compilation error on clock_gettime and CLOCK_MONOTONIC

Question

I'm using clock_gettime in a program. I've tried including as well as but neither works. I have also added -lrt to my compiler arguments but still I get the same errors.

This is on

CentOS Linux release 7.2.1511 (Core)
gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4)
GNU ld version 2.23.52.0.1-55.el7 20130226
ldd (GNU libc) 2.17

Compiler output:

gcc -o main packet.c connect.c transport.c accept.c main.c close.c util.c receive.c send.c congestion.c -Wall -g -std=c99 -lrt
util.c: In function ‘millis’:
util.c:42:21: error: storage size of ‘t’ isn’t known
     struct timespec t;
                     ^
util.c:43:5: warning: implicit declaration of function ‘clock_gettime’ [-Wimplicit-function-declaration]
     clock_gettime(CLOCK_MONOTONIC_RAW, &t);
     ^
util.c:43:19: error: ‘CLOCK_MONOTONIC_RAW’ undeclared (first use in this function)
     clock_gettime(CLOCK_MONOTONIC_RAW, &t);
               ^

Makefile

CFLAGS = -Wall -g -std=c99
LIBS = -lrt

# Should be equivalent to your list of C files, if you don't build selectively
SRC=$(wildcard *.c)

main: $(SRC)
    gcc -o $@ $^ $(CFLAGS) $(LIBS)

Top of util.h

#ifndef UTILS_438_H_
#define UTILS_438_H_

#include "const.h"
#include "data.h"
#include "transport.h"

#include <time.h>

Top of util.c

#include "util.h"

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

Please let me know if I can supply more information to help answer this

Answer 1

Before including the header(<time.h>), do

#define _POSIX_C_SOURCE 199309L

---

http://man7.org/linux/man-pages/man2/clock_gettime.2.html

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   clock_getres(), clock_gettime(), clock_settime():
          _POSIX_C_SOURCE >= 199309L

---

http://man7.org/linux/man-pages/man7/feature_test_macros.7.html

_POSIX_C_SOURCE Defining this macro causes header files to expose definitions as follows:

  ·  The value 1 exposes definitions conforming to POSIX.1-1990 and
     ISO C (1990).

  ·  The value 2 or greater additionally exposes definitions for
     POSIX.2-1992.

  ·  The value 199309L or greater additionally exposes definitions
     for POSIX.1b (real-time extensions).

  ·  The value 199506L or greater additionally exposes definitions
     for POSIX.1c (threads).

  ·  (Since glibc 2.3.3) The value 200112L or greater additionally
     exposes definitions corresponding to the POSIX.1-2001 base
     specification (excluding the XSI extension).  This value also
     causes C95 (since glibc 2.12) and C99 (since glibc 2.10)
     features to be exposed (in other words, the equivalent of
     defining _ISOC99_SOURCE).

  ·  (Since glibc 2.10) The value 200809L or greater additionally
     exposes definitions corresponding to the POSIX.1-2008 base
     specification (excluding the XSI extension).

Answer 2

Simply replace -std=c99 with -std=gnu99. Thisway you do not have to add _POSIX_C_SOURCE