3.1. Simple Log Module

3.1.1. Concepts

  • Logs don’t lie

  • Log files are written to /var/log/ or /var/log/<application>, for example:

    • /var/log/messages: generic system activity logs
    • /var/log/cron.log: Crond logs (cron job)
    • /var/log/auth.log: Authentication log
    • /var/log/httpd/: Apache access and error logs directory
    • …

  • Log files and directories are rotated to limit the space used by older logs (see logrotate)

  • Log messages can be filtered away by implementing a treshold on log (severity) level

3.1.2. Assignment

  • By default, log to the connected terminal
  • Make functions to open, close, and print to a log file
  • Support the syslog logging levels
  • Write the logs with a timestamp of format [2017/10/18 14:40:35.666]
  • Provide support for setting a global log treshold
  • Create a header file for reuse in other programs
  • Use snprintf and vsnsprintf

3.1.3. Module API

#ifndef LOG_HDR
#define LOG_HDR

// mapping of syslog log levels
#define LOG_LEVEL_EMERGENCY     0
#define LOG_LEVEL_ALERT         1
#define LOG_LEVEL_CRITICAL      2
#define LOG_LEVEL_ERROR         3
#define LOG_LEVEL_WARNING       4
#define LOG_LEVEL_NOTICE        5
#define LOG_LEVEL_INFO          6
#define LOG_LEVEL_DEBUG         7

int log_init(void);
int log_set_level(int lvl);

int log_console_println(int lvl, const char *str);
int log_console_printf(int lvl, const char *fmt, ...);

int log_file_open(const char *path, int truncate);
int log_file_close(int fd);
int log_file_println(int fd, int lvl, const char *str);
int log_file_printf(int fd, int lvl, const char *fmt, ...);

#endif

3.1.4. Example Output

[2017/10/18 14:40:35.666][INFO     ] HTTP GET /index.html 192.168.157.21
[2017/10/18 14:40:36.666][INFO     ] HTTP GET /index.html 192.168.157.133
[2017/10/18 14:40:37.666][ERROR    ] HTTP PUT /upload/badfile.txt 192.168.157.154
[2017/10/18 14:40:38.666][WARNING  ] HTTP GET /index.php 192.168.157.154 - error 404
[2017/10/18 14:40:39.666][INFO     ] HTTP GET /index.html 192.168.157.100
[2017/10/18 14:40:39.666][EMERGENCY] error - out-of-memory, shutting down daemon

3.1.5. Questions

  • Why is snprintf() superior to strcpy() and sprintf()?

3.1.6. Extension (Optional)

  • Colorize the outpur on the console regarding the log level (red = error for example)

  • The terminal will interpret ANSI control characters as text formatting settings

  • It is possible to check if a fd is a terminal: isatty()

  • ANSI terminal control characters:

    #define ANSI_RESET              "\x1b[0m"
#define ANSI_BOLD               "\x1b[1m"
#define ANSI_UNDERLINE          "\x1b[4m"
#define ANSI_COLOR_TXT_BLACK    "\x1b[30m"
#define ANSI_COLOR_TXT_RED      "\x1b[31m"
#define ANSI_COLOR_TXT_GREEN    "\x1b[32m"
#define ANSI_COLOR_TXT_YELLOW   "\x1b[33m"
#define ANSI_COLOR_TXT_BLUE     "\x1b[34m"
#define ANSI_COLOR_TXT_MAGENTA  "\x1b[35m"
#define ANSI_COLOR_TXT_CYAN     "\x1b[36m"
#define ANSI_COLOR_TXT_WHITE    "\x1b[37m"
#define ANSI_COLOR_BKG_BLACK    "\x1b[40m"
#define ANSI_COLOR_BKG_RED      "\x1b[41m"
#define ANSI_COLOR_BKG_GREEN    "\x1b[42m"
#define ANSI_COLOR_BKG_YELLOW   "\x1b[43m"
#define ANSI_COLOR_BKG_BLUE     "\x1b[44m"
#define ANSI_COLOR_BKG_MAGENTA  "\x1b[45m"
#define ANSI_COLOR_BKG_CYAN     "\x1b[46m"
#define ANSI_COLOR_BKG_WHITE    "\x1b[47m"

  • For example: print ‘hello world’ formatted bold + red:

    dprintf(STDOUT_FILENO, ANSI_BOLD ANSI_COLOR_TXT_RED);
dprintf(STDOUT_FILENO, "hello world\n");
dprintf(STDOUT_FILENO, ANSI_RESET);