Question

I’m trying to pass pointers to two struct timevals to a function that would output the elapsed time between the two in a C program. However, even if I dereference these pointers, nvcc throws the error “expression must have class type” (this is a CUDA program). Here is the relevant code from main():

struct timeval begin, end;
if (tflag) { HostStartTimer(&begin) };
// CUDA Kernel execution
if (tflag) { HostStopTimer(&begin, &end); }

And the function definition for HostStopTimer():

void HostStopTimer(struct timeval *begin, stuct timeval *end) {
    long elapsed;
    gettimeofday(end, NULL);
    elapsed = ((*end.tv_sec - *begin.tv_sec)*1000000 + *end.tv_usec - *begin.tv_usec);
    printf("Host elapsed time: %ldus\n", elapsed);
 }

The line causing the error is the assignment to elapsed. I don’t have much experience using structs in C, much less passing pointers to structs to functions, so I’m not sure what is causing the error.

Answer 1

The . operator has higher precedence than the * operator, so expressions like *end.tv_sec attempt to first evaluate end.tv_sec (which isn’t possible since end is a pointer) and then dereference the result.

You should use (*end).tv_sec or end->tv_sec instead.