Under Linux and most other Un*x systems, you can rely on the directory /tmp being the system scratch space. As such, you can pretty much use /tmp without fear.

On OS X, each user has its own scratch space. Use the NSTemporaryDirectory() function to retrieve the directory name.

Here is a small bit of Objective C example code.

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

// print the current user's scratch directory.
NSLog(NSTemporaryDirectory());

[pool drain];
return 0;
}

Update 2009-07-27:
On the command line, the TMPDIR environment variable holds the same value that NSTemporaryDirectory() returns.

1. Harley Pebley Says:
   July 30th, 2009 at 6:53 am

   That’s cool that each user has their own /tmp space. Hope as sys admin you can specify where it is (like /home). Once found out the hard way /tmp should not be a mount point on / (the default, at least on the dist I was on). Since then I always give it its own partition.

2. James Reuben Knowles Says:
   July 30th, 2009 at 10:28 am

   “Once found out the hard way /tmp should not be a mount point on /”

   You, too, eh? War story time.

   I haven’t dug into system internals enough to know if there’s a simple way to tell OS X to use a separate partition for temp directories.

   Interestingly, /etc /tmp and /var are all symlinks into /private, which appears to be marked with extended attributes. I don’t know enough about BSD-specific intricacies to know what’s going on, but the per-user configurability make me suspect that union filesystem features may be used — but that’s pure speculation.