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What API do I call to get the system uptime?

Posted by: admin November 30, 2017 Leave a comment


I would like to get the system uptime from within a C application running on a linux-based system. I don’t want to call uptime(1) and parse the output, I’d like to call the underlying C API I suspect exists. Anyone know if there is such a call, or does uptime(1) simply process records obtained from wtmp?


The system call you’re looking for is sysinfo().

It’s defined in sys/sysinfo.h

Its signature is:
int sysinfo(struct sysinfo *info)

Since kernel 2.4, the structure has looked like this:

struct sysinfo {
    long uptime;             /* Seconds since boot */
    unsigned long loads[3];  /* 1, 5, and 15 minute load averages */
    unsigned long totalram;  /* Total usable main memory size */
    unsigned long freeram;   /* Available memory size */
    unsigned long sharedram; /* Amount of shared memory */
    unsigned long bufferram; /* Memory used by buffers */
    unsigned long totalswap; /* Total swap space size */
    unsigned long freeswap;  /* swap space still available */
    unsigned short procs;    /* Number of current processes */
    unsigned long totalhigh; /* Total high memory size */
    unsigned long freehigh;  /* Available high memory size */
    unsigned int mem_unit;   /* Memory unit size in bytes */
    char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding for libc5 */

Have fun!


Read the file /proc/uptime and take the first decimal number as the uptime in seconds.

From man 5 proc:

          This file contains two numbers: the uptime of the  system  (sec‐
          onds), and the amount of time spent in idle process (seconds).


That would be something like this.

#include <stdio.h>
#include <errno.h>
#include <linux/unistd.h>       /* for _syscallX macros/related stuff */
#include <linux/kernel.h>       /* for struct sysinfo */
#include <sys/sysinfo.h>

long get_uptime()
    struct sysinfo s_info;
    int error = sysinfo(&s_info);
    if(error != 0)
        printf("code error = %d\n", error);
    return s_info.uptime;

See “man sysinfo” for more info.


There’s also clock_gettime (probably needs -lrt). The behavior I’ve seen (I’m not going to claim it’s guaranteed) but given CLOCK_MONOTONIC as the clk_id is that it returns the system uptime in the given struct timespec * parameter.

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

int main(int argc, char* argv[]) {
  struct timespec t;
  clock_gettime(CLOCK_MONOTONIC, &t);
  printf("tv_sec=%llu tv_nsec=%llu\n",
    (unsigned long long)t.tv_sec,
    (unsigned long long)t.tv_nsec);
  return 0;

#include <sys/sysinfo.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <grp.h>

int main() {
  struct sysinfo sys_info;
  struct group* grp;

    gid_t gid;
    char** users;

  int days, hours, mins, x = 1;

  printf("3[4;40m            Systems information                   3[0;m \n");
  if(sysinfo(&sys_info) != 0)

  days = sys_info.uptime / 86400;
  hours = (sys_info.uptime / 3600) - (days * 24);
  mins = (sys_info.uptime / 60) - (days * 1440) - (hours * 60);

  printf("3[1;33m  Uptime: 3[0;36m %ddays, %dhours, %dminutes, %ldseconds 3[0;m \n",
                      days, hours, mins, sys_info.uptime % 60);

  printf("3[1;33m  Load Avgs: 3[0;m 1min(%ld) 5min(%ld) 15min(%ld) \n",
          sys_info.loads[0], sys_info.loads[1], sys_info.loads[2]);

  printf("3[1;33m  Total Ram: 3[0;m %ldk \t Free: %ldk \n", sys_info.totalram / 1024, sys_info.freeram / 1024);
  printf(" 3[1;33m Shared Ram: 3[0;m %ldk ", sys_info.sharedram / 1024);
  printf("  Buffered Ram: %ldk \n", sys_info.bufferram / 1024);
  printf("3[1;33m  Total Swap: 3[0;m %ldk \t Free swap: %ldk \n", sys_info.totalswap / 1024, sys_info.freeswap / 1024);
  printf("3[1;33m  Total High Memory: 3[0;m %ldk  Free high memory: %ldk 3[0;m \n", sys_info.totalhigh / 1024, sys_info.freehigh / 1024);
  printf(" \n");
  printf("3[1;44m Total Number of processes: %d 3[0;m \n", sys_info.procs);  
  gid = getgid();
    printf("  Group ID: 3[031m %d", gid);
    if((grp = getgrgid(gid)) == NULL ) return 1;
    printf("3[0;m Group %s ", grp->gr_name );
    printf("\n  Users in your group ");
    for( users = grp->gr_mem; *users != NULL; users++,++x ); printf( "%d", ++x);     
    if(strcmp(grp->gr_passwd,"x") == 0) printf("  Password is protected by shadow file. \n");
    else printf("Password: %s ", grp->gr_passwd);   

    return 0;