These interfaces are specified in POSIX.1-2008.

[thirdparty/man-pages.git] / man3 / realpath.3

.\" Copyright (C) 1999 Andries Brouwer (aeb@cwi.nl)
.\"
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.\" Rewritten old page, 990824, aeb@cwi.nl
.\" 2004-12-14, mtk, added discussion of resolved_path == NULL
.\"
.TH REALPATH 3  2007-07-26 "" "Linux Programmer's Manual"
.SH NAME
realpath \- return the canonicalized absolute pathname
.SH SYNOPSIS
.nf
.B #include <limits.h>
.B #include <stdlib.h>
.sp
.BI "char *realpath(const char *" path ", char *" resolved_path );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR realpath ():
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
.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
and stores the canonicalized absolute pathname in the buffer of size
.B PATH_MAX
named by
.IR resolved_path .
The resulting path will have no symbolic link,
.I "/./"
or
.I "/../"
components.
.SH "RETURN VALUE"
If there is no error,
.BR realpath ()
returns a pointer to the
.IR resolved_path .

Otherwise it returns a NULL pointer, and the contents
of the array
.I resolved_path
are undefined.
The global variable
.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
Either
.I path
or
.I resolved_path
is NULL.
(In libc5 this would just cause a segfault.)
But, see NOTES below.
.TP
.B EIO
An I/O error occurred while reading from the file system.
.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 ENOTDIR
A component of the path prefix is not a directory.
.SH VERSIONS
On Linux this function appeared in libc 4.5.21.
.SH "CONFORMING TO"
4.4BSD, POSIX.1-2001.

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
.LP
.in +4n
.nf
#ifdef PATH_MAX
  path_max = PATH_MAX;
#else
  path_max = pathconf(path, _PC_PATH_MAX);
  if (path_max <= 0)
    path_max = 4096;
#endif
.fi
.in
.LP
(But see the BUGS section.)
.LP
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 NOTES
The glibc implementation of
.BR realpath ()
provides a non-standard extension.
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 HISTORY
.\" The
.\" .BR realpath ()
.\" function first appeared in 4.4BSD, contributed by Jan-Simon Pendry.
.SH BUGS
Avoid using this function.
It is broken by design since (unless
using the non-standard
.I "resolved_path\ ==\ NULL"
feature) it is
impossible to determine a suitable size for the output buffer,
.IR resolved_path .
According to POSIX 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.
.LP
The libc4 and libc5 implementation contains a buffer overflow
(fixed in libc-5.4.13).
Thus, set-user-ID programs like
.BR mount (8)
need a private version.
.SH "SEE ALSO"
.BR readlink (2),
.BR canonicalize_file_name (3),
.BR getcwd (3),
.BR pathconf (3),
.BR sysconf (3)