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.TH READLINK "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

readlink, readlinkat

\(em read the contents of a symbolic link

.SH SYNOPSIS

.LP

.nf

#include <unistd.h>

.P

ssize_t readlink(const char *restrict \fIpath\fP, char *restrict \fIbuf\fP,

    size_t \fIbufsize\fP);

ssize_t readlinkat(int \fIfd\fP, const char *restrict \fIpath\fP,

    char *restrict \fIbuf\fP, size_t \fIbufsize\fP);

.fi

.SH DESCRIPTION

The

\fIreadlink\fR()

function shall place the contents of the symbolic link referred to by

.IR path

in the buffer

.IR buf

which has size

.IR bufsize .

If the number of bytes in the symbolic link is less than

.IR bufsize ,

the contents of the remainder of

.IR buf

are unspecified. If the

.IR buf

argument is not large enough to contain the link content, the first

.IR bufsize

bytes shall be placed in

.IR buf .

.P

If the value of

.IR bufsize

is greater than

{SSIZE_MAX},

the result is implementation-defined.

.P

Upon successful completion,

\fIreadlink\fR()

shall mark for update the last data access timestamp of the symbolic

link.

.P

The

\fIreadlinkat\fR()

function shall be equivalent to the

\fIreadlink\fR()

function except in the case where

.IR path

specifies a relative path. In this case the symbolic link whose content

is read is relative to the directory associated with the file

descriptor

.IR fd

instead of the current working directory. If the file descriptor was

opened without O_SEARCH, the function shall check whether directory

searches are permitted using the current permissions of the directory

underlying the file descriptor. If the file descriptor was opened with

O_SEARCH, the function shall not perform the check.

.P

If

\fIreadlinkat\fR()

is passed the special value AT_FDCWD in the

.IR fd

parameter, the current working directory shall be used and the behavior

shall be identical to a call to

\fIreadlink\fR().

.SH "RETURN VALUE"

Upon successful completion, these functions shall return the count of

bytes placed in the buffer. Otherwise, these functions shall return a

value of \(mi1, leave the buffer unchanged, and set

.IR errno

to indicate the error.

.SH ERRORS

These functions shall fail if:

.TP

.BR EACCES

Search permission is denied for a component of the path prefix of

.IR path .

.TP

.BR EINVAL

The

.IR path

argument names a file that is not a symbolic link.

.TP

.BR EIO

An I/O error occurred while reading from the file system.

.TP

.BR ELOOP

A loop exists in symbolic links encountered during resolution of the

.IR path

argument.

.TP

.BR ENAMETOOLONG

.br

The length of a component of a pathname is longer than

{NAME_MAX}.

.TP

.BR ENOENT

A component of

.IR path

does not name an existing file or

.IR path

is an empty string.

.TP

.BR ENOTDIR

A component of the path prefix names an existing file that is neither

a directory nor a symbolic link to a directory, or the

.IR path

argument contains at least one non-\c

<slash>

character and ends with one or more trailing

<slash>

characters and the last pathname component names an existing file that

is neither a directory nor a symbolic link to a directory.

.br

.P

The

\fIreadlinkat\fR()

function shall fail if:

.TP

.BR EACCES

.IR fd

was not opened with O_SEARCH and the permissions of the directory

underlying

.IR fd

do not permit directory searches.

.TP

.BR EBADF

The

.IR path

argument does not specify an absolute path and the

.IR fd

argument is neither AT_FDCWD nor a valid file descriptor open for reading

or searching.

.TP

.BR ENOTDIR

The

.IR path

argument is not an absolute path and

.IR fd

is a file descriptor associated with a non-directory file.

.P

These functions may fail if:

.TP

.BR ELOOP

More than

{SYMLOOP_MAX}

symbolic links were encountered during resolution of the

.IR path

argument.

.TP

.BR ENAMETOOLONG

.br

The length of a pathname exceeds

{PATH_MAX},

or pathname resolution of a symbolic link produced an intermediate

result with a length that exceeds

{PATH_MAX}.

.LP

.IR "The following sections are informative."

.SH EXAMPLES

.SS "Reading the Name of a Symbolic Link"

.P

The following example shows how to read the name of a symbolic link

named

.BR /modules/pass1 .

.sp

.RS 4

.nf

\fB

#include <unistd.h>

.P

char buf[1024];

ssize_t len;

\&...

if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1)

    buf[len] = '\e0';

.fi \fR

.P

.RE

.SH "APPLICATION USAGE"

Conforming applications should not assume that the returned contents of

the symbolic link are null-terminated.

.SH RATIONALE

The type associated with

.IR bufsiz

is a

.BR size_t

in order to be consistent with both the ISO\ C standard and the definition of

\fIread\fR().

The behavior specified for

\fIreadlink\fR()

when

.IR bufsiz

is zero represents historical practice. For this case, the standard

developers considered a change whereby

\fIreadlink\fR()

would return the number of non-null bytes contained in the symbolic

link with the buffer

.IR buf

remaining unchanged; however, since the

.BR stat

structure member

.IR st_size

value can be used to determine the size of buffer necessary to contain

the contents of the symbolic link as returned by

\fIreadlink\fR(),

this proposal was rejected, and the historical practice retained.

.P

The purpose of the

\fIreadlinkat\fR()

function is to read the content of symbolic links in directories other

than the current working directory without exposure to race conditions.

Any part of the path of a file could be changed in parallel to a call

to

\fIreadlink\fR(),

resulting in unspecified behavior. By opening a file descriptor for

the target directory and using the

\fIreadlinkat\fR()

function it can be guaranteed that the symbolic link read is located

relative to the desired directory.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "\fIfstatat\fR\^(\|)",

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

.P

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

.IR "\fB<unistd.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 .