Has this C code a defined behavior? -

this thought experiment, not production code nor coding style.

suppose have function

int find_process_pid_by_name(char* name, int* threads_in_process); 

that return pid of named process , store threads_in_process number of threads running in said process.

a lazy programmer, interested on pid, writes code

int pid = find_process_pid_by_name("a process name", &pid); 

does trigger undefined behavior?

no — don't think undefined behaviour. there's sequence point before function called (after arguments , expression denotes called function have been evaluated), , there's before called function returns. side-effects on pid performed called function have been completed before function finishes returning. result of function assigned pid. there's no question of location assigned being changed function. see nothing invokes undefined behaviour.

i assuming called function treats int * argument write-only pointer single value. if reads single value, need know pid initialized (formally; in practice, won't matter). in context, pid has not been initialized; result of function initialize it. so, if function reads pointer argument, technically, have undefined behaviour. if function treats pointer start of multi-element array , accesses beyond zeroth element, there problems. these issues outside intended scope of question/discussion.