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.TH STRERROR "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH 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.

.SH NAME

strerror,

strerror_l,

strerror_r

\(em get error message string

.SH SYNOPSIS

.LP

.nf

#include <string.h>

.P

char *strerror(int \fIerrnum\fR);

char *strerror_l(int \fIerrnum\fR, locale_t \fIlocale\fR);

int strerror_r(int \fIerrnum\fR, char *\fIstrerrbuf\fR, size_t \fIbuflen\fR);

.fi

.SH DESCRIPTION

For

\fIstrerror\fR():

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\(hy2008 defers to the ISO\ C standard.

.P

The

\fIstrerror\fR()

function shall map the error number in

.IR errnum

to a locale-dependent error message string and shall return a pointer

to it. Typically, the values for

.IR errnum

come from

.IR errno ,

but

\fIstrerror\fR()

shall map any value of type

.BR int

to a message.

.P

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

\fIstrerror\fR(),

or by a subsequent call to

\fIstrerror_l\fR()

in the same thread.

.P

The string may be overwritten by a subsequent call to

\fIstrerror_l\fR()

in the same thread.

.P

The contents of the error message strings returned by

\fIstrerror\fR()

should be determined by the setting of the

.IR LC_MESSAGES

category in the current locale.

.P

The implementation shall behave as if no function defined in this volume of POSIX.1\(hy2008

calls

\fIstrerror\fR().

.P

The

\fIstrerror\fR()

and

\fIstrerror_l\fR()

functions shall not change the setting of

.IR errno

if successful.

.P

Since no return value is reserved to indicate an error of

\fIstrerror\fR(),

an application wishing to check for error situations should set

.IR errno

to 0, then call

\fIstrerror\fR(),

then check

.IR errno .

Similarly, since

\fIstrerror_l\fR()

is required to return a string for some errors, an application wishing

to check for all error situations should set

.IR errno

to 0, then call

\fIstrerror_l\fR(),

then check

.IR errno .

.P

The

\fIstrerror\fR()

function need not be thread-safe.

.P

The

\fIstrerror_l\fR()

function shall map the error number in

.IR errnum

to a locale-dependent error message string in the locale represented by

.IR locale

and shall return a pointer to it.

.P

The

\fIstrerror_r\fR()

function shall map the error number in

.IR errnum

to a locale-dependent error message string and shall return the string

in the buffer pointed to by

.IR strerrbuf ,

with length

.IR buflen .

.P

If the value of

.IR errnum

is a valid error number, the message string shall indicate what error

occurred; if the value of

.IR 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.

.P

The behavior is undefined if the

.IR locale

argument to

\fIstrerror_l\fR()

is the special locale object LC_GLOBAL_LOCALE or is not a valid locale

object handle.

.SH "RETURN VALUE"

Upon completion, whether successful or not,

\fIstrerror\fR()

shall return a pointer to the generated message string.

On error

.IR errno

may be set, but no return value is reserved to indicate an error.

.P

Upon successful completion,

\fIstrerror_l\fR()

shall return a pointer to the generated message string. If

.IR errnum

is not a valid error number,

.IR errno

may be set to

.BR [EINVAL] ,

but a pointer to a message string shall still be returned. If any other

error occurs,

.IR errno

shall be set to indicate the error and a null pointer shall be

returned.

.P

Upon successful completion,

\fIstrerror_r\fR()

shall return 0. Otherwise, an error number shall be returned to

indicate the error.

.SH ERRORS

These functions may fail if:

.TP

.BR EINVAL

The value of

.IR errnum

is neither a valid error number nor zero.

.P

The

\fIstrerror_r\fR()

function may fail if:

.TP

.BR ERANGE

Insufficient storage was supplied via

.IR strerrbuf

and

.IR buflen

to contain the generated message string.

.LP

.IR "The following sections are informative."

.SH EXAMPLES

None.

.SH "APPLICATION USAGE"

Historically in some implementations, calls to

\fIperror\fR()

would overwrite the string that the pointer returned by

\fIstrerror\fR()

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.

.SH RATIONALE

The

\fIstrerror_l\fR()

function is required to be thread-safe, thereby eliminating the

need for an equivalent to the

\fIstrerror_r\fR()

function.

.P

Earlier versions of this standard did not explicitly require that the

error message strings returned by

\fIstrerror\fR()

and

\fIstrerror_r\fR()

provide any information about the error. This version of the standard

requires a meaningful message for any successful completion.

.P

Since no return value is reserved to indicate a

\fIstrerror\fR()

error, but all calls (whether successful or not) must return a pointer

to a message string, on error

\fIstrerror\fR()

can return a pointer to an empty string or a pointer to a meaningful

string that can be printed.

.P

Note that the

.BR [EINVAL] 

error condition is a may fail error. If an invalid error number is

supplied as the value of

.IR errnum ,

applications should be prepared to handle any of the following:

.IP " 1." 4

Error (with no meaningful message):

.IR errno

is set to

.BR [EINVAL] ,

the return value is a pointer to an empty string.

.IP " 2." 4

Successful completion:

.IR errno

is unchanged and the return value points to a string like

.BR \(dqunknown error\(dq 

or

.BR \(dqerror number xxx\(dq 

(where

.IR xxx

is the value of

.IR errnum ).

.IP " 3." 4

Combination of #1 and #2:

.IR errno

is set to

.BR [EINVAL] 

and the return value points to a string like

.BR \(dqunknown error\(dq 

or

.BR \(dqerror number xxx\(dq 

(where

.IR xxx

is the value of

.IR errnum ).

Since applications frequently use the return value of

\fIstrerror\fR()

as an argument to functions like

\fIfprintf\fR()

(without checking the return value) and since applications have no way

to parse an error message string to determine whether

.IR errnum

represents a valid error number, implementations are encouraged to

implement #3. Similarly, implementations are encouraged to have

\fIstrerror_r\fR()

return

.BR [EINVAL] 

and put a string like

.BR \(dqunknown error\(dq 

or

.BR \(dqerror number xxx\(dq 

in the buffer pointed to by

.IR strerrbuf

when the value of

.IR errnum

is not a valid error number.

.P

Some applications rely on being able to set

.IR errno

to 0 before calling a function with no reserved value to indicate an

error, then call

.IR strerror ( errno )

afterwards to detect whether an error occurred (because

.IR errno

changed) or to indicate success (because

.IR errno

remained zero). This usage pattern requires that

.IR strerror (0)

succeed with useful results. Previous versions of the standard did not

specify the behavior when

.IR errnum

is zero.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "\fIperror\fR\^(\|)"

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "\fB<string.h>\fP"

.SH COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form

from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology

-- Portable Operating System Interface (POSIX), The Open Group Base

Specifications Issue 7, Copyright (C) 2013 by the Institute of

Electrical and Electronics Engineers, Inc and The Open Group.

(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the

event of any discrepancy between this version and the original IEEE and

The Open Group Standard, the original IEEE and The Open Group Standard

is the referee document. The original Standard can be obtained online at

http://www.unix.org/online.html .

Any typographical or formatting errors that appear

in this page are most likely

to have been introduced during the conversion of the source files to

man page format. To report such errors, see

https://www.kernel.org/doc/man-pages/reporting_bugs.html .