From d15775ce40a2bf2c99b762a2a893b71de516ea98 Mon Sep 17 00:00:00 2001 From: Mattias Andrée Date: Tue, 13 Oct 2015 19:03:57 +0200 Subject: m + info: error reporting facilities from errno.h, string.h and stdio.h MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Signed-off-by: Mattias Andrée --- doc/info/chap/error-reporting.texinfo | 259 +++++++++++++++++++++++++++++++++- 1 file changed, 257 insertions(+), 2 deletions(-) (limited to 'doc') diff --git a/doc/info/chap/error-reporting.texinfo b/doc/info/chap/error-reporting.texinfo index 08571cd..7769015 100644 --- a/doc/info/chap/error-reporting.texinfo +++ b/doc/info/chap/error-reporting.texinfo @@ -16,6 +16,8 @@ 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 @@ -48,6 +50,259 @@ 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. +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{}. + +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}. + +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{}. + +@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{}. 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{} 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{}. 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{} (derived from @sc{BSD} +@code{libc}) and @file{} (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{} (derived from @sc{BSD} +@code{libc}) and @file{} (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{}, 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{}, 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. + +@end table -- cgit v1.2.3-70-g09d2