用于守护进程的出错处理函数

/*
* Error routines for programs that can run as a daemon.
*/

#include "apue.h"
#include <errno.h>    /* for definition of errno */
#include <stdarg.h>    /* ISO C variable arguments */
#include <syslog.h>

static void log_doit(int, int, const char *, va_list ap);

/*
* Caller must define and set this: nonzero if
* interactive, zero if daemon
*/
extern int log_to_stderr;;

/*
* Initialize syslog(), is running as daemon.
*/
void 
log_open(const char *ident, int option, int facility)
{
    if(log_to_stderr == 0)
        openlog(ident, option, facility);
}

/*
* Nonfatal error related to a system call.
* Print a message with the system's errno value and return.
*/
void 
log_ret(const char *fmt, ...)
{
    va_list        ap;
    va_start(ap, fmt);
    log_doit(1, LOG_ERR, fmt, ap);
    va_end(ap);
}

/*
* Fatal error realted to a system call.
* Print a message and terminate.
*/
void
log_sys(const char *fmt, ...)
{
    va_list        ap;
    
    va_start(ap, fmt);
    log_doit(1, LOG_ERR, fmt, ap);
    va_end(ap);
    exit(2);
}

/*
* Nonfatal error unrelated to a system call.
* Print a message and return.
*/
void
log_msg(const char *fmt, ...)
{
    va_list        ap;
    
    va_start(ap, fmt);
    log_doit(0, LOG_ERR, fmt, ap);
    va_end(ap);
}

/*
* Fatal error unrelated to a system call.
* Print a message and terminate.
*/
void
log_quit(const char *fmt, ...)
{
    va_list        ap;
    
    va_start(ap, fmt);
    log_doit(0, LOG_ERR, fmt, ap);
    va_end(ap);
    exit(2);
}

/*
* Print a message and return to caller.
* Caller specifies "errnoflag" and "priority". 
*/
static void
log_doit(int errnoflag, int priority, const char *fmt, va_list ap)
{
    int     errno_save;
    char     buf[MAXLINE];

    errno_save = errno;    /* value caller might want printed */
    vsnprintf(buf, MAXLINE, fmt, ap);
    if(errnoflag)
        snprintf(buf+strlen(buf), MAXLINE-strlen(buf), ": %s", strerror(errno_save));
    strcat(buf, "
");
    if(log_to_stderr)
    {
        fflush(stdout);
        fputs(buf, stderr);    
        fflush(stderr);
    }
    else
    {
        syslog(priority, buf);
    }
}

本篇博文内容摘自《UNIX环境高级编程》（第二版），仅作个人学习记录所用。关于本书可参考：http://www.apuebook.com/。