Flexible Logging

Background

Logging is often an afterthought. Trace, info, warnings, and errors, all sent to stdout or stderr. The C-Craft library provides a flexible logging system that can be quickly changed to target a file, or syslog, or disabled completely or partially with just one or two lines of code.

File Logs

#include <c-craft/app.h>

void main(int argc, char **argv)
{
    MEM_SCOPE mem = sm_create(0);
    LOG log = log_file_create(mem, "myapp-log.txt");

    const char *app = get_program_name();
    const char *host = get_host_name();

    log_printf(log, L_INFO, "'%s' starting up on %s", app, host);

    . . .

    log_printf(log, L_INFO, "'%s' on %s.  Processing complete.", app, host);

    log_close(log);
    sm_free(mem);
}

This example writes two lines to the log file ‘myapp-log.txt’ with severity and timestamp.

If, at a later time you decide to write to the syslog instead, you change one line of code.

LOG log = log_sys_create(mem, 0, LOG_USER);

Other Logs

There are other options too.

/* Log to FILE stream. */
LOG log = log_fp_create(mem, stdout);

/* Log to standard output/error.  Writes L_FATAL, L_ERROR, and L_WARN
 * to stderr, and L_INFO, L_TRACE to stdout.
 */
LOG log = log_std_create(mem);

Note that it is good practise to close a log. The log_fp is created with an open FILE, so closing a log_fp does not close the open FILE.

Custom Logs

If you need your own custom logging.

/* Log to a custom log. */
void my_log_write(LOG_CTRL log, LOG_LEVEL level, const char* message);
void my_log_close(LOG_CTRL log);
LOG_CTRL ctrl = my_log;

LOG log = log_gen_create(mem, my_log, my_log_write, my_log_close);

/* Here my_log is a pointer to your own struct that carries data about
 * your log, and is passed to the my_log_write and my_log_close functions.
 * It might be a database reference, for example, depending on your log.
 */

Now when you write using log_printf(log, …) the logging system will call your custom write function. The close function can be NULL, if it is meaningless for your logger.