How to printf a time_t variable as a floating point number?

I'm using a time_t variable in C (openMP enviroment) to keep cpu execution time...I define a float value sum_tot_time to sum time for all cpu's...I mea开发者_运维知识库n sum_tot_time is the sum of cpu's time_t values. The problem is that printing the value sum_tot_time it appear as an integer or long, by the way without its decimal part!

I tried in these ways:

1. to printf sum_tot_time as a double being a double value
2. to printf sum_tot_time as float being a float value
3. to printf sum_tot_time as double being a time_t value
4. to printf sum_tot_time as float being a time_t value

The resolution of time_t is at most one second on most platforms. That is, on most platforms, time_t will be an integer (32- or 64-bit) value counting the number of seconds elapsed since midnight of Jan 1st 1970 (UTC), and can only achieve one-second resolution.

Therefore, a sum of time_t values will also only exhibit one-second resolution (no decimal part, even after converting to double.)

The above having been said, what native or OpenMP call are you using to obtain the time_t values that you are attempting to accumulate?

If using either the native *nix getrusage() call to fill out an rusage structure (provided your platform supports it) with user/kernel times, or if using gettimeofday() to get wall time, then use both the tv_sec and tv_usec fields of struct timeval to generate a double value (of millisecond-or-better resolution, typically), and use that instead of time_t in your calculations:

struct timeval {
        time_t          tv_sec;         /* seconds */
        suseconds_t     tv_usec;        /* microseconds */
};

Correspondingly, you can use GetThreadTimes/GetProcessTimes for user/kernel times or _ftime for wall time on Windows platforms, then combine FILETIME::dwHighDateTime/dwLowDateTime.

I'm not sure if you have access to standard *nix system calls ( or if this is relevant to specifically to what you're doing ), but if you do you can use the timeval struct and gettimeofday. For example, to print out a timestamp with six decimal places of precision which produces a tcpdump style time stamp ( courtesy of Steven UNP )

#include    "unp.h"
#include    <time.h>

char *
gf_time(void)
{
    struct timeval  tv;
    time_t          t;
    static char     str[30];
    char            *ptr;

    if (gettimeofday(&tv, NULL) < 0)
        err_sys("gettimeofday error");

    t = tv.tv_sec;  /* POSIX says tv.tv_sec is time_t; some BSDs don't agree. */
    ptr = ctime(&t);
    strcpy(str, &ptr[11]);
        /* Fri Sep 13 00:00:00 1986\n\0 */
        /* 0123456789012345678901234 5  */
    snprintf(str+8, sizeof(str)-8, ".%06ld", tv.tv_usec);

    return(str);
}