path: root/man3/realpath.3

'\" t
.\" Copyright (C) 1999 Andries Brouwer (aeb@cwi.nl)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Rewritten old page, 990824, aeb@cwi.nl
.\" 2004-12-14, mtk, added discussion of resolved_path == NULL
.\"
.TH realpath 3 (date) "Linux man-pages (unreleased)"
.SH NAME
realpath \- return the canonicalized absolute pathname
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <limits.h>
.B #include <stdlib.h>
.PP
.BI "char *realpath(const char *restrict " path ,
.BI "               char *restrict " resolved_path );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR realpath ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE
.fi
.SH DESCRIPTION
.BR realpath ()
expands all symbolic links and resolves references
to
.IR "/./" ", " "/../"
and extra \[aq]/\[aq]
characters in the null-terminated string named by
.I path
to produce a canonicalized absolute pathname.
The resulting pathname is stored as a null-terminated string,
up to a maximum of
.B PATH_MAX
bytes,
in the buffer pointed to by
.IR resolved_path .
The resulting path will have no symbolic link,
.I "/./"
or
.I "/../"
components.
.PP
If
.I resolved_path
is specified as NULL, then
.BR realpath ()
uses
.BR malloc (3)
to allocate a buffer of up to
.B PATH_MAX
bytes to hold the resolved pathname,
and returns a pointer to this buffer.
The caller should deallocate this buffer using
.BR free (3).
.\" Even if we use resolved_path == NULL, then realpath() will still
.\" return ENAMETOOLONG if the resolved pathname would exceed PATH_MAX
.\" bytes -- MTK, Dec 04
.SH RETURN VALUE
If there is no error,
.BR realpath ()
returns a pointer to the
.IR resolved_path .
.PP
Otherwise, it returns NULL, the contents
of the array
.I resolved_path
are undefined, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
Read or search permission was denied for a component of the path prefix.
.TP
.B EINVAL
.I path
is NULL.
.\" (In libc5 this would just cause a segfault.)
(Before glibc 2.3,
this error is also returned if
.I resolved_path
is NULL.)
.TP
.B EIO
An I/O error occurred while reading from the filesystem.
.TP
.B ELOOP
Too many symbolic links were encountered in translating the pathname.
.TP
.B ENAMETOOLONG
A component of a pathname exceeded
.B NAME_MAX
characters, or an entire pathname exceeded
.B PATH_MAX
characters.
.TP
.B ENOENT
The named file does not exist.
.TP
.B ENOMEM
Out of memory.
.TP
.B ENOTDIR
A component of the path prefix is not a directory.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR realpath ()
T}	Thread safety	MT-Safe
.TE
.sp 1
.SH VERSIONS
.SS GNU extensions
If the call fails with either
.B EACCES
or
.B ENOENT
and
.I resolved_path
is not NULL, then the prefix of
.I path
that is not readable or does not exist is returned in
.IR resolved_path .
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
4.4BSD, POSIX.1-2001, Solaris.
.PP
POSIX.1-2001 says that the behavior if
.I resolved_path
is NULL is implementation-defined.
POSIX.1-2008 specifies the behavior described in this page.
.PP
In 4.4BSD and Solaris, the limit on the pathname length is
.B MAXPATHLEN
(found in \fI<sys/param.h>\fP).
SUSv2 prescribes
.B PATH_MAX
and
.BR NAME_MAX ,
as found in \fI<limits.h>\fP or provided by the
.BR pathconf (3)
function.
A typical source fragment would be
.PP
.in +4n
.EX
#ifdef PATH_MAX
  path_max = PATH_MAX;
#else
  path_max = pathconf(path, _PC_PATH_MAX);
  if (path_max <= 0)
    path_max = 4096;
#endif
.EE
.in
.PP
(But see the BUGS section.)
.\".PP
.\"     2012-05-05, According to Casper Dik, the statement about
.\"     Solaris was not true at least as far back as 1997, and
.\"     may never have been true.
.\"
.\" The 4.4BSD, Linux and SUSv2 versions always return an absolute
.\" pathname.
.\" Solaris may return a relative pathname when the
.\" .I path
.\" argument is relative.
.\" The prototype of
.\" .BR realpath ()
.\" is given in \fI<unistd.h>\fP in libc4 and libc5,
.\" but in \fI<stdlib.h>\fP everywhere else.
.SH BUGS
The POSIX.1-2001 standard version of this function is broken by design,
since it is impossible to determine a suitable size for the output buffer,
.IR resolved_path .
According to POSIX.1-2001 a buffer of size
.B PATH_MAX
suffices, but
.B PATH_MAX
need not be a defined constant, and may have to be obtained using
.BR pathconf (3).
And asking
.BR pathconf (3)
does not really help, since, on the one hand POSIX warns that
the result of
.BR pathconf (3)
may be huge and unsuitable for mallocing memory,
and on the other hand
.BR pathconf (3)
may return \-1 to signify that
.B PATH_MAX
is not bounded.
The
.I "resolved_path\ ==\ NULL"
feature, not standardized in POSIX.1-2001,
but standardized in POSIX.1-2008, allows this design problem to be avoided.
.\" .LP
.\" The libc4 and libc5 implementation contained a buffer overflow
.\" (fixed in libc-5.4.13).
.\" Thus, set-user-ID programs like
.\" .BR mount (8)
.\" needed a private version.
.SH SEE ALSO
.BR realpath (1),
.BR readlink (2),
.BR canonicalize_file_name (3),
.BR getcwd (3),
.BR pathconf (3),
.BR sysconf (3)