Why does each child process generate the same "random" number when using rand() in c? -

i trying spawn n child processes, let each child process request random number of resources. however, each child requests identical number of resources, though number changes each time run program.

/* create appropriate number of processes */ int pid; for(int = 0; < numberofprocesses; i++){     pid = fork();     if(pid < 0){         fprintf(stderr, "fork failed");         exit(1);     }     else if(pid == 0){         time_t t;         srand((unsigned) time(&t));          printf("child (%d): %d.", i+1, getpid());          /* generate random number [0, max_resources] of resources request */          int requestnum = rand() % (max_resources + 1);         printf(" requesting %d resources\n", requestnum);          exit(0);     }     else{ wait(null); } } 

update: following seems have solved issue. thank help, commented!

time_t t; srand((int)time(&t) % getpid()); 

you seeding random number generator current time. it's same time children. forking child processes, checking clock in parallel. whole point of parallelism multiple things @ same time.

from man time():

time() returns time number of seconds since epoch, 1970-01-01 00:00:00 +0000 (utc).

notice in seconds. second long time running process. if tried sequentially (not parallel) on , over, highly 2 processes ask time, , same result.

design better way seed random number generator based on unique each process. could, example, add process id of child seed (getpid(), not pid).