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manual: Update documentation of strerror and related functions

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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.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
This commit is contained in:
Florian Weimer 2023-07-03 12:36:56 +02:00
parent 9651b06940
commit e18c293af0
1 changed files with 98 additions and 26 deletions
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124
manual/errno.texi
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@ -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}.
@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.
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{}}
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
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
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