Programming :: O_SYNC Parameter In Open Command?

I have a small board that has a static ram board attached to it. I have two different programs that each write to a byte of memory in the static ram board. I get a file descriptor to the device memory with the following code:

int phymemfd;
phymemfd=open("dev/mem, O_RDWR | O_SYNC);

This opens the physical memory for both reading and writing. Next I get a pointer to the beginning of my ram board.

volatile unsigned char *novram;
novram = (unsigned char *) mmap(0, getpagesize() * 31,
PROT_READ|PROT_WRITE, MAP_SHARED, phymemfd, 0x11AA0000);

I do this same code in two processes that are both running and this works fine. Now, if I take out the O_SYNC parameter, one of the programs gets into some sort of weird state.My understanding of the O_SYNC parameter is that it cause the process accessing the memory location to block, not allow another process to run, until the first process has finished writing to it.I can see that if my program doesn't block, I wouldn't really know what was in the memory location, but would I can't see how it would cause any other type of system problem.