Problem with C code for timing measurements

Question

Hi,

EDIT: Thanks for the answers so far, at least I can compile it now, but I still get a segmentation error.

For compilation I use the following line:

gcc -g -O0 -I../include -L../ test.c -static -lrt

Source Code is as follows:

#include <sys/time.h>
#include <time.h>
#include <stdio.h>


struct timespec *diff(struct timespec *start, struct timespec *end);

int main()
{
struct timespec time1, time2;
    int i;
int temp = 0;
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1);
for (i = 0; i< 242000000; i++)
    temp+=temp;
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2);
    printf("sec: %d, nsec: %f",diff(&time1,&time2)->tv_sec,    diff(&time1,&time2)->tv_nsec);
//cout<<diff(time1,time2).tv_sec<<":"<<diff(time1,time2).tv_nsec<<endl;
return 0;
}

struct timespec *diff(struct timespec *start, struct timespec *end)
{
struct timespec *temp;
if ((end->tv_nsec-start->tv_nsec)<0) {
    temp->tv_sec = end->tv_sec-start->tv_sec-1;
    temp->tv_nsec = 1000000000+end->tv_nsec-start->tv_nsec;
} else {
    temp->tv_sec = end->tv_sec-start->tv_sec;
    temp->tv_nsec = end->tv_nsec-start->tv_nsec;
}
return temp;
}

I get now the following warning:

test.c: In function ‘main’: test.c:17: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘__time_t’ test.c:17: warning: format ‘%f’ expects type ‘double’, but argument 3 has type ‘long int’

Segmentation fault is surely caused by my handling of the structures. It is quite a while ago that i last had to deal with C....

Many thanks, Marcus

Answer 1

Your diff function's signature is timespec diff(timespec start, timespec end), where it should be struct timespec diff(struct timespec start, struct timespec end).

---

EDIT Your diff function is assigning to an uninitialized struct pointer, you might want to change it to something like the following.

void diff(struct timespec *start, struct timespec *end,
    struct timespec *result);

int main()
{
    struct timespec time1, time2, result;
    int i;
    int temp = 0;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1);
    for (i = 0; i< 242000000; i++)
        temp+=temp;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2);

    diff(&time1, &time2, &result);

    printf("sec: %ld, nsec: %ld", (long int)result.tv_sec,
        (long int)result.tv_nsec);
    return 0;
}

void diff(struct timespec *start, struct timespec *end,
    struct timespec * result)
{
    if ((end->tv_nsec-start->tv_nsec)<0) {
        result->tv_sec = end->tv_sec-start->tv_sec-1;
        result->tv_nsec = 1000000000+end->tv_nsec-start->tv_nsec;
    } else {
        result->tv_sec = end->tv_sec-start->tv_sec;
        result->tv_nsec = end->tv_nsec-start->tv_nsec;
    }
}

Answer 2

You have to add the struct keyword before the timespec's. E.g.

struct timespec diff(struct timespec start, struct timespec end);

Answer 3

You are compiling it as C (gcc and .c extension), but looks like you are coding in C++ (where struct could be committed when declaring variables).

Also, in C we usually don't pass structures by value.

Another hint is to always compile with high warning level, at least -Wall -pedantic, to catch problems early.

Answer 4

I think you want diff() to return either a dynamically or statically allocated pointer.

Try (suggestions in comments):

struct timespec *diff(struct timespec *start, struct timespec *end)
{
struct timespec *temp;

/* allocate temp to be sizeof(struct timespec) and zero it out */
temp = malloc(sizeof(struct timespec));
/* Of course, deal with malloc (or calloc) failing */
memset(tmp, 0, sizeof(struct timespec));

if ((end->tv_nsec-start->tv_nsec)<0) {
    temp->tv_sec = end->tv_sec-start->tv_sec-1;
    temp->tv_nsec = 1000000000+end->tv_nsec-start->tv_nsec;
} else {
    temp->tv_sec = end->tv_sec-start->tv_sec;
    temp->tv_nsec = end->tv_nsec-start->tv_nsec;
}
return temp;
/* Make sure caller frees the returned pointer */
}

If dynamic allocation is taboo, then:

static struct timespec temp;
/*zero it out, watch concurrency too! */

....

temp.tv_sec = end->tv_sec-start->tv_sec;
/* Make sure caller does NOT free (or modify) the pointer */
return temp;

You need to return a pointer there (however allocated) that can actually be reached by the caller. Or, make *temp global.

Answer 5

Depending on your needs, you might just want to generate the elapsed time like this:

double t_ns = (double)(end.tv_sec - start.tv_sec) * 1.0e9 + (double)(end.tv_nsec - start.tv_nsec); // get elapsed time in ns

Also, FWIW, I use CLOCK_PROCESS_CPUTIME_ID - it seems to give better accuracy and resolution on the various Linux-based systems that I've tried it on:

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);

// ... stuff ... //

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);