path: root/doc/info/chap/error-reporting.texinfo

@node Error reporting
@chapter Error reporting

Many functions, especially those involving
system calls, detect and report exceptional
conditions, including error conditions and
interruptions. Exception conditions must often
be checked and handled, often by retrying,
doing something else, or reporting the error
to the user and exit.

Unless otherwise specified, exceptional
conditions are reporting using the facilities
described in this chapter.


@menu
* Error checking::                            Checking whether an exception condition occurred.
* Error codes::                               Values for @code{errno}.
* Error messages::                            Reporting errors to the user.
@end menu



@node Error checking
@section Error checking

@cpindex Checking for errors
@cpindex Errors checking
@cpindex Exception checking
Most functions return a sentinel value when an
exceptional condition occurs, typically a negative
value (actually @code{-1}), a constant such as
@code{EOF}, or @code{NULL}@. This return value only
tells you that an exeception condition has occurred.

@hfindex errno.h
@lvindex errno
By including the header file @file{<errno.h>}, the
modifiable lvalue @code{errno} is made available.
@sc{ISO}@tie{}C specifies that @code{errno} is not
necessarily a variable, but rather just an modifiable
lvalue. In fact, must @code{libc}:s, including
@code{slibc} defines it as a macro that calls a
function and uses the unary @code{*}-operator on the
return.

@code{errno} is often thought of as a variable
declared as @code{int errno}, or @code{volatile int errno}.
But the value is thread-local, and is thus more
like @code{_Thread_local volatile int errno}. Because
of this, the @code{errno} is defined as a macro that
calls a function. Otherwise, it would not be
implementable without @sc{ISO}@tie{}C11.

The value of @code{errno} is describes the error,
if and only if the function returned a value indicating
an error. Some, function can return the sentinel
value both on success and error. Therefore, the value
@code{0} on @code{errno} means that the function
returned after successful completion rather than on
failure. Exceptional conditions are describe by
non-zero values listed in @ref{Error codes}.

The state of @code{errno} is undefined unless otherwise
specified after a succesful completion of a function call.
This means that you, generally, cannot check whether an
error has occurred by checking the value of @code{errno}
rather than the return value.



@node Error codes
@section Error codes

TODO: The list of error codes have not be added to slibc yet.



@node Error messages
@section Error messages

@cpindex Error messages
@cpindex Printing errors
@cpindex Errors, printing
The C standard library provides a several mechanisms
for reporting errors to the user, spread out over
a few header files.

@table @code
@item void perror(const char* prefix)
@fnindex perror
@hfindex stdio.h
The @sc{POSIX}.1-2001 standard
(@code{_POSIX_C_SOURCE >= 200112L}), the
@sc{ISO}@tie{}C89 standard, and 4.3 @sc{BSD}
(@code{_BSD_SOURCE}) added this function
to the header file @file{<stdio.h>}.

This function prints an error message
for the current value on @code{errno},
to stderr. If @code{prefix} is neither
@code{NULL} or an empty string, @code{prefix}
followed by a colon and a blank space,
is prepended to the ouput. The function
will also (always) add a line feed to
the end of the message.

This function does not detect or report
errors. Thus, you should not use this
function if you want to be able to tell
whether the messages was actually printed.
Of course, if it was not, your only option
is to print it to another file descriptor
which you normally do not want to do.

This function is subject to race condition
over @code{stderr}.

Because successful function calls may change
the value on @code{errno}, its value must
be saved if there are function calls between
the failure and the call to @code{perror}.

@lvindex program_invocation_name
@lvindex program_invocation_short_name
It is customary to set @code{prefix} to
a value equivalent to @code{program_invocation_name},
or alternatively @code{program_invocation_short_name}.
Sometiems, the name of the called function is
used, this is to simplify debugging. But the
standard is that programs shall print there
them when reporting an error so that the user
knows that program failed.

TODO: Not implemented.

@item char* strerror(int errnum)
@fnindex strerror
@hfindex string.h
The @sc{POSIX}.1-2001 standard
(@code{_POSIX_C_SOURCE >= 200112L}) and
@sc{ISO}@tie{}C89 standard, defines this
function in the header file @file{<string.h>}.

@vrindex LC_MESSAGES
This function returns a description of
the error code whose number is @code{errnum}.
Description may or may not (it is in @command{slibc})
be localised using the @env{LC_MESSAGES} part of
the current locale.

