How to programmatically prevent a Mac from going to sleep?

Here is the official Apple documentation (including code snippet):
Technical Q&A QA1340 – How to I prevent sleep?

Quote: Preventing sleep using I/O Kit in Mac OS X 10.6 Snow Leopard:

#import <IOKit/pwr_mgt/IOPMLib.h>

// kIOPMAssertionTypeNoDisplaySleep prevents display sleep,
// kIOPMAssertionTypeNoIdleSleep prevents idle sleep

// reasonForActivity is a descriptive string used by the system whenever it needs 
// to tell the user why the system is not sleeping. For example, 
// "Mail Compacting Mailboxes" would be a useful string.

// NOTE: IOPMAssertionCreateWithName limits the string to 128 characters. 
CFStringRef* reasonForActivity= CFSTR("Describe Activity Type");

IOPMAssertionID assertionID;
IOReturn success = IOPMAssertionCreateWithName(kIOPMAssertionTypeNoDisplaySleep, 
                                    kIOPMAssertionLevelOn, reasonForActivity, &assertionID); 
if (success == kIOReturnSuccess)
{
    //  Add the work you need to do without 
    //  the system sleeping here.

    success = IOPMAssertionRelease(assertionID);
    //  The system will be able to sleep again. 
}

For older OSX version, check the following:
Technical Q&A QA1160 – How can I prevent system sleep while my application is running?

Quote: Example usage of UpdateSystemActivity (the canonical way for < 10.6)

#include <CoreServices/CoreServices.h>

void
MyTimerCallback(CFRunLoopTimerRef timer, void *info)
{
    UpdateSystemActivity(OverallAct);
}


int
main (int argc, const char * argv[])
{
    CFRunLoopTimerRef timer;
    CFRunLoopTimerContext context = { 0, NULL, NULL, NULL, NULL };

    timer = CFRunLoopTimerCreate(NULL, CFAbsoluteTimeGetCurrent(), 30, 0, 0, MyTimerCallback, &context);
    if (timer != NULL) {
        CFRunLoopAddTimer(CFRunLoopGetCurrent(), timer, kCFRunLoopCommonModes);
    }

    /* Start the run loop to receive timer callbacks. You don't need to
    call this if you already have a Carbon or Cocoa EventLoop running. */
    CFRunLoopRun();

    CFRunLoopTimerInvalidate(timer);
    CFRelease(timer);

    return (0);
}