raise() of signal is not waking a sleeping process - c

Question

I have a single thread program that sleeps to conserve battery power while waiting for hardware events. main() registers a signal handler, then sleeps. Hardware interrupt handlers may wake main by calling a function which calls raise(), and seeing any signal should wake main from sleep.
Now, for development purposes, I am running on a desktop, and instead of getting true hardware events, there are a couple of other processes which send fake hardware events. Instead of the interrupt handler making the aforementioned function calls, these tasks do.
A very simple demo case works fine, I can provide that code if needed, but don't want this post to be too long.
I have declared this signal handler:
void sig_handler( int signo )
{
//To confirm that this process is the same one that is sleeping
printf("sig_handler: my PID is %d\n", getpid());
if ( SIGUSR1 == signo)
{
printf("received SIGUSR1\n");
}
else
{
printf("received some other signal\n");
}
}
There is lot of other code, but the bit in question is that during init, I call:
sSigAction.sa_handler = sig_handler;
sigemptyset( &sSigAction.sa_mask );
sigaction( SIGUSR1, &sSigAction, NULL );
In a function called by another process (or interrupt handler in real system):
printf( "raising signal!\n");
raise( SIGUSR1 );
In main:
printf("main: my PID is %d\n", getpid());
while(1)
{
printf("sleeping forever\n");
usleep(-1);
printf("main is awake"\n);
//Do work we were waiting to do
}
The output looks like this:
main: my PID is 17250
sleeping forever
raising signal!
sig_handler: my PID is 17250
received SIGUSR1
Note the critical lack of "main is awake". As I said, doing these same steps does work, in a simple demo case. Since I am doing the same setup, I don't understand why it's not working. In both cases, I perform the same signal handler registration, usleep(), and raise(), but the demo case works and this does not.
Any ideas of what I could be missing?
Follow-up: I have found that if I use $kill -s USR1 then the task wakes as expected. So the mechanism is sound, but why does $kill work and raise() not?
Follow-up 2: I now have the function calling raise() also print getpid() and it matches the PID of main().
Resolution: While in the end I will have a single thread, and raise() would be appropriate right now I have multiple, and so must use kill(). It's that simple.