Measure wall clock time before and after a function - c

Question

I am measuring wall time through the use of clock_gettime() found in . It works perfectly fine when i use it in main() but not the way i am attempting to use it.
I am familiarizing myself with the linux scheduler and i am measuring performance on different parts.
I want to be able to measure Waiting time which is defined by "the total time a thread spends in the ready queue" (how long until it starts executing the function).
Easily enough i can measure this by setting a clock_gettime() before the thread function and another right inside the function. However the problem i am having is that the time inside the thread function is lower than the one outside, giving us a negative time.
I am running this on my windows pc through ubuntu.
what could the problem be?
code:
clock_gettime(CLOCK_REALTIME,&data.before);
thread_array[i-1] = data;
if(pthread_create(&tids[i],&attr,workLoad,(void*) &data) != 0){
perror("Could not create thread");
return 1;
}
}
for(int i = 1;i < threadAmount; i++){
if(pthread_join(tids[i],NULL)!= 0){
perror("Thread could not wait");
return 1;
}
}
and here is my threadfunc:
void *workLoad(void *args)
{
threadData* data = (threadData*) args;
clock_gettime(CLOCK_REALTIME,&data->after);
int loopAmount = data->loopAmount;
int counter = 0;
for(int i = 0; i < loopAmount; i++){
counter++;
}
return NULL;
}
result of time intervall

Answer 1

In the following code:
clock_gettime(CLOCK_REALTIME,&data.before);
thread_array[i-1] = data;
if(pthread_create(&tids[i],&attr,workLoad,(void*) &data) != 0){
data seems to be a local variable whose address you pass to the thread. You also copy this variable into thread_array[i-1]. If you then do thread_array[i-1].after - thread_array[i-1].before then that means that the thread updates a wrong variable. You need to pass &thread_array[i-1] to that thread, e.g.:
if(pthread_create(&tids[i],&attr,workLoad,(void*)&thread_array[i-1]) != 0){