Logger

The logger library provides a tool to write some logging messages into a file.

Use case

The following snippet explains how to use the logger library (see test/main.c to try this example) :

#include "logger.h"

int	main(void)
{
	if (logger_init(D_TRACE, "/tmp/out.log") != 0)
		printf("failed to open the logger\n");
	log_fatal("%s", "fatal message");
	log_error("%s", "error message");
	log_warn("%s", "warning message");
	log_success("%s", "success message");
	log_info("%s", "info message");
	log_debug("%s", "debug message");
	log_trace("%s", "trace message");
	logger_close();
	return (0);
}

Initialization and exit

• The purpose of int logger_init(int level, char *log_file); is to setup the logger with some parameters. It returns 0 on success, and a negative value otherwise.

if (logger_init(D_TRACE, "/tmp/out.log") != 0)
	printf("failed to open the logger\n");

• To exit properly the logger, use the close function int logger_close(void); :

logger_close();

Logger level

The log level must be one of the following macros, if it is not, the default D_OFF level will be used :

# define D_OFF			0
# define D_FATAL		1
# define D_ERROR		2
# define D_WARN			3
# define D_SUCCESS		4
# define D_INFO			5
# define D_DEBUG		6
# define D_TRACE		7

The logic of the log level in the initializing function is the following:

• if the flag is D_OFF flag, the logger will not display anything
• if the flag is D_FATAL, the logger will display only the D_FATAL logging messages
• if the flag is D_ERROR, the logger will display the D_FATAL and D_ERROR logging messages (but not D_WARN to D_TRACE logging messages)
• ...
• if the flag is D_TRACE, all the logging messages will be display

Output file

• It depends on the purpose of the logger, but for development, it seems to be a good idea to put the log file into /tmp or everywhere out of the project folder (the amount of access may be important).
• The function will setup a file with the following flags : O_WRONLY | O_APPEND | O_CREAT. It means that :
• the file is open in write-only mode
• the new log message will be written at the end of the file
• the file will be created if it doesn't exist yet

Logger functions

Use the following functions to display a log message :

void	log_fatal(const char * restrict format, ...);
void	log_error(const char * restrict format, ...);
void	log_warn(const char * restrict format, ...);
void	log_success(const char * restrict format, ...);
void	log_info(const char * restrict format, ...);
void	log_debug(const char * restrict format, ...);
void	log_trace(const char * restrict format, ...);

The arguments are the same as man 3 printf function.

if the log level is D_WARN and the code contains a log_debug(...), this debug message will be simply ignored.

Each functions will provide a different output, here is a sample of the execution of the snippet at the top of the document :

>>>>>>>>>>>>>>>>>> NEW INSTANCE OF THE APPLICATION <<<<<<<<<<<<<<<<<<

[ 12:50:30 ] (l. 26) main.c               -> fatal message
[ 12:50:30 ] (l. 27) main.c               -> error message
[ 12:50:30 ] (l. 28) main.c               -> warning message
[ 12:50:30 ] (l. 29) main.c               -> success message
[ 12:50:30 ] (l. 30) main.c               -> info message
[ 12:50:30 ] (l. 31) main.c               -> debug message
[ 12:50:30 ] (l. 32) main.c               -> trace message

>>>>>>>>>>>>>>>>>> END OF THE APPLICATION INSTANCE <<<<<<<<<<<<<<<<<<

Notice that the logger appends a \n at the end of the message, so it is not necessary to add it.

Purpose of logging level

There is no rules for the using of the levels, but we can considered that :

• FATAL will produce the end of the execution of the program (ex : wrong configuration)
• ERROR is a code failure (ex : cannot open a non-critical configuration file)
• WARN is not an error, but change the behavior of the program (ex : stub part of code)
• SUCCESS inform that an operation has exited with success
• INFO is like warn, but it is not dangerous as a warning (ex : configuration file loaded)
• DEBUG is for developers, like INFO but not for the user (ex : display the size of a window)
• TRACE is for 'low level' developers (ex : display the i of a while loop)

The Team

• Adrien Nouvel @anouvel
• Gabriel Kuma @gabkk
• Jean-Michel Gigault @jgigault
• Matthias Leconte @Tix6