man3/realpath.3

   1 .\" Copyright (C) 1999 Andries Brouwer (aeb@cwi.nl)
   2 .\"
   3 .\" %%%LICENSE_START(VERBATIM)
   4 .\" Permission is granted to make and distribute verbatim copies of this
   5 .\" manual provided the copyright notice and this permission notice are
   6 .\" preserved on all copies.
   7 .\"
   8 .\" Permission is granted to copy and distribute modified versions of this
   9 .\" manual under the conditions for verbatim copying, provided that the
  10 .\" entire resulting derived work is distributed under the terms of a
  11 .\" permission notice identical to this one.
  12 .\"
  13 .\" Since the Linux kernel and libraries are constantly changing, this
  14 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  15 .\" responsibility for errors or omissions, or for damages resulting from
  16 .\" the use of the information contained herein.  The author(s) may not
  17 .\" have taken the same level of care in the production of this manual,
  18 .\" which is licensed free of charge, as they might when working
  19 .\" professionally.
  20 .\"
  21 .\" Formatted or processed versions of this manual, if unaccompanied by
  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\" %%%LICENSE_END
  24 .\"
  25 .\" Rewritten old page, 990824, aeb@cwi.nl
  26 .\" 2004-12-14, mtk, added discussion of resolved_path == NULL
  27 .\"
  28 .TH REALPATH 3  2017-09-15 "" "Linux Programmer's Manual"
  29 .SH NAME
  30 realpath \- return the canonicalized absolute pathname
  31 .SH SYNOPSIS
  32 .nf
  33 .B #include <limits.h>
  34 .B #include <stdlib.h>
  35 .PP
  36 .BI "char *realpath(const char *" path ", char *" resolved_path );
  37 .fi
  38 .PP
  39 .in -4n
  40 Feature Test Macro Requirements for glibc (see
  41 .BR feature_test_macros (7)):
  42 .in
  43 .PP
  44 .BR realpath ():
  45 .ad l
  46 .RS 4
  47 _XOPEN_SOURCE\ >=\ 500
  48 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  49     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  50     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  51 .RE
  52 .ad
  53 .SH DESCRIPTION
  54 .BR realpath ()
  55 expands all symbolic links and resolves references
  56 to
  57 .IR "/./" ", " "/../"
  58 and extra \(aq/\(aq
  59 characters in the null-terminated string named by
  60 .I path
  61 to produce a canonicalized absolute pathname.
  62 The resulting pathname is stored as a null-terminated string,
  63 up to a maximum of
  64 .B PATH_MAX
  65 bytes,
  66 in the buffer pointed to by
  67 .IR resolved_path .
  68 The resulting path will have no symbolic link,
  69 .I "/./"
  70 or
  71 .I "/../"
  72 components.
  73 .PP
  74 If
  75 .I resolved_path
  76 is specified as NULL, then
  77 .BR realpath ()
  78 uses
  79 .BR malloc (3)
  80 to allocate a buffer of up to
  81 .B PATH_MAX
  82 bytes to hold the resolved pathname,
  83 and returns a pointer to this buffer.
  84 The caller should deallocate this buffer using
  85 .BR free (3).
  86 .\" Even if we use resolved_path == NULL, then realpath() will still
  87 .\" return ENAMETOOLONG if the resolved pathname would exceed PATH_MAX
  88 .\" bytes -- MTK, Dec 04
  89 .\" .SH HISTORY
  90 .\" The
  91 .\" .BR realpath ()
  92 .\" function first appeared in 4.4BSD, contributed by Jan-Simon Pendry.
  93 .SH RETURN VALUE
  94 If there is no error,
  95 .BR realpath ()
  96 returns a pointer to the
  97 .IR resolved_path .
  98 .PP
  99 Otherwise, it returns NULL, the contents
 100 of the array
 101 .I resolved_path
 102 are undefined, and
 103 .I errno
 104 is set to indicate the error.
 105 .SH ERRORS
 106 .TP
 107 .B EACCES
 108 Read or search permission was denied for a component of the path prefix.
 109 .TP
 110 .B EINVAL
 111 .I path
 112 is NULL.
