GNU Info
(libc.info)Error Messages
Error Messages
==============
The library has functions and variables designed to make it easy for
your program to report informative error messages in the customary
format about the failure of a library call. The functions `strerror'
and `perror' give you the standard error message for a given error
code; the variable `program_invocation_short_name' gives you convenient
access to the name of the program that encountered the error.
- Function: char * strerror (int ERRNUM)
The `strerror' function maps the error code (Note: Checking for
Errors) specified by the ERRNUM argument to a descriptive error
message string. The return value is a pointer to this string.
The value ERRNUM normally comes from the variable `errno'.
You should not modify the string returned by `strerror'. Also, if
you make subsequent calls to `strerror', the string might be
overwritten. (But it's guaranteed that no library function ever
calls `strerror' behind your back.)
The function `strerror' is declared in `string.h'.
- Function: char * strerror_r (int ERRNUM, char *BUF, size_t N)
The `strerror_r' function works like `strerror' but instead of
returning the error message in a statically allocated buffer
shared by all threads in the process, it returns a private copy
for the thread. This might be either some permanent global data or
a message string in the user supplied buffer starting at BUF with
the length of N bytes.
At most N characters are written (including the NUL byte) so it is
up to the user to select the buffer large enough.
This function should always be used in multi-threaded programs
since there is no way to guarantee the string returned by
`strerror' really belongs to the last call of the current thread.
This function `strerror_r' is a GNU extension and it is declared in
`string.h'.
- Function: void perror (const char *MESSAGE)
This function prints an error message to the stream `stderr'; see
Note: Standard Streams. The orientation of `stderr' is not
changed.
If you call `perror' with a MESSAGE that is either a null pointer
or an empty string, `perror' just prints the error message
corresponding to `errno', adding a trailing newline.
If you supply a non-null MESSAGE argument, then `perror' prefixes
its output with this string. It adds a colon and a space
character to separate the MESSAGE from the error string
corresponding to `errno'.
The function `perror' is declared in `stdio.h'.
`strerror' and `perror' produce the exact same message for any given
error code; the precise text varies from system to system. On the GNU
system, the messages are fairly short; there are no multi-line messages
or embedded newlines. Each error message begins with a capital letter
and does not include any terminating punctuation.
*Compatibility Note:* The `strerror' function was introduced in
ISO C89. Many older C systems do not support this function yet.
Many programs that don't read input from the terminal are designed to
exit if any system call fails. By convention, the error message from
such a program should start with the program's name, sans directories.
You can find that name in the variable `program_invocation_short_name';
the full file name is stored the variable `program_invocation_name'.
- Variable: char * program_invocation_name
This variable's value is the name that was used to invoke the
program running in the current process. It is the same as
`argv[0]'. Note that this is not necessarily a useful file name;
often it contains no directory names. Note: Program Arguments.
- Variable: char * program_invocation_short_name
This variable's value is the name that was used to invoke the
program running in the current process, with directory names
removed. (That is to say, it is the same as
`program_invocation_name' minus everything up to the last slash,
if any.)
The library initialization code sets up both of these variables
before calling `main'.
*Portability Note:* These two variables are GNU extensions. If you
want your program to work with non-GNU libraries, you must save the
value of `argv[0]' in `main', and then strip off the directory names
yourself. We added these extensions to make it possible to write
self-contained error-reporting subroutines that require no explicit
cooperation from `main'.
Here is an example showing how to handle failure to open a file
correctly. The function `open_sesame' tries to open the named file for
reading and returns a stream if successful. The `fopen' library
function returns a null pointer if it couldn't open the file for some
reason. In that situation, `open_sesame' constructs an appropriate
error message using the `strerror' function, and terminates the
program. If we were going to make some other library calls before
passing the error code to `strerror', we'd have to save it in a local
variable instead, because those other library functions might overwrite
`errno' in the meantime.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE *
open_sesame (char *name)
{
FILE *stream;
errno = 0;
stream = fopen (name, "r");
if (stream == NULL)
{
fprintf (stderr, "%s: Couldn't open file %s; %s\n",
program_invocation_short_name, name, strerror (errno));
exit (EXIT_FAILURE);
}
else
return stream;
}
Using `perror' has the advantage that the function is portable and
available on all systems implementing ISO C. But often the text
`perror' generates is not what is wanted and there is no way to extend
or change what `perror' does. The GNU coding standard, for instance,
requires error messages to be preceded by the program name and programs
which read some input files should should provide information about the
input file name and the line number in case an error is encountered
while reading the file. For these occasions there are two functions
available which are widely used throughout the GNU project. These
functions are declared in `error.h'.
- Function: void error (int STATUS, int ERRNUM, const char *FORMAT,
...)
The `error' function can be used to report general problems during
program execution. The FORMAT argument is a format string just
like those given to the `printf' family of functions. The
arguments required for the format can follow the FORMAT parameter.
Just like `perror', `error' also can report an error code in
textual form. But unlike `perror' the error value is explicitly
passed to the function in the ERRNUM parameter. This elimintates
the problem mentioned above that the error reporting function must
be called immediately after the function causing the error since
otherwise `errno' might have a different value.
The `error' prints first the program name. If the application
defined a global variable `error_print_progname' and points it to a
function this function will be called to print the program name.