The returned value must not be modified the
the program, by it may be modified by subsequent
calls to @code{strerror}-functions. If the error
code is not recognised some implementations
will return @code{NULL}, other implementations,
such as @command{slibc}'s, will return a message
that states to number of the error code.

According to @sc{POSIX}, @code{errno} must not
be modified if the call is successful. It may
however set @code{errno} on failure. However,
the @command{slibc} implementation cannot fail.

@command{slibc}'s implementation of this function
is threadsafe, this is however not true for
all implementions.

@item int strerror_r(int errnum, char* buffer, size_t size)
@fnindex strerror_r
@hfindex string.h
The @sc{POSIX}.1-2001 standard added this function
to the header file @file{<string.h>}. It is a
reenterant variant fo @code{strerror}. Instead of
returning the error message, it saves it to
@code{buffer}. The allocation size of @code{buffer}
is pass to the function via the argument @code{size}.
If it is too small, @code{ERANGE} is returned.

The function returns a positive value, that is
appropriate for @code{errno}, on error, rather than
setting the value of @code{errno}. Upon successful
completion, @code{0} is returned.

This function is not available if @code{_PORTABLE_SOURCE}
or @code{_LIBRARY_HEADER} is defined, because it is
conflicting with the @sc{GNU}-version. Without knowning
which version that is used, it is impossible to
check for errors.

This function is also non-portable even if the
@sc{GNU}-version is not used, that is, if
@code{_GNU_SOURCE} is not defined and
@code{(_POSIX_C_SOURCE >= 200112L) || (_XOPEN_SOURCE >= 600)}.
Old versions of @sc{GNU} C Library, returned @code{-1}
on failure and set @code{errno}, instead of returned
a value for @code{errno}.

Whilst @code{strerror_r} is not portable, the most
portable use of @code{strerror_r} is
@example
int r;
r = strerror_r(errnum, buffer, size);
if (r > 0 ? (errno = r) : r)
  goto fail;
@end example

Note that, unlike @code{strerror}, there is no
restriction against @code{strerror_r} to set
@code{errno} on success.

@item char* strerror_r(int errnum, char* buffer, size_t size)
@fnindex strerror_r
@hfindex string.h
This is a @sc{GNU} extension, defined in the header file
@file{<string.h>} and is available, instead of the
@sc{XSI}-compliant version, if @code{_GNU_SOURCE} is
defined or if
@code{(_POSIX_C_SOURCE < 200112L) && (_XOPEN_SOURCE < 600)}.
It is identical to the @sc{XSI}-compliant version,
except, rather than returning an error code, it
will set @code{errno} and return @code{NULL} on
failure. On successful completion, @code{buffer}
is returned.

This function is not available if @code{_PORTABLE_SOURCE}
or @code{_LIBRARY_HEADER} is defined, because it is
conflicting with the @sc{XSI}-compliant version. Without
knowning which version that is used, it is impossible
to check for errors.

@item char* strerror_l(int errnum, locale_t locale)
@fnindex strerror_l
@hfindex string.h
The @sc{POSIX}.1-2008 standard
(@code{_POSIX_C_SOURCE >= 200809L}),
defines this function in the header file
@file{<string.h>}. It is identical to
@code{strerror_l} with two exceptions:
it is thread save, and it uses a specified
locale.

@lvindex LC_GLOBAL_LOCALE
The behaviour of this function is undefined
if @code{locale} is @code{LC_GLOBAL_LOCALE}@.

@item const char* const sys_errlist[]
This variable is defined in the header files
@file{<errno.h>} (derived from @sc{BSD}
@code{libc}) and @file{<stdio.h>} (derived
from @sc{GNU} @code{libc}) if @code{_BSD_SOURCE}
is defined.

This is a list, index by error codes, of
error messages.

This list is deprecated, and all error codes
may not have been added in all @code{libc}
implementations.

TODO: Not implemented.

@item int sys_nerr
This variable is defined in the header files
@file{<errno.h>} (derived from @sc{BSD}
@code{libc}) and @file{<stdio.h>} (derived
from @sc{GNU} @code{libc}) if @code{_BSD_SOURCE}
is defined.

This is the number of error codes that are
defined, including @code{0} (successful completion.)
That is, the value of the error code with
highest value, plus 1.

TODO: Not implemented.

@item char* program_invocation_name
@lvindex program_invocation_name
@hfindex errno.h
@cpindex Process name
This variable is defined in the header file
@file{<errno.h>}, if @code{_GNU_SOURCE} or
@code{_SLIBC_SOURCE} is defined.

