Add `teprintf()` function

A new function to print to stderr prefixed with a timestamp. The idea is
to make the printed line's format match the Rust logger's.

The Rust logger prints timestamps like this:

    2018-11-04T03:16:31.938+01:00

`teprintf` does:

    2018-11-04T15:02:21+0100

Unfortunately, I wasn't able to reproduce it exactly. It's missing the
milliseconds because I didn't want to deal with using `gettimeofday`,
and the time zone format isn't right because I didn't know how to format
it easily with a colon.

Resources:
Getting the time in C:
https://stackoverflow.com/questions/9596945/how-to-get-appropriate-timestamp-in-c-for-logs#9596994

Variadic functions in C:
https://stackoverflow.com/questions/205529/passing-variable-number-of-arguments-around

Getting milliseconds in C:
https://stackoverflow.com/questions/10192903/time-in-milliseconds#10192994
https://stackoverflow.com/questions/3673226/how-to-print-time-in-format-2009-08-10-181754-811
https://stackoverflow.com/questions/361363/how-to-measure-time-in-milliseconds-using-ansi-c

Concatenating strings in C:
https://stackoverflow.com/questions/308695/how-do-i-concatenate-const-literal-strings-in-c#308712
https://stackoverflow.com/questions/8465006/how-do-i-concatenate-two-strings-in-c

3 files changed, 41 insertions, 0 deletions

diff --git a/DomeKey.xcodeproj/project.pbxproj b/DomeKey.xcodeproj/project.pbxproj
index 3da05f9..110ee5b 100644
--- a/DomeKey.xcodeproj/project.pbxproj
+++ b/DomeKey.xcodeproj/project.pbxproj
@@ -11,6 +11,7 @@
 		D110C94A2122F1ED0094F963 /* libddhid.a in Frameworks */ = {isa = PBXBuildFile; fileRef = D160C32221197983007D1B50 /* libddhid.a */; };
 		D11184622125206E00961687 /* AppDelegate.m in Sources */ = {isa = PBXBuildFile; fileRef = D11184612125206E00961687 /* AppDelegate.m */; };
 		D11CD06D217FECD9001FCB97 /* AquaticPrime.c in Sources */ = {isa = PBXBuildFile; fileRef = D11CD067217FECD9001FCB97 /* AquaticPrime.c */; };
+		D12B5B76218F31E300C6F7FC /* log.c in Sources */ = {isa = PBXBuildFile; fileRef = D12B5B71218F31E200C6F7FC /* log.c */; };
 		D131C4D321663B6500801267 /* Mappings.m in Sources */ = {isa = PBXBuildFile; fileRef = D131C4D221663B6500801267 /* Mappings.m */; };
 		D160C2A12118EF9D007D1B50 /* main.m in Sources */ = {isa = PBXBuildFile; fileRef = D160C2A02118EF9D007D1B50 /* main.m */; };
 		D18255B12180018C00ABC1E0 /* LicenseHandler.m in Sources */ = {isa = PBXBuildFile; fileRef = D18255B02180018C00ABC1E0 /* LicenseHandler.m */; };
@@ -77,6 +78,7 @@
 		D11184612125206E00961687 /* AppDelegate.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = AppDelegate.m; sourceTree = "<group>"; };
 		D11CD067217FECD9001FCB97 /* AquaticPrime.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = AquaticPrime.c; path = lib/AquaticPrime/Source/CoreFoundation/AquaticPrime.c; sourceTree = SOURCE_ROOT; };
 		D11CD06C217FECD9001FCB97 /* AquaticPrime.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = AquaticPrime.h; path = lib/AquaticPrime/Source/CoreFoundation/AquaticPrime.h; sourceTree = SOURCE_ROOT; };
+		D12B5B71218F31E200C6F7FC /* log.c */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.c; path = log.c; sourceTree = "<group>"; };
 		D131C4D121663B6500801267 /* Mappings.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = Mappings.h; sourceTree = "<group>"; };
 		D131C4D221663B6500801267 /* Mappings.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = Mappings.m; sourceTree = "<group>"; };
 		D160C29D2118EF9D007D1B50 /* DomeKey */ = {isa = PBXFileReference; explicitFileType = "compiled.mach-o.executable"; includeInIndex = 0; path = DomeKey; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -141,6 +143,7 @@
 				D18255B02180018C00ABC1E0 /* LicenseHandler.m */,
 				D1EAA7A221803BB300A0AC35 /* XDG.h */,
 				D1EAA7A321803BB300A0AC35 /* XDG.m */,
+				D12B5B71218F31E200C6F7FC /* log.c */,
 			);
 			path = DomeKey;
 			sourceTree = "<group>";
@@ -273,6 +276,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				D12B5B76218F31E300C6F7FC /* log.c in Sources */,
 				D131C4D321663B6500801267 /* Mappings.m in Sources */,
 				D1EAA7A421803BB300A0AC35 /* XDG.m in Sources */,
 				D1C80CA021877E8200011088 /* Sounds.m in Sources */,
diff --git a/DomeKey/log.c b/DomeKey/log.c
new file mode 100644
index 0000000..f39e2bf
--- /dev/null
+++ b/DomeKey/log.c
@@ -0,0 +1,26 @@
+#include "log.h"
+
+void teprintf(char const *fmt, ...) {
+    // Format the current time
+    time_t raw_time;
+    size_t buffer_size = 35;
+    char buffer[buffer_size];
+    time(&raw_time);
+    struct tm *now = localtime(&raw_time);
+    strftime(buffer, buffer_size, "%Y-%m-%dT%H:%M:%S%z", now);
+
+    // Prefix the time to the format string
+    char *timestampped_fmt = malloc(strlen(buffer) + strlen(fmt) + 5);
+    strcpy(timestampped_fmt, buffer);
+    strcat(timestampped_fmt, " - ");
+    strcat(timestampped_fmt, fmt);
+    strcat(timestampped_fmt, "\n");
+
+    // Print the log message
+    va_list args;
+    va_start(args, fmt);
+    vfprintf(stderr, timestampped_fmt, args);
+    va_end(args);
+
+    free(timestampped_fmt);
+}
diff --git a/DomeKey/log.h b/DomeKey/log.h
index 7684143..e285f60 100644
--- a/DomeKey/log.h
+++ b/DomeKey/log.h
@@ -1,10 +1,21 @@
 #ifndef LOG_H
 #define LOG_H
 
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
 #define LogDebug(...) \
     if ([[[[NSProcessInfo processInfo] environment] \
         objectForKey:@"DOME_KEY_DEBUG"] isEqualToString:@"1"]) { \
         NSLog(__VA_ARGS__); \
     }
 
+
+// Print a log message prefixed with a timestamp to stderr. Format:
+// 2018-11-04T14:35:58+0100 - Format string
+void teprintf(char const *fmt, ...);
+
 #endif /* LOG_H */