Interpreting time command output on a multi threaded program - linux

Question

I have a multi threaded program and I am profiling time taken starting before all pthread_create's and after all pthread_join's.
Now I find that this time, lets call it X, which is shown below in "Done in xms" is actually user + sys time of time output. In my app the number argument to a.out controls how many threads to spawn. ./a.out 1 spawn 1 pthread and ./a.out 2 spawns 2 threads where each thread does the same amount of work.
I was expecting X to be the real time instead of user + sys time. Can someone please tell me why this is not so? Then this really means my app is indeed running parallel without any locking between threads.
[jithin#whatsoeverclever tests]$ time ./a.out 1
Done in 320ms
real 0m0.347s
user 0m0.300s
sys 0m0.046s
[jithin#whatsoeverclever tests]$ time ./a.out 2
Done in 450ms
real 0m0.266s
user 0m0.383s
sys 0m0.087s
[jithin#whatsoeverclever tests]$ time ./a.out 3
Done in 630ms
real 0m0.310s
user 0m0.532s
sys 0m0.105s
Code
int main(int argc, char **argv) {
//Read the words
getWords();
//Set number of words to use
int maxWords = words.size();
if(argc > 1) {
int numWords = atoi(argv[1]);
if(numWords > 0 && numWords < maxWords) maxWords = numWords;
}
//Init model
model = new Model(MODEL_PATH);
pthread_t *threads = new pthread_t[maxWords];
pthread_attr_t attr;
void *status;
// Initialize and set thread joinable
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
int rc;
clock_t startTime = clock();
for(unsigned i=0; i<maxWords; i++) {
//create thread
rc = pthread_create(&threads[i], NULL, processWord, (void *)&words[i] );
if (rc){
cout << "Error:unable to create thread: " << i << "," << rc << endl;
exit(-1);
}
}
// free attribute and wait for the other threads
pthread_attr_destroy(&attr);
for(unsigned i=0; i<maxWords; i++) {
rc = pthread_join(threads[i], &status);
if (rc){
cout << "Error:unable to join thread: " << i << "," << rc << endl;
exit(-1);
}
}
clock_t endTime = clock();
float diff = (((float)endTime - (float)startTime) / 1000000.0F ) * 1000;
cout<<"Done in "<< diff << "ms\n";
delete[] threads;
delete model;
}

Answer 1

The clock function is specifically documented to return the processor time used by a process. If you want to measure wall time elapsed, it's not the right function.