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// gcc -Wall -arch i386 -arch ppc -mmacosx-version-min=10.4 -Os -framework AppKit -o relaunch relaunch.m

#import <AppKit/AppKit.h>

@interface TerminationListener : NSObject
{
const char *executablePath;
pid_t parentProcessId;
}

- (void) relaunch;

@end

@implementation TerminationListener

- (id) initWithExecutablePath:(const char *)execPath parentProcessId:(pid_t)ppid
{
self = [super init];
if (self != nil) {
executablePath = execPath;
parentProcessId = ppid;

// This adds the input source required by the run loop
[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self selector:@selector(applicationDidTerminate:) name:NSWorkspaceDidTerminateApplicationNotification object:nil];
if (getppid() == 1) {
// ppid is launchd (1) => parent terminated already
[self relaunch];
}
}
return self;
}

- (void) applicationDidTerminate:(NSNotification *)notification
{
if (parentProcessId == [[[notification userInfo] valueForKey:@"NSApplicationProcessIdentifier"] intValue]) {
// parent just terminated
[self relaunch];
}
}

- (void) relaunch
{
[[NSWorkspace sharedWorkspace] launchApplication:[NSString stringWithUTF8String:executablePath]];
exit(0);
}

@end

int main (int argc, const char * argv[])
{
if (argc != 3) return EXIT_FAILURE;

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

[[[TerminationListener alloc] initWithExecutablePath:argv[1] parentProcessId:atoi(argv[2])] autorelease];
[[NSApplication sharedApplication] run];

[pool release];

return EXIT_SUCCESS;
}

source