получить строку сообщения об ошибке (get error message string)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
strerror, strerror_l, strerror_r — get error message string
Синопсис (Synopsis)
#include <string.h>
char *strerror(int errnum);
char *strerror_l(int errnum, locale_t locale);
int strerror_r(int errnum, char *strerrbuf, size_t buflen);
Описание (Description)
For strerror(): The functionality described on this reference
page is aligned with the ISO C standard. Any conflict between the
requirements described here and the ISO C standard is
unintentional. This volume of POSIX.1‐2017 defers to the ISO C
standard.
The strerror() function shall map the error number in errnum to a
locale-dependent error message string and shall return a pointer
to it. Typically, the values for errnum come from errno, but
strerror() shall map any value of type int to a message.
The application shall not modify the string returned. The
returned string pointer might be invalidated or the string
content might be overwritten by a subsequent call to strerror(),
or by a subsequent call to strerror_l() in the same thread. The
returned pointer and the string content might also be invalidated
if the calling thread is terminated.
The string may be overwritten by a subsequent call to
strerror_l() in the same thread.
The contents of the error message strings returned by strerror()
should be determined by the setting of the LC_MESSAGES category
in the current locale.
The implementation shall behave as if no function defined in this
volume of POSIX.1‐2017 calls strerror().
The strerror() and strerror_l() functions shall not change the
setting of errno if successful.
Since no return value is reserved to indicate an error of
strerror(), an application wishing to check for error situations
should set errno to 0, then call strerror(), then check errno.
Similarly, since strerror_l() is required to return a string for
some errors, an application wishing to check for all error
situations should set errno to 0, then call strerror_l(), then
check errno.
The strerror() function need not be thread-safe.
The strerror_l() function shall map the error number in errnum to
a locale-dependent error message string in the locale represented
by locale and shall return a pointer to it.
The strerror_r() function shall map the error number in errnum to
a locale-dependent error message string and shall return the
string in the buffer pointed to by strerrbuf, with length buflen.
If the value of errnum is a valid error number, the message
string shall indicate what error occurred; if the value of errnum
is zero, the message string shall either be an empty string or
indicate that no error occurred; otherwise, if these functions
complete successfully, the message string shall indicate that an
unknown error occurred.
The behavior is undefined if the locale argument to strerror_l()
is the special locale object LC_GLOBAL_LOCALE or is not a valid
locale object handle.
Возвращаемое значение (Return value)
Upon completion, whether successful or not, strerror() shall
return a pointer to the generated message string. On error errno
may be set, but no return value is reserved to indicate an error.
Upon successful completion, strerror_l() shall return a pointer
to the generated message string. If errnum is not a valid error
number, errno may be set to [EINVAL], but a pointer to a message
string shall still be returned. If any other error occurs, errno
shall be set to indicate the error and a null pointer shall be
returned.
Upon successful completion, strerror_r() shall return 0.
Otherwise, an error number shall be returned to indicate the
error.
Ошибки (Error)
These functions may fail if:
EINVAL The value of errnum is neither a valid error number nor
zero.
The strerror_r() function may fail if:
ERANGE Insufficient storage was supplied via strerrbuf and buflen
to contain the generated message string.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
Historically in some implementations, calls to perror() would
overwrite the string that the pointer returned by strerror()
points to. Such implementations did not conform to the ISO C
standard; however, application developers should be aware of this
behavior if they wish their applications to be portable to such
implementations.
Обоснование (Rationale)
The strerror_l() function is required to be thread-safe, thereby
eliminating the need for an equivalent to the strerror_r()
function.
Earlier versions of this standard did not explicitly require that
the error message strings returned by strerror() and strerror_r()
provide any information about the error. This version of the
standard requires a meaningful message for any successful
completion.
Since no return value is reserved to indicate a strerror() error,
but all calls (whether successful or not) must return a pointer
to a message string, on error strerror() can return a pointer to
an empty string or a pointer to a meaningful string that can be
printed.
Note that the [EINVAL] error condition is a may fail error. If an
invalid error number is supplied as the value of errnum,
applications should be prepared to handle any of the following:
1. Error (with no meaningful message): errno is set to [EINVAL],
the return value is a pointer to an empty string.
2. Successful completion: errno is unchanged and the return
value points to a string like "unknownerror" or
"errornumberxxx" (where xxx is the value of errnum).
3. Combination of #1 and #2: errno is set to [EINVAL] and the
return value points to a string like "unknownerror" or
"errornumberxxx" (where xxx is the value of errnum). Since
applications frequently use the return value of strerror() as
an argument to functions like fprintf() (without checking the
return value) and since applications have no way to parse an
error message string to determine whether errnum represents a
valid error number, implementations are encouraged to
implement #3. Similarly, implementations are encouraged to
have strerror_r() return [EINVAL] and put a string like
"unknownerror" or "errornumberxxx" in the buffer pointed to
by strerrbuf when the value of errnum is not a valid error
number.
Some applications rely on being able to set errno to 0 before
calling a function with no reserved value to indicate an error,
then call strerror(errno) afterwards to detect whether an error
occurred (because errno changed) or to indicate success (because
errno remained zero). This usage pattern requires that
strerror(0) succeed with useful results. Previous versions of the
standard did not specify the behavior when errnum is zero.
Будущие направления (Future directions)
None.
Смотри также (See also)
perror(3p)
The Base Definitions volume of POSIX.1‐2017, string.h(0p)