This is the name that was used to invoke the
program running in the current process. This
is the value @code{argv[0]} from the @code{main}
function (where @code{argv} is the second parameter.)
If @code{argc} (the first parameter) is zero, this
variable will have the value @code{NULL}@. This is
not necessarily a proper command name. For example,
login shells are usually prefixes with a dash,
for example @code{-bash}, despite that there is
no such command. Often, but not always, this will
not contain directory.

It is recommended for portable programs
to saved the value of @code{argv[0]} from
@code{main} to a global variable (often
named @code{argv0}) instead of using this
variable.

@item char* program_invocation_short_name
@lvindex program_invocation_short_name
@hfindex errno.h
@cpindex Process name
This variable is defined in the header file
@file{<errno.h>}, if @code{_GNU_SOURCE} is
defined. It is similar to @code{program_invocation_name},
but it only contains the basename, that is,
the part after the last slash.

If @code{program_invocation_name} is edited,
@code{program_invocation_short_name} may be
modified too. However, if @code{error_print_progname}
from @file{<error.h>} is not @code{NULL},
it will be used to pritn the process name.

@item void warn(const char* format, ...)
@fnindex warn
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is similar
to @code{printf} except it prints the message
to stderr, and a description of the error
specified by the current value on @code{errno}
is also printed.

It is specified that the last component of
the process name shall be printed, however,
the @command{slibc}'s implementation will
print the full process name, or more precisely,
it prints @code{program_invocation_name}.

@item void vwarn(const char* format, va_list args)
@fnindex vwarn
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{warn} except it uses @code{va_list}
instead of being a variadic function.

@item void warnx(const char* format, ...)
@fnindex warnx
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{warn} except it does not print a
description of the value of @code{errno}.

@item void vwarnx(const char* format, va_list args)
@fnindex vwarnx
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{warnx} except it uses @code{va_list}
instead of being a variadic function.

@item noreturn void err(int status, const char* format, ...)
@fnindex err
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{warn} except will return with the
exit value @code{status & 255}.

@item noreturn void verr(int status, const char* format, va_list args)
@fnindex verr
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{err} except it uses @code{va_list}
instead of being a variadic function.

@item noreturn void errx(int status, const char* format, ...)
@fnindex errx
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{err} except it does not print a
description of the value of @code{errno}.

@item noreturn void verrx(int status, const char* format, va_list args)
@fnindex verrx
@hfindex err.h
This function is a non-standard @sc{BSD}
extension, and requires that @code{_BSD_SOURCE}
is defined. It is made available by including
the header file @file{<err.h>}. It is identical
to @code{errx} except it uses @code{va_list}
instead of being a variadic function.

@item void error(int status, int errnum, const char* format, ...)
@fnindex error
@hfindex error.h
This function is a @sc{GNU}-extension (@code{_GNU_SOURCE})
that is made available by including the header
file @file{<error.h>}.

This function flushes stdout and prints an
error message to stderr. The message begins
with the process name (more precisely:
@code{program_invocation_name}), a colon,
and a blank space. The message, continues with
a desciption of the @code{errno}-value pass
via the argument @code{errnum}. However, if
@code{errnum} is @code{0}, the string described
by @code{format} and the following arguments
are printed --- using @code{fprintf} --- instead
of the description of @code{errnum}.

Unless @code{status} is @code{0}, the process
will exit with the exit value @code{status & 255}.

@item void verror(int status, int errnum, const char* format, va_list args)
@fnindex verror
@hfindex error.h
This function is made available, if both
@code{_GNU_SOURCE} and @code{_SLIBC_SOURCE} are
defined, by including the header file
@file{<error.h>}. It is identical to
@code{error} except it uses @code{va_list}
instead of being a variadic function.

@item void error_at_line(int status, int errnum, const char* filename, unsigned int linenum, const char* format, ...)
@fnindex error_at_line
@hfindex error.h
This function is a @sc{GNU}-extension (@code{_GNU_SOURCE})
that is made available by including the header
file @file{<error.h>}. It is a variant of @code{error}.
It will also print the filename and the line where
there error occured.

A way to use this function is to utilise the
C preprocessor.
@example
#include <error.h>
#include <errno.h>
#define if(...)  if (error_line = __LINE__, __VA_ARGS__)

int main(void)
@{
  int error_line;

  if (something_that_may_fail())
    goto fail;

  return 0;
 fail:
  error_at_line(1, errno, __FILE__, error_line, NULL);
@}
@end example

@item void verror_at_line(int status, int errnum, const char* filename, unsigned int linenum, const char* format, va_list args)
@fnindex verror_at_line
@hfindex error.h
This function is made available, if both
@code{_GNU_SOURCE} and @code{_SLIBC_SOURCE} are
defined, by including the header file
@file{<error.h>}. It is identical to
@code{error_at_line} except it uses @code{va_list}
instead of being a variadic function.

