/tmp/ccNLM6c6.o: In function `main':
 unlink.c:(.text+0x2e): undefined reference to `err_sys(char const*, ...)'
 unlink.c:(.text+0x52): undefined reference to `err_sys(char const*, ...)'
 collect2: error: ld returned 1 exit status

把下面的代码复制到 apue.h 中 ，记得包含这个头文件

#include <errno.h> /* for definition of errno */
#include <stdarg.h> /* ISO C variable aruments */

static void err_doit(int, int, const char *, va_list);

/*
 * Nonfatal error related to a system call.
 * Print a message and return.
 */
void err_ret(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(1, errno, fmt, ap);
    va_end(ap);
}


/*
 * Fatal error related to a system call.
 * Print a message and terminate.
 */
void err_sys(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(1, errno, fmt, ap);
    va_end(ap);
    exit(1);
}


/*
 * Fatal error unrelated to a system call.
 * Error code passed as explict parameter.
 * Print a message and terminate.
 */
void err_exit(int error, const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(1, error, fmt, ap);
    va_end(ap);
    exit(1);
}


/*
 * Fatal error related to a system call.
 * Print a message, dump core, and terminate.
 */
void err_dump(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(1, errno, fmt, ap);
    va_end(ap);
    abort(); /* dump core and terminate */
    exit(1); /* shouldn't get here */
}


/*
 * Nonfatal error unrelated to a system call.
 * Print a message and return.
 */
void err_msg(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(0, 0, fmt, ap);
    va_end(ap);
}


/*
 * Fatal error unrelated to a system call.
 * Print a message and terminate.
 */
void err_quit(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(0, 0, fmt, ap);
    va_end(ap);
    exit(1);
}


/*
 * Print a message and return to caller.
 * Caller specifies "errnoflag".
 */
static void err_doit(int errnoflag, int error, const char *fmt, va_list ap)
{
    char buf[MAXLINE];
   vsnprintf(buf, MAXLINE, fmt, ap);
   if (errnoflag)
       snprintf(buf+strlen(buf), MAXLINE-strlen(buf), ": %s",
         strerror(error));
   strcat(buf, "\n");
   fflush(stdout); /* in case stdout and stderr are the same */
   fputs(buf, stderr);
   fflush(NULL); /* flushes all stdio output streams */
}