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readlink.3p (7077B)

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.TH READLINK "3P" 2017 "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);
.P
#include <fcntl.h>
.P
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 access mode of the
open file description associated with the file descriptor is not
O_SEARCH, the function shall check whether directory searches are
permitted using the current permissions of the directory underlying
the file descriptor. If the access mode is 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 \-1, 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
The access mode of the open file description associated with
.IR fd
is not 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
#include <unistd.h>
.P
char buf[1024];
ssize_t len;
\&...
if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1)
    buf[len] = \(aq\e0\(aq;
.fi
.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\(hy2017,
.IR "\fB<fcntl.h>\fP",
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
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.opengroup.org/unix/online.html .
.PP
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 .