@item volatile unsigned int error_message_count
@lvindex error_message_count
@hfindex error.h
This variable is a @sc{GNU}-extension (@code{_GNU_SOURCE})
that is made available by including the header
file @file{<error.h>}. Counts the number of times
@code{error} (or @code{verror}) and @code{error_at_line}
(@code{verror_at_line}) has returned. This variable
is global and shared by all threads.

@item volatile int error_one_per_line
@lvindex error_message_count
@hfindex error.h
This variable is a @sc{GNU}-extension (@code{_GNU_SOURCE})
that is made available by including the header
file @file{<error.h>}.

If set to non-zero (the default), @code{error_at_line}
and @code{verror_at_line} restrict themself to only
print an error the first time it appears. This is
determined by the uniqueness of the combination of
the values on @code{filename} and @code{linenum}.
However it will forget the line the next time the
function is called from a different line. This
variable is global and shared by all threads. As a
@code{slibc} modification, @code{errnum} is also
remembered, meaning if the error code is different
(zero is considered an error code), the error is
printed even if it occurred on the same line as the
last time.

@item void (*volatile error_print_progname)(void)
@lvindex error_message_count
@hfindex error.h
This variable is a @sc{GNU}-extension (@code{_GNU_SOURCE})
that is made available by including the header
file @file{<error.h>}.

If not @code{NULL}, this function is used instead
of flushing stdout and printing the process name to
stderr, by @code{error} (@code{verror}) and
@code{verror_at_line} (@code{verror_at_line}). This
variable is global and shared by all threads.
@end table

In @code{slibc}, functions defined in @file{<err.h>}
use functions defined in @file{<error.h>}.

@hfindex slibc-error.h
@code{slibc} adds the header file @file{<slibc-error.h>}.
It includes @file{<errno.h>}. It does not require
the @code{_SLIBC_SOURCE} is defined, but it does
require that neither @code{_PORTABLE_SOURCE} nor
@code{_LIBRARY_HEADER}@. @file{<slibc-error.h>} also
uses C preprocessors directives added by the
@sc{ISO}@tie{}C99 standard. Additionally, the
C language extensions @code{(@{ @})} and @code{typeof}.
These extensions are available in @sc{GCC}@.

@table @code
@item FAILABLE_CALL(expression)
@fnindex FAILABLE_CALL
Store the current line, so we know which line
failed, when @code{PRINT_CUSTOM_FAILURE},
@code{PRINT_FAILURE} or @code{slibc_perror} is
called. @code{expression} will be evaluated
exaclty once, and is returned without casting.

Currently, the expression is not stored. However,
if it is found in the future that it is preferable
to have the expression printed on failure, this may
change in the future. Therefore, it is advisable
to only include the function call (and perhaps a
variable assigment) in @code{expression}, and not
the failure check.

Example usage:
@example
int my_function(void)
@{
  size_t n;
  void* array;
  /* ... */
  if (FAILABLE_CALL(array = malloc(n)) == NULL)
    goto fail;
  /* ... */
  return 0;

 fail:
  PRINT_FAILURE(NULL);
  /* ... */
  return errno = 0, -1;
@}
@end example

A more scalable example:
@example
#define fail_if (call_and_check)  \
  do @{ \
    if (FAILABLE_CALL(call_and_check)) \
      goto fail; \
  @} while (0)
/* @w{@xrm{}`@xtt{}try@xrm{}` or simply `@xtt{}t@xrm{}` may be even better.@xtt{}} */

#define fail_unless (call_and_check)  \
  fail_if(!(call_and_check))
/* @w{@xrm{}`@xtt{}f@xrm{}` may be even better.@xtt{}} */

int my_function(void)
@{
  size_t n;
  void* array;
  /* ... */
  fail_if (!(array = malloc(n)));
  /* @w{@xrm{}or perhaps, `@xtt{}fail_unless (array = malloc(n));@xrm{}`@xtt{}} */
  /* ... */
  return 0;

 fail:
  PRINT_FAILURE(NULL);
  /* ... */
  return errno = 0, -1;
@}
@end example

@item FAILURE_CHECK(condition, offset)
@fnindex FAILURE_CHECK
Check whether an error occurred. If an
error occurred, stored the line, for
@code{PRINT_CUSTOM_FAILURE} or
@code{PRINT_FAILURE}, where the error occurred.

