c - Synchronising processes with semaphores -

i'm having tricky time understanding how alternate control between 2 processes using semaphores. here's contrived example of process handling code.

int pid = fork();  if (pid) {   int counter = 0;   while (true) {     counter += 1;     printf("p%d = %d", pid, counter);   } } else {   int counter = 0;   while (true) {     counter += 1;     printf("p%d = %d", pid, counter);   } } 

i expecting above code run in parallel, seems control flow continues instantly forked process , later resumes parent process.

this fundamentally breaking existing code uses semaphore control process can act.

int id = get_semaphore(1); int pid = fork();  if (pid) {   int counter = 0;   while (true) {     sem_wait(id);     counter += 1;     printf("p%d = %d\n", pid, counter);     sem_signal(id);   } } else {   int counter = 0;   while (true) {     sem_wait(id);     counter += 1;     printf("p%d = %d\n", pid, counter);     sem_signal(id);   } } 

• the sem_wait helper subtracts 1 semaphore value , blocks until result > 0 (uses semop under hood).

• the sem_signal helper adds 1 semaphore value (uses semop under hood).

i'd code alternate between 2 processes, using sem_wait block until other process releases resources sem_signal. desired output be:

p1 = 0 p0 = 0 p1 = 1 p0 = 1 ... 

however, because of initial execution delay between processes, child process takes available semaphore resource, uses print number, restores , loops — @ point resource available again, continues without ever waiting other process.

what's best way prevent process using resources if released them itself?

it seems control flow continues instantly forked process , later resumes parent process

that because stream io buffers output on stdout until either

• the buffer full
• fflush() called on stdout
• a newline (\n) encountered

in program, each process fill buffer before sending contents stdout giving appearance of 1 process running long time, other. terminate format strings of printf statements \n , you'll see behaviour in first program more expect.

i not sure why semaphore thing isn't working - i'm not knowledgeable system v semaphores seems red flag me getting semaphore after have forked. more common posix semaphores, semaphore has in memory both processes can see otherwise it's 2 semaphores.

anyway, assuming get_semaphore() function right thing share semaphore, there still problem because there no guarantee that, when 1 process signals semaphore, other 1 start enough grab again before first process loops round , grabs itself.

you need 2 semaphores, 1 parent , 1 child. before print each process should wait on own semaphore. after print, each process should signal other semaphore. also, 1 semaphore should initialised count of 1 , other should initialised count of 0.