very useful log

First of all, the log is not comprehensive, only the error is implemented, the others can be implemented by referring to the source code:

#ifndef log_h_
#define log_h_

#include <stdarg.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

#define EVENT_LOG_DEBUG 0
#define EVENT_LOG_MSG   1
#define EVENT_LOG_WARN  2
#define EVENT_LOG_ERR   3
#define _EVENT_LOG_DEBUG EVENT_LOG_DEBUG
#define _EVENT_LOG_MSG EVENT_LOG_MSG
#define _EVENT_LOG_WARN EVENT_LOG_WARN
#define _EVENT_LOG_ERR EVENT_LOG_ERR

typedef void (*event_log)(int severity, const char *msg)

//We mainly call him
void event_error(const char* fmt,...);
void event_warnx(const char *fmt, ...);
void event_msgx(const char *fmt, ...);
///The following function is our internal, we can put it into a header file separately
void _warn_helper(int severity,const char* errstr,char* fmt,va_list ap);
void event_log(int severity, const char *msg);
int evutil_snprintf(char* buf,int size ,char* fmt,...);
int evutil_vsnprintf(char* buf,int size,char* fmt,va_list ap);

#endif

Implementation of .c file

#include "build_libevent_log.h"
//define global variable
event_log func = NULL;
//Set user-defined output format
void event_set_log_callback(event_log cb)
{
		func = cb;
}

void event_warnx(const char *fmt, ...)
{
	va_list ap;
	va_start(ap, fmt);
	_warn_helper(EVENT_LOG_WARN, NULL, fmt, ap);
	va_end(ap);
}
void event_msgx(const char *fmt, ...)
{
	va_list ap;
	va_start(ap, fmt);
	_warn_helper(EVENT_LOG_MSG, NULL, fmt, ap);
	va_end(ap);
}
void event_error(const char* fmt,...)
{
	va_list ap;
	va_start(ap,fmt);
	//This is a proxy function, other priority functions can also call him
	_warn_helper(EVENT_LOG_ERR,strerror(errno),fmt,ap);
	va_end(ap);
}

void _warn_helper(int severity,const char* errstr,char* fmt,va_list ap)
{
	char buf[1024];
	if(fmt != NULL)
	{
		//Call the splicing function of the string. The format of the function is (char* buf, int size, char* fmt, va_list ap);
		evutil_vsnprintf(buf,sizeof(buf),fmt,ap);
	}
	if(errstr != NULL)
	{
		int len ​​= strlen(buf);
		//The format of this function is (char* buf, int size, char* fmt,...); in fact, the above function is also the underlying implementation function of our function
		evutil_snprintf(buf + len,sizeof(buf) - len,": %s",errstr);
	}
	event_log(severity,buf);
}
int evutil_snprintf(char* buf,int size ,char* fmt,...)
{
	int ret;
	va_list ap;
	va_start(ap,fmt);
	ret = evutil_vsnprintf(buf,size,fmt,ap);
	va_end(ap);
	return ret;
}
int evutil_vsnprintf(char* buf,int size,char* fmt,va_list ap)
{
	int ret = vsnprintf(buf,size,fmt,ap);
	buf[size -1] = '\0';
	return ret;
}
void event_log(int severity, const char *msg)
{
	if(func)
	{
		func(severity,msg);
	}
	else{
		const char *severity_str;
		
		switch (severity)
		{
			case _EVENT_LOG_DEBUG:
				severity_str = "debug";
				break;
			case _EVENT_LOG_MSG:
				severity_str = "msg";
				break;
			case _EVENT_LOG_WARN:
				severity_str = "warn";
				break;
			case _EVENT_LOG_ERR:
				severity_str = "error";
				break;
			default:
				break;
		}

		fprintf(stderr, "[%s] %s\n", severity_str, msg);
	}
}