The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


strerror, strerror_r - get error message string


#include <string.h>

char *strerror(int

[TSF] [Option Start] int strerror_r(int errnum, char *strerrbuf, size_t buflen); [Option End]


For strerror(): [CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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 string pointed to shall not be modified by the application, but may be overwritten by a subsequent call to strerror() [CX] [Option Start]  or perror(). [Option End]

[CX] [Option Start] 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. [Option End]

The implementation shall behave as if no function defined in this volume of IEEE Std 1003.1-2001 calls strerror().

[CX] [Option Start] The strerror() function shall not change the setting of errno if successful.

Since no return value is reserved to indicate an error, an application wishing to check for error situations should set errno to 0, then call strerror(), then check errno.

The strerror() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe. [Option End]

[TSF] [Option Start] 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. [Option End]


Upon successful completion, 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.

[TSF] [Option Start] Upon successful completion, strerror_r() shall return 0. Otherwise, an error number shall be returned to indicate the error. [Option End]


These functions may fail if:

The value of errnum is not a valid error number.

The strerror_r() function may fail if:

[TSF] [Option Start] Insufficient storage was supplied via strerrbuf and buflen to contain the generated message string. [Option End]

The following sections are informative.










perror(), the Base Definitions volume of IEEE Std 1003.1-2001, <string.h>


First released in Issue 3.

Issue 5

The DESCRIPTION is updated to indicate that errno is not changed if the function is successful.

A note indicating that this function need not be reentrant is added to the DESCRIPTION.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The strerror_r() function is added in response to IEEE PASC Interpretation 1003.1c #39.

The strerror_r() function is marked as part of the Thread-Safe Functions option.

End of informative text.

UNIX ® is a registered Trademark of The Open Group.
POSIX ® is a registered Trademark of The IEEE.
[ Main Index | XBD | XCU | XSH | XRAT ]