Otherwise the string from the global variable `program_name' is
used. The program name is followed by a colon and a space which
in turn is followed by the output produced by the format string.
If the ERRNUM parameter is non-zero the format string output is
followed by a colon and a space, followed by the error message for
the error code ERRNUM. In any case is the output terminated with
a newline.
The output is directed to the `stderr' stream. If the `stderr'
wasn't oriented before the call it will be narrow-oriented
afterwards.
The function will return unless the STATUS parameter has a
non-zero value. In this case the function will call `exit' with
the STATUS value for its parameter and therefore never return. If
`error' returns the global variable `error_message_count' is
incremented by one to keep track of the number of errors reported.
- Function: void error_at_line (int STATUS, int ERRNUM, const char
*FNAME, unsigned int LINENO, const char *FORMAT, ...)
The `error_at_line' function is very similar to the `error'
function. The only difference are the additional parameters FNAME
and LINENO. The handling of the other parameters is identical to
that of `error' except that between the program name and the string
generated by the format string additional text is inserted.
Directly following the program name a colon, followed by the file
name pointer to by FNAME, another colon, and a value of LINENO is
printed.
This additional output of course is meant to be used to locate an
error in an input file (like a programming language source code
file etc).
If the global variable `error_one_per_line' is set to a non-zero
value `error_at_line' will avoid printing consecutive messages for
the same file anem line. Repetition which are not directly
following each other are not caught.
Just like `error' this function only returned if STATUS is zero.
Otherwise `exit' is called with the non-zero value. If `error'
returns the global variable `error_message_count' is incremented
by one to keep track of the number of errors reported.
As mentioned above the `error' and `error_at_line' functions can be
customized by defining a variable named `error_print_progname'.
- Variable: void (* error_print_progname ) (void)
If the `error_print_progname' variable is defined to a non-zero
value the function pointed to is called by `error' or
`error_at_line'. It is expected to print the program name or do
something similarly useful.
The function is expected to be print to the `stderr' stream and
must be able to handle whatever orientation the stream has.
The variable is global and shared by all threads.
- Variable: unsigned int error_message_count
The `error_message_count' variable is incremented whenever one of
the functions `error' or `error_at_line' returns. The variable is
global and shared by all threads.
- Variable: int error_one_per_line
The `error_one_per_line' variable influences only `error_at_line'.
Normally the `error_at_line' function creates output for every
invocation. If `error_one_per_line' is set to a non-zero value
`error_at_line' keeps track of the last file name and line number
for which an error was reported and avoid directly following
messages for the same file and line. This variable is global and
shared by all threads.
A program which read some input file and reports errors in it could look
like this:
{
char *line = NULL;
size_t len = 0;
unsigned int lineno = 0;
error_message_count = 0;
while (! feof_unlocked (fp))
{
ssize_t n = getline (&line, &len, fp);
if (n <= 0)
/* End of file or error. */
break;
++lineno;
/* Process the line. */
...
if (Detect error in line)
error_at_line (0, errval, filename, lineno,
"some error text %s", some_variable);
}
if (error_message_count != 0)
error (EXIT_FAILURE, 0, "%u errors found", error_message_count);
}
`error' and `error_at_line' are clearly the functions of choice and
enable the programmer to write applications which follow the GNU coding
standard. The GNU libc additionally contains functions which are used
in BSD for the same purpose. These functions are declared in `err.h'.
It is generally advised to not use these functions. They are included
only for compatibility.
- Function: void warn (const char *FORMAT, ...)
The `warn' function is roughly equivalent to a call like
error (0, errno, format, the parameters)
except that the global variables `error' respects and modifies are
not used.
- Function: void vwarn (const char *FORMAT, va_list)
The `vwarn' function is just like `warn' except that the
parameters for the handling of the format string FORMAT are passed
in as an value of type `va_list'.
- Function: void warnx (const char *FORMAT, ...)
The `warnx' function is roughly equivalent to a call like
error (0, 0, format, the parameters)
except that the global variables `error' respects and modifies are
not used. The difference to `warn' is that no error number string
is printed.
- Function: void vwarnx (const char *FORMAT, va_list)
The `vwarnx' function is just like `warnx' except that the
parameters for the handling of the format string FORMAT are passed
in as an value of type `va_list'.
- Function: void err (int STATUS, const char *FORMAT, ...)
The `err' function is roughly equivalent to a call like
error (status, errno, format, the parameters)
except that the global variables `error' respects and modifies are
not used and that the program is exited even if STATUS is zero.
- Function: void verr (int STATUS, const char *FORMAT, va_list)
The `verr' function is just like `err' except that the parameters
for the handling of the format string FORMAT are passed in as an
value of type `va_list'.
- Function: void errx (int STATUS, const char *FORMAT, ...)
The `errx' function is roughly equivalent to a call like
error (status, 0, format, the parameters)
except that the global variables `error' respects and modifies are
not used and that the program is exited even if STATUS is zero.
The difference to `err' is that no error number string is printed.
- Function: void verrx (int STATUS, const char *FORMAT, va_list)
The `verrx' function is just like `errx' except that the
parameters for the handling of the format string FORMAT are passed
in as an value of type `va_list'.
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