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Colin Robertson
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Address thread-safety issue 1827
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‎docs/c-runtime-library/reference/strerror-strerror-wcserror-wcserror.md‎

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title: "strerror, _strerror, _wcserror, __wcserror"
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ms.date: "11/04/2016"
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description: "Describes the Microsoft C Runtime Library (CRT) functions strerror, _strerror, _wcserror, and __wcserror."
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ms.date: "01/07/2020"
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api_name: ["strerror", "_strerror", "_wcserror", "__wcserror"]
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api_location: ["msvcrt.dll", "msvcr80.dll", "msvcr90.dll", "msvcr100.dll", "msvcr100_clr0400.dll", "msvcr110.dll", "msvcr110_clr0400.dll", "msvcr120.dll", "msvcr120_clr0400.dll", "ucrtbase.dll", "api-ms-win-crt-runtime-l1-1-0.dll"]
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api_type: ["DLLExport"]
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### Parameters
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*errnum*<br/>
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*errnum*\
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Error number.
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*strErrMsg*<br/>
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*strErrMsg*\
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User-supplied message.
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## Return Value
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## Return value
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All of these functions return a pointer to the error-message string. Subsequent calls can overwrite the string.
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All of these functions return a pointer to an error-message string, in a thread-local storage buffer owned by the runtime. Later calls on the same thread can overwrite this string.
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## Remarks
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The **strerror** function maps *errnum* to an error-message string and returns a pointer to the string. Neither **strerror** nor **_strerror** actually prints the message: For that, you have to call an output function such as [fprintf](fprintf-fprintf-l-fwprintf-fwprintf-l.md):
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The **strerror** function maps *errnum* to an error-message string and returns a pointer to the string. The **strerror** and **_strerror** functions don't actually print the message. To print, call an output function such as [fprintf](fprintf-fprintf-l-fwprintf-fwprintf-l.md):
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```C
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if (( _access( "datafile",2 )) == -1 )
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if (( _access( "datafile", 2 )) == -1 )
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fprintf( stderr, _strerror(NULL) );
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```
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If *strErrMsg* is passed as **NULL**, **_strerror** returns a pointer to a string that contains the system error message for the last library call that produced an error. The error-message string is terminated by the newline character ('\n'). If *strErrMsg* is not equal to **NULL**, then **_strerror** returns a pointer to a string that contains (in order) your string message, a colon, a space, the system error message for the last library call that produces an error, and a newline character. Your string message can be, at most, 94 characters long.
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If *strErrMsg* is passed as **NULL**, **_strerror** returns a pointer to a string. It contains the system error message for the last library call that produced an error. The error-message string is terminated by the newline character ('\n'). When *strErrMsg* isn't **NULL**, the string contains, in order: your *strErrMsg* string, a colon, a space, the system error message, and a newline character. Your string message can be, at most, 94 characters long, in either narrow (**_strerror**) or wide (**__wcserror**) characters.
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The actual error number for **_strerror** is stored in the variable [errno](../../c-runtime-library/errno-doserrno-sys-errlist-and-sys-nerr.md). To produce accurate results, call **_strerror** immediately after a library routine returns with an error. Otherwise, subsequent calls to **strerror** or **_strerror** can overwrite the **errno** value.
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The actual error number for **_strerror** is stored in the variable [errno](../../c-runtime-library/errno-doserrno-sys-errlist-and-sys-nerr.md). To produce accurate results, call **_strerror** immediately after a library routine returns an error. Otherwise, later calls to library routines may overwrite the **errno** value.
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**_wcserror** and **__wcserror** are wide-character versions of **strerror** and **_strerror**, respectively.
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**_strerror**, **_wcserror**, and **__wcserror** are not part of the ANSI definition; they are Microsoft extensions and we recommend that you do not use them where you want portable code. For ANSI compatibility, use **strerror** instead.
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**_strerror**, **_wcserror**, and **__wcserror** are Microsoft-specific, not part of the Standard C library. We don't recommend you use them where you want portable code. For Standard C compatibility, use **strerror** instead.
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To get error strings, we recommend **strerror** or **_wcserror** instead of the deprecated macros [_sys_errlist](../../c-runtime-library/errno-doserrno-sys-errlist-and-sys-nerr.md) and [_sys_nerr](../../c-runtime-library/errno-doserrno-sys-errlist-and-sys-nerr.md) and the deprecated internal functions **__sys_errlist** and **__sys_nerr**.
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### Generic-Text Routine Mappings
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### Generic-text routine mappings
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|TCHAR.H routine|_UNICODE & _MBCS not defined|_MBCS defined|_UNICODE defined|
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|---------------------|------------------------------------|--------------------|-----------------------|
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## See also
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[String Manipulation](../../c-runtime-library/string-manipulation-crt.md)<br/>
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[clearerr](clearerr.md)<br/>
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[ferror](ferror.md)<br/>
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[perror, _wperror](perror-wperror.md)<br/>
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[String manipulation](../../c-runtime-library/string-manipulation-crt.md)\
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[clearerr](clearerr.md)\
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[ferror](ferror.md)\
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[perror, _wperror](perror-wperror.md)

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