[v2] manual: Update documentation of strerror and related functions

The current implementation of strerror is thread-safe, but this
has implications for the lifetime of the return string.

Describe the strerror_l function.  Describe both variants of the
strerror_r function.  Mention the lifetime of the returned string
for strerrorname_np and strerrordesc_np.  Clarify that perror
output depends on the current locale.

---
v2: Address review comments from Carlos O'Donell and Shani Leviim.
 manual/errno.texi | 124 ++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 98 insertions(+), 26 deletions(-)


base-commit: 21fbc0a19366f89638a30eef2b53c6d4baafdb88

On 6/26/23 09:00, Florian Weimer via Libc-alpha wrote:
> The current implementation of strerror is thread-safe, but this
> has implications for the lifetime of the return string.
> 
> Describe the strerror_l function.  Describe both variants of the
> strerror_r function.  Mention the lifetime of the returned string
> for strerrorname_np and strerrordesc_np.  Clarify that perror
> output depends on the current locale.

v2 LGTM.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>


> ---
> v2: Address review comments from Carlos O'Donell and Shani Leviim.
>  manual/errno.texi | 124 ++++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 98 insertions(+), 26 deletions(-)
> 
> diff --git a/manual/errno.texi b/manual/errno.texi
> index 28dd871caa..ce07806bf5 100644
> --- a/manual/errno.texi
> +++ b/manual/errno.texi
> @@ -1147,42 +1147,111 @@ name of the program that encountered the error.
>  
>  @deftypefun {char *} strerror (int @var{errnum})
>  @standards{ISO, string.h}
> -@safety{@prelim{}@mtunsafe{@mtasurace{:strerror}}@asunsafe{@ascuheap{} @ascuintl{}}@acunsafe{@acsmem{}}}
> -@c Calls strerror_r with a static buffer allocated with malloc on the
> -@c first use.
> +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @ascuintl{}}@acunsafe{@acsmem{}}}
>  The @code{strerror} function maps the error code (@pxref{Checking for
>  Errors}) specified by the @var{errnum} argument to a descriptive error
> -message string.  The return value is a pointer to this string.
> +message string.  The string is translated according to the current
> +locale.  The return value is a pointer to this string.
>  
>  The value @var{errnum} normally comes from the variable @code{errno}.
>  
>  You should not modify the string returned by @code{strerror}.  Also, if
> -you make subsequent calls to @code{strerror}, the string might be
> -overwritten.  (But it's guaranteed that no library function ever calls
> -@code{strerror} behind your back.)
> +you make subsequent calls to @code{strerror} or @code{strerror_l}, or
> +the thread that obtained the string exits, the returned pointer will be
> +invalidated.
> +
> +As there is no way to restore the previous state after calling
> +@code{strerror}, library code should not call this function because it
> +may interfere with application use of @code{strerror}, invalidating the
> +string pointer before the application is done using it.  Instead,
> +@code{strerror_r}, @code{snprintf} with the @samp{%m} or @samp{%#m}
> +specifiers, @code{strerrorname_np}, or @code{strerrordesc_np} can be
> +used instead.
> +
> +The @code{strerror} function preserves the value of @code{errno} and
> +cannot fail.
>  
>  The function @code{strerror} is declared in @file{string.h}.
>  @end deftypefun
>  
> +@deftypefun {char *} strerror_l (int @var{errnum}, locale_t @var{locale})
> +@standards{POSIX, string.h}
> +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @ascuintl{}}@acunsafe{@acsmem{}}}
> +This function is like @code{strerror}, except that the returned string
> +is translated according to @var{locale} (instead of the current locale
> +used by @code{strerror}).  Note that calling @code{strerror_l}
> +invalidates the pointer returned by @code{strerror} and vice versa.
> +
> +The function @code{strerror_l} is defined by POSIX and is declared in
> +@file{string.h}.

OK.

> +@end deftypefun
> +
>  @deftypefun {char *} strerror_r (int @var{errnum}, char *@var{buf}, size_t @var{n})
>  @standards{GNU, string.h}
>  @safety{@prelim{}@mtsafe{}@asunsafe{@ascuintl{}}@acunsafe{}}
> +The following description is for the GNU variant of the function,
> +used if @code{_GNU_SOURCE} is defined.  @xref{Feature Test Macros}.
> +
>  The @code{strerror_r} function works like @code{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 @var{buf} with the
> -length of @var{n} bytes.
> +returning a pointer to a string that is managed by @theglibc{}, it can
> +use the user supplied buffer starting at @var{buf} for storing the
> +string.
>  
> -At most @var{n} characters are written (including the NUL byte) so it is
> -up to the user to select a buffer large enough.
> +At most @var{n} characters are written (including the NUL byte) to
> +@var{buf}, so it is up to the user to select a buffer large enough.
> +Whether returned pointer points to the @var{buf} array or not depends on
> +the @var{errnum} argument.  If the result string is not stored in
> +@var{buf}, the string will not change for the remaining execution
> +of the program.

