Demonstrate Application Logging in C Programming Language

basic_application_logging.c

/*
 * Copyright (c) 2015 Neeraj Sharma. All rights reserved.
 *
 * Licensed under GNU GPL v3 License.
 * License: https://www.gnu.org/licenses/gpl-3.0.txt
 *
 * Code style based on the Linux Kernel, see
 * https://www.kernel.org/doc/Documentation/CodingStyle
 * for more details.
 *
 * indent with the following options
 * -kr -i8 -ts8 -sob -l80 -ss -bs -psl
 */
#include <stdio.h>    /* fprintf() and friends */
#include <string.h>   /* strncpy() */
#include <sys/time.h>   /* gmtime_r() */
#include <sys/types.h>
#include <time.h>   /* time() */
#include <unistd.h>   /* getpid(), gethostname() */


#define MAX_PROG_NAME_LEN 40
#define MAX_HOSTNAME_LEN 20
static char g_progname[MAX_PROG_NAME_LEN] = {0};
static char g_hostname[MAX_HOSTNAME_LEN] = {0};
static int g_pid = 0;
#define LOG_ERROR(msg) logmsg(__func__, __LINE__, "ERROR", (msg))
/*
 * Get the current time in c string with '\0' termination
 * and returns the length of the string.
 * The current implementation follows the ISO 8601 date and time
 * combined format with a resolution of seconds in the UTC timezone.
 * In case of error -1 is retured.
 */
int
time_to_cstr(time_t *t, char *timestr, int maxlen)
{
  struct tm tms;
  gmtime_r(t, &tms);
  snprintf(timestr, maxlen, "%04d-%02d-%02dT%02d:%02d:%02d+00:00",
     tms.tm_year + 1900, tms.tm_mon + 1, tms.tm_mday,
     tms.tm_hour, tms.tm_min, tms.tm_sec);
}
void
logmsg(const char *funcname, int linenum, const char *level, const char *msg)
{
  /* ISO 8601 date and time format with sec */
  const int time_str_len = 26;
  char time_str[time_str_len];
  time_t now;
  int len = 0;
  int ret = 0;


  time(&now);
  len = time_to_cstr(&now, time_str, time_str_len);
  if (len < 0) {
    /* huh! I'd like to crash at this point but
     * lets just log the message, which is a must.
     */
    return;
  }
  /* <HEADER> <MESSAGE>
   * <HEADER> = <TIMESTAMP> <HOSTNAME>
   * <MESSAGE> = <TAG> <LEVEL> <CONTENT>
   *   <TAG> = <PROGRAM>[<PID>]
   *   <LEVEL> = DEBUG | INFO | WARNING | ERROR
   *   <CONTENT> = <FUNCTION/MODULE>: <APPLICATION_MESSAGE>
   */
  fprintf(stderr, "%s %s %s[%d] %s %s(%d): %s\n", time_str, g_hostname,
    g_progname, g_pid, level, funcname, linenum, msg);
}
void display_welcome(void)
{
  logmsg(__func__, __LINE__, "ERROR", "a log entry");
  LOG_ERROR("Another log message");
}
int
main(int argc, char *argv[])
{
  int ret = 0;
  strncpy(g_progname, argv[0], MAX_PROG_NAME_LEN);
  ret = gethostname(g_hostname, MAX_HOSTNAME_LEN);
  if (ret < 0) {
    strncpy(g_hostname, "unknown", MAX_HOSTNAME_LEN);
  }
  g_pid = (int)getpid();
  display_welcome();
  return 0;
}