timespec の足し算引き算

#include <time.h>   // clock_gettime
#include <stdio.h>  // printf


struct timespec *timessub(const struct timespec *A, const struct timespec *B, struct timespec *C)
{
    C->tv_sec  = A->tv_sec  - B->tv_sec;
    C->tv_nsec = A->tv_nsec - B->tv_nsec;
    if(C->tv_nsec<0){
        C->tv_sec--;
        C->tv_nsec += 1000000000;
    }
    
    return C;
}
struct timespec *timesadd(const struct timespec *A, const struct timespec *B, struct timespec *C)
{
    C->tv_sec  = A->tv_sec  + B->tv_sec;
    C->tv_nsec = A->tv_nsec + B->tv_nsec;
    if(C->tv_nsec>=1000000000){
        C->tv_sec++;
        C->tv_nsec -= 1000000000;
    }
    
    return C;
}


int main()
{
    struct timespec A = { 3, 500000000 };
    struct timespec B = { 1, 600000000 };
    struct timespec C;

    timessub(&A,&B,&C);
    printf("tv_sec  = %ld\n", C.tv_sec);
    printf("tv_nsec = %ld\n", C.tv_nsec);

    timesadd(&A,&B,&C);
    printf("tv_sec  = %ld\n", C.tv_sec);
    printf("tv_nsec = %ld\n", C.tv_nsec);

    return 0;
}