c - Why strange behavior of fork system call? -

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• printf anomaly after “fork()” 3 answers

int main(void) {     printf("abc");     fork();     return 0; } 

output of code :

abcabc 

why printing twice, when fork system call after printf statement?

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because stdout buffered, line-buffered. , in program buffer flushed @ exit time, or when returning main (and happens "twice" when fork don't fail).

try adding fflush(null); before fork(); (which should do)

btw, should keep result of fork , handle three cases: fork failure, in child, in parent.

so fork behaving should, printf has not naive immediate side-effect imagine: buffering real output may happen latter. you'll observe buffering replacing fork() sleep(15) (the output happening @ exit time, or @ end of main, after sleep, , 15 seconds program won't apparently output anything)

you might use strace(1) understanding system calls happening...