@code{condition} should evaluate to non-zero
on and only on error. @code{offset} is the
number of lines above the current line
that was the line that failed, if @code{condition}
evaluates to non-zero. @code{!!condition} is returned.

Example usage:
@example
int my_function(void)
@{
  char* buf;
  size_t bufsize;
  ssize_t r;
  int fd;
  /* ... */
  for (;;)
    @{
      r = read(fd, buf, bufsize);
      if (FAILURE_CHECK(r < 0, 1))
        @{
          if (errno != EINTR)
            /* @w{@xrm{}Or `@xtt{}FAILURE_CHECK(errno != EINTR, 3)@xrm{}`.@xtt{}} */
            goto fail;
          continue;
        @}
      /* ... */
    @}
  /* ... */
  return 0;

 fail:
  PRINT_FAILURE(NULL);
  /* ... */
  return errno = 0, -1;
@}
@end example

@item PRINT_FAILURE(...)
@fnindex PRINT_FAILURE
This macro print an error message describing the
error, and where the error happened. Optionally
with detailed description of the action that failed.

This function must be called in the same file and
function as the the expansion of @code{FAILABLE_CALL},
@code{FAILURE_CHECK}, or @code{slibc_perror}. It is
recommended that this is done all the way down the
call-stack to @code{main}. This is however problematic
(although it is possible) if you do not want the
process to exit.

Example of recursion:
@example
static int b(void)
@{
  if (FAILABLE_CALL(some_function()) < 0)
    goto fail;
  return 0;
 fail:
  return PRINT_FAILURE(NULL), -1;
@}
static int a(void)
@{
  if (FAILABLE_CALL(b()) < 0)
    goto fail;
  return 0;
 fail:
  return PRINT_FAILURE(NULL), -1;
@}
int main(void)
@{
  if (FAILABLE_CALL(a()) < 0)
    goto fail;
  return 0;
 fail:
  return PRINT_FAILURE(NULL), 1;
@}
@end example

This macro will set @code{errno} to zero. If you
have statements after the call to this function that
may modify @code{errno}, be sure to set @code{errno}
to zero before returning. A good way to do this is
to have the return statement look like
@code{return errno = 0, -1}.

The arguments for this macro should be a
@code{printf}-formatting-string followed by
formatting-arguments used to describe what action
failed. If the first argument is @code{NULL}, this
is not printed and only the location of the error
and @code{errno} is used to describe the error.

@item PRINT_CUSTOM_FAILURE(error_code, error_string, ...)
@fnindex PRINT_CUSTOM_FAILURE
This macro is is a generalisation of
@code{PRINT_FAILURE}@. It is useful if the error
is not described with @code{errno} and @code{strerror}.
It adds two parameters at the beginning:

@table @code
@item error_code
The error code. This must be a modifiable lvalue,
as it will be modified to zero to indicate that the
error string has already been printed. Thus, on the
original error location this value must not be zero.

@item error_string
Textual representation of the error. If @code{errno}
indicates the error, this should usually be
@code{strerror(errno)}.
@end table

Usage example:
@example
PRINT_CUSTOM_FAILURE(r, gai_strerror(r), 
  _("fail to connect to server: %s"), host)
@end example

@item void slibc_perror(const char* progname, const char* filename, int linenum, const char* function, int* error_code, const char* error_string, const char* format, ...)
@fnindex slibc_perror
This function print a description of an error, and
where the error occurred. This function is capable
of printed the full call-stack.

It is not recommended to used this function directly.
Use the macros in @file{<slibc-error.h>} instead.

This function has a number of argument:
@table @code
@item progname
The name of the program. @code{`NULL} or an empty
string to use @code{program_invocation_name}.

@item filename
The source code file where the error occurred.

@item linenum
The line in the source code where the error occurred.

@item function
The function in the source code where the
error occurred.

@item error_code
Pointer the variable that stores the error
code, will zeroed.

@item error_string
Textual description of the error code. @code{NULL}
if @code{strerror} should be used. This parameter
is unused if @code{error_code} is zero.

@item format
Formatting-string for a description of the action
that failed. @code{NULL} if no description other
than the location and @code{error_string} should
be printed. It is followed by the formatting-arguments.
@end table

@end table