How do I get the unix timestamp in C as an int?

I would like to get the current timestamp and print it out using fprintf.

For 32-bit systems:

fprintf(stdout, "%u\n", (unsigned)time(NULL)); 

For 64-bit systems:

fprintf(stdout, "%lu\n", (unsigned long)time(NULL)); 

Is just casting the value returned by time()

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

int main(void) {
    printf("Timestamp: %d\n",(int)time(NULL));
    return 0;
}

what you want?

$ gcc -Wall -Wextra -pedantic -std=c99 tstamp.c && ./a.out
Timestamp: 1343846167

To get microseconds since the epoch, from C11 on, the portable way is to use

int timespec_get(struct timespec *ts, int base)

Unfortunately, C11 is not yet available everywhere, so as of now, the closest to portable is using one of the POSIX functions clock_gettime or gettimeofday (marked obsolete in POSIX.1-2008, which recommends clock_gettime).

The code for both functions is nearly identical:

#include <stdio.h>
#include <time.h>
#include <stdint.h>
#include <inttypes.h>

int main(void) {

    struct timespec tms;

    /* The C11 way */
    /* if (! timespec_get(&tms, TIME_UTC)) { */

    /* POSIX.1-2008 way */
    if (clock_gettime(CLOCK_REALTIME,&tms)) {
        return -1;
    }
    /* seconds, multiplied with 1 million */
    int64_t micros = tms.tv_sec * 1000000;
    /* Add full microseconds */
    micros += tms.tv_nsec/1000;
    /* round up if necessary */
    if (tms.tv_nsec % 1000 >= 500) {
        ++micros;
    }
    printf("Microseconds: %"PRId64"\n",micros);
    return 0;
}

With second precision, you can print tv_sec field of timeval structure that you get from gettimeofday() function. For example:

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

int main()
{
	struct timeval tv;
	gettimeofday(&tv, NULL);
	printf("Seconds since Jan. 1, 1970: %ld\n", tv.tv_sec);
	return 0;
}

Example of compiling and running:

$ gcc -Wall -o test ./test.c 
$ ./test 
Seconds since Jan. 1, 1970: 1343845834

Note, however, that its been a while since epoch and so long int is used to fit a number of seconds these days.

There are also functions to print human-readable times. See this manual page for details. Here goes an example using ctime():

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

int main()
{
	time_t clk = time(NULL);
	printf("%s", ctime(&clk));
	return 0;
}

Example run & output:

$ gcc -Wall -o test ./test.c 
$ ./test 
Wed Aug  1 14:43:23 2012
$ 

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

int main ()
{
   time_t seconds;

   seconds = time(NULL);
   printf("Seconds since January 1, 1970 = %ld\n", seconds);
  
   return(0);
}

And will get similar result:
Seconds since January 1, 1970 = 1476107865

An important point is to consider if you perform tasks based on difference between 2 timestamps because you will get odd behavior if you generate it with gettimeofday(), and even clock_gettime(CLOCK_REALTIME,..) at the moment where you will set the time of your system.

To prevent such problem, use clock_gettime(CLOCK_MONOTONIC_RAW, &tms) instead.