STRERROR(3)

NAME
perror, strerror, strerror_l, strerror_r, sys_errlist, sys_nerr - system error messages

LIBRARY
libc

SYNOPSIS
#include <stdio.h>
void
perror(const char *string);
extern const char * const sys_errlist[];
extern const int sys_nerr;
#include <string.h>
char *
strerror(int errnum);
char *
strerror_l(int errnum, locale_t);
int
strerror_r(int errnum, char *strerrbuf, size_t buflen);

DESCRIPTION
The strerror(), strerror_l(), strerror_r(), and perror() functions look up the error message string corresponding to an error number.

The strerror() function accepts an error number argument errnum and returns a pointer to the corresponding message string in the current locale. strerror() is not thread-safe. It returns a pointer to an internal static buffer that could be overwritten by a strerror() call from another thread.

The strerror_l() function accepts errnum error number and locale locale handle arguments and returns a pointer to a string corresponding to the specified error in the given locale. strerror_l() is thread-safe, its result can be only overwritten by another call to strerror_l() from the current thread.

The strerror_r() function renders the same result into strerrbuf for a maximum of buflen characters and returns 0 upon success.

The perror() function finds the error message corresponding to the current value of the global variable errno (intro(2)) and writes it, followed by a newline, to the standard error file descriptor. If the argument string is non-NULL and does not point to the null character, this string is prepended to the message string and separated from it by a colon and space (“: ”); otherwise, only the error message string is printed.

If the error number is not recognized, these functions return an error message string containing “Unknown error: ” followed by the error number in decimal. The strerror() and strerror_r() functions return EINVAL as a warning. Error numbers recognized by this implementation fall in the range 0 < errnum < sys_nerr. The number 0 is also recognized, although applications that take advantage of this are likely to use unspecified values of errno.

If insufficient storage is provided in strerrbuf (as specified in buflen) to contain the error string, strerror_r() returns ERANGE and strerrbuf will contain an error message that has been truncated and NUL terminated to fit the length specified by buflen.

The message strings can be accessed directly using the external array sys_errlist. The external value sys_nerr contains a count of the messages in sys_errlist. The use of these variables is deprecated; strerror(), strerror_l(), or strerror_r() should be used instead.

EXAMPLES
The following example shows how to use perror() to report an error.

    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    int
    main(void)
    {
    	int fd;
    	if ((fd = open("/nonexistent", O_RDONLY)) == -1) {
    		perror("open()");
    		exit(1);
    	}
    	printf("File descriptor: %d\n", fd);
    	return (0);
    }

When executed, the program will print an error message along the lines of ‘open(): No such file or directory’.

SEE ALSO
intro(2), err(3), psignal(3)

STANDARDS
The perror() and strerror() functions conform to isoC-99. The strerror_r() function conforms to p1003.1-2001. The strerror_l() function conforms to p1003.1-2008.

HISTORY
The strerror() and perror() functions first appeared in 4.4BSD. The strerror_r() function was implemented in FreeBSD 4.4 by Wes Peters <wes@FreeBSD.org>. The strerror_l() function was added in FreeBSD 13.0.

BUGS
The strerror() function returns its result in a static buffer which will be overwritten by subsequent calls.

Programs that use the deprecated sys_errlist variable often fail to compile because they declare it inconsistently. Size of the sys_errlist object might increase during FreeBSD lifetime, breaking some ABI stability guarantees.