n", (int)pid);
+        Log("Unexpectedly woke up for PID %d", (int)pid);
       } else if (!should_be_running) {
-        fprintf(stderr, "Unexpectedly woke up for PID 0 despite no WNOHANG.\n");
+        Log("Unexpectedly woke up for PID 0 despite no WNOHANG");
       }
       // Otherwise, we're still alive.
     } while (!should_be_running && saver_child_pid[index] != 0);
@@ -98,7 +95,7 @@ void WatchSaverChild(Display* dpy, Window w, int index, const char* executable,
   if (should_be_running && saver_child_pid[index] == 0) {
     pid_t pid = fork();
     if (pid == -1) {
-      perror("fork");
+      LogErrno("fork");
     } else if (pid == 0) {
       // Child process.
       setsid();
@@ -111,7 +108,7 @@ void WatchSaverChild(Display* dpy, Window w, int index, const char* executable,
 
       ExportWindowID(w);
       execl(executable, executable, NULL);  // Flawfinder: ignore
-      perror("execl");
+      LogErrno("execl");
       exit(EXIT_FAILURE);
     } else {
       // Parent process after successful fork.
diff --git a/test/break-compositor.c b/test/break-compositor.c
index 5b3011ee..cceb6517 100644
--- a/test/break-compositor.c
+++ b/test/break-compositor.c
@@ -1,6 +1,6 @@
 #include <X11/X.h>
-#include <X11/extensions/Xcomposite.h>
 #include <X11/Xlib.h>
+#include <X11/extensions/Xcomposite.h>
 #include <stdio.h>
 #include <unistd.h>
 
diff --git a/test/get-compositor.c b/test/get-compositor.c
index f5d41525..abc866c1 100644
--- a/test/get-compositor.c
+++ b/test/get-compositor.c
@@ -9,10 +9,10 @@ int main() {
     fprintf(stderr, "Could not connect to $DISPLAY.\n");
     return 1;
   }
-  char buf[32];  // Flawfinder: ignore
+  char buf[32];                                 // Flawfinder: ignore
   snprintf(buf, sizeof(buf), "_NET_WM_CM_S%d",  // Flawfinder: ignore
            (int)DefaultScreen(display));
-  buf[sizeof(buf)-1] = 0;
+  buf[sizeof(buf) - 1] = 0;
   Atom atom = XInternAtom(display, buf, False);
   Window w = XGetSelectionOwner(display, atom);
   if (w == None) {
diff --git a/xscreensaver_api.c b/xscreensaver_api.c
index ebe18ceb..fe3a7ec0 100644
--- a/xscreensaver_api.c
+++ b/xscreensaver_api.c
@@ -21,6 +21,7 @@ limitations under the License.
 #include <stdlib.h>  // for setenv
 
 #include "env_settings.h"  // for GetUnsignedLongLongSetting
+#include "logging.h"
 
 void ExportWindowID(Window w) {
   char window_id_str[32];  // Flawfinder: ignore
@@ -28,7 +29,7 @@ void ExportWindowID(Window w) {
       snprintf(window_id_str, sizeof(window_id_str),  // Flawfinder: ignore
                "%llu", (unsigned long long)w);
   if (window_id_len <= 0 || (size_t)window_id_len >= sizeof(window_id_str)) {
-    fprintf(stderr, "Window ID doesn't fit into buffer.\n");
+    Log("Window ID doesn't fit into buffer");
     return;
   }
   setenv("XSCREENSAVER_WINDOW", window_id_str, 1);