OK. Corrected. Looks good.

>  
> -This function should always be used in multi-threaded programs since
> -there is no way to guarantee the string returned by @code{strerror}
> -really belongs to the last call of the current thread.
> +The function @code{strerror_r} as described above is a GNU extension and
> +it is declared in @file{string.h}.  There is a POSIX variant of this
> +function, described next.
> +@end deftypefun
>  
> -The function @code{strerror_r} is a GNU extension and it is declared in
> -@file{string.h}.
> +@deftypefun int strerror_r (int @var{errnum}, char *@var{buf}, size_t @var{n})
> +@standards{POSIX, string.h}
> +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuintl{}}@acunsafe{}}

OK. Corrected, @safety present.

> +
> +This variant of the @code{strerror_r} function is used if a standard is
> +selected that includes @code{strerror_r}, but @code{_GNU_SOURCE} is not
> +defined.  This POSIX variant of the function always writes the error

OK. Corrected, "the function" present.

> +message to the specified buffer @var{buf} of size @var{n} bytes.
> +
> +Upon success, @code{strerror_r} returns 0.  Two more return values are
> +used to indicate failure.
> +
> +@vtable @code
> +@item EINVAL
> +The @var{errnum} argument does not correspond to a known error constant.
> +
> +@item ERANGE
> +The buffer size @var{n} is not large enough to store the entire error message.
> +@end vtable
> +
> +Even if an error is reported, @code{strerror_r} still writes as much of

OK. Corrected, "is reported" present.

> +the error message to the output buffer as possible.  After a call to
> +@code{strerror_r}, the value of @code{errno} is unspecified.
> +
> +If you want to use the always-copying POSIX semantics of
> +@code{strerror_r} in a program that is potentially compiled with
> +@code{_GNU_SOURCE} defined, you can use @code{snprintf} with the
> +@samp{%m} conversion specifier, like this:
> +
> +@smallexample
> +int saved_errno = errno;
> +errno = errnum;
> +int ret = snprintf (buf, n, "%m");
> +errno = saved_errno;
> +if (strerrorname_np (errnum) == NULL)
> +  return EINVAL;
> +if (ret >= n)
> +  return ERANGE:
> +return 0;
> +@end smallexample
> +
> +This function is declared in @file{string.h} if it is declared at all.
> +It is a POSIX extension.
>  @end deftypefun
>  
>  @deftypefun void perror (const char *@var{message})
> @@ -1212,7 +1281,8 @@ The function @code{perror} is declared in @file{stdio.h}.
>  @safety{@mtsafe{}@assafe{}@acsafe{}}
>  This function returns the name describing the error @var{errnum} or
>  @code{NULL} if there is no known constant with this value (e.g "EINVAL"
> -for @code{EINVAL}).
> +for @code{EINVAL}).  The returned string does not change for the
> +remaining execution of the program.
>  
>  @pindex string.h
>  This function is a GNU extension, declared in the header file @file{string.h}.
> @@ -1223,18 +1293,20 @@ This function is a GNU extension, declared in the header file @file{string.h}.
>  @safety{@mtsafe{}@assafe{}@acsafe{}}
>  This function returns the message describing the error @var{errnum} or
>  @code{NULL} if there is no known constant with this value (e.g "Invalid
> -argument" for @code{EINVAL}).  Different than @code{strerror} the returned
> -description is not translated.
> +argument" for @code{EINVAL}).  Different than @code{strerror} the
> +returned description is not translated, and the returned string does not
> +change for the remaining execution of the program.
>  
>  @pindex string.h
>  This function is a GNU extension, declared in the header file @file{string.h}.
>  @end deftypefun
>  
>  @code{strerror} and @code{perror} produce the exact same message for any
> -given error code; the precise text varies from system to system.  With
> -@theglibc{}, 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.
> +given error code under the same locale; the precise text varies from
> +system to system.  With @theglibc{}, 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.
>  
>  @cindex program name
>  @cindex name of running program
> 
> base-commit: 21fbc0a19366f89638a30eef2b53c6d4baafdb88
>