What is a .app?
In macOS, I browse to my /Applications directory to see lots of files. Keynote.app, Spotify.app, Vidrio.app, etc. What are these things? These “applications” can’t be launched in the normal fashion:
$ /Applications/Spotify.app/
-bash: /Applications/Spotify.app/: is a directory
They’re not files, they’re directories!
$ cd /Applications/Spotify.app
$ tree -L 2
.
└── Contents
├── Frameworks
├── Info.plist
├── MacOS
├── Resources
└── _CodeSignature
Despite being directories, we can open them to launch the application. What program is used to run this?
$ pstree -p 38699
-+= 00001 root /sbin/launchd
\-+= 38699 jim /Applications/Spotify.app/Contents/MacOS/Spotify
|--- 38705 jim /Applications/Spotify.app/Contents/Frameworks/Spotify Helper.app/Contents/MacOS/Spotify Helper --type=gpu-process --lang=en-US --log-file=/
\--- 38710 jim /Applications/Spotify.app/Contents/Frameworks/Spotify Helper.app/Contents/MacOS/Spotify Helper --type=renderer --disable-pinch --primordial
The process 1, /sbin/launchd, started /Applications/Spotify.app/Contents/MacOS/Spotify.
The real program binary is at Contents/MacOS/Spotify within the .app directory. We can run this program ourselves, but it doesn’t run correctly for some reason:
$ /Applications/Spotify.app/Contents/MacOS/Spotify
2017-05-07 12:41:40.858 Spotify[39249:1666301] NSWindow warning: adding an unknown subview: <_NSThemeCloseWidget: 0x7f8ca4482130>. Break on NSLog to debug.
2017-05-07 12:41:40.859 Spotify[39249:1666301] Call stack:
(
0 AppKit 0x00007fffbfa56ebd -[NSThemeFrame addSubview:] + 109
1 Spotify 0x0000000108524c02 Spotify + 1461250
2 Spotify 0x0000000108524958 Spotify + 1460568
...
We can make an “application bundle” from scratch. Make any binary, like so:
#include <unistd.h>
#include <stdio.h>
char log_line[] = "log line\n";
int main() {
FILE * f = fopen("/tmp/log", "w");
for (;;) {
size_t num_written = fwrite(log_line, sizeof(log_line), 1, f);
printf("num_written: %zu\n", num_written);
int flush_result = fflush(f);
sleep(1);
}
return 0;
}
Compile this and construct the application bundle directories:
$ mkdir -p Foo.app/Contents/MacOS
$ clang Foo.c -o Foo.app/Contents/MacOS/Foo
$ open Foo.app/ &
$ cat /tmp/log
log line
log line
log line
log line
It seems the expected path for the binary in <Name>.app is /Contents/MacOS/<Name>.
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