 113 .\" (In libc5 this would just cause a segfault.)
 114 (In glibc versions before 2.3,
 115 this error is also returned if
 116 .IR resolved_path
 117 is NULL.)
 118 .TP
 119 .B EIO
 120 An I/O error occurred while reading from the filesystem.
 121 .TP
 122 .B ELOOP
 123 Too many symbolic links were encountered in translating the pathname.
 124 .TP
 125 .B ENAMETOOLONG
 126 A component of a pathname exceeded
 127 .B NAME_MAX
 128 characters, or an entire pathname exceeded
 129 .B PATH_MAX
 130 characters.
 131 .TP
 132 .B ENOENT
 133 The named file does not exist.
 134 .TP
 135 .B ENOMEM
 136 Out of memory.
 137 .TP
 138 .B ENOTDIR
 139 A component of the path prefix is not a directory.
 140 .SH ATTRIBUTES
 141 For an explanation of the terms used in this section, see
 142 .BR attributes (7).
 143 .TS
 144 allbox;
 145 lb lb lb
 146 l l l.
 147 Interface       Attribute       Value
 148 T{
 149 .BR realpath ()
 150 T}      Thread safety   MT-Safe
 151 .TE
 152 .SH CONFORMING TO
 153 4.4BSD, POSIX.1-2001.
 154 .PP
 155 POSIX.1-2001 says that the behavior if
 156 .I resolved_path
 157 is NULL is implementation-defined.
 158 POSIX.1-2008 specifies the behavior described in this page.
 159 .SH NOTES
 160 In 4.4BSD and Solaris, the limit on the pathname length is
 161 .B MAXPATHLEN
 162 (found in \fI<sys/param.h>\fP).
 163 SUSv2 prescribes
 164 .B PATH_MAX
 165 and
 166 .BR NAME_MAX ,
 167 as found in \fI<limits.h>\fP or provided by the
 168 .BR pathconf (3)
 169 function.
 170 A typical source fragment would be
 171 .PP
 172 .in +4n
 173 .EX
 174 #ifdef PATH_MAX
 175   path_max = PATH_MAX;
 176 #else
 177   path_max = pathconf(path, _PC_PATH_MAX);
 178   if (path_max <= 0)
 179     path_max = 4096;
 180 #endif
 181 .EE
 182 .in
 183 .PP
 184 (But see the BUGS section.)
 185 .PP
 186 .\"     2012-05-05, According to Casper Dik, the statement about
 187 .\"     Solaris was not true at least as far back as 1997, and
 188 .\"     may never have been true.
 189 .\"
 190 .\" The 4.4BSD, Linux and SUSv2 versions always return an absolute
 191 .\" pathname.
 192 .\" Solaris may return a relative pathname when the
 193 .\" .I path
 194 .\" argument is relative.
 195 .\" The prototype of
 196 .\" .BR realpath ()
 197 .\" is given in \fI<unistd.h>\fP in libc4 and libc5,
 198 .\" but in \fI<stdlib.h>\fP everywhere else.
 199 .SS GNU extensions
 200 If the call fails with either
 201 .BR EACCES
 202 or
 203 .BR ENOENT
 204 and
 205 .I resolved_path
 206 is not NULL, then the prefix of
 207 .I path
 208 that is not readable or does not exist is returned in
 209 .IR resolved_path .
 210 .SH BUGS
 211 The POSIX.1-2001 standard version of this function is broken by design,
 212 since it is impossible to determine a suitable size for the output buffer,
 213 .IR resolved_path .
 214 According to POSIX.1-2001 a buffer of size
 215 .B PATH_MAX
 216 suffices, but
 217 .B PATH_MAX
 218 need not be a defined constant, and may have to be obtained using
 219 .BR pathconf (3).
 220 And asking
 221 .BR pathconf (3)
 222 does not really help, since, on the one hand POSIX warns that
 223 the result of
 224 .BR pathconf (3)
 225 may be huge and unsuitable for mallocing memory,
 226 and on the other hand
 227 .BR pathconf (3)
 228 may return \-1 to signify that
 229 .B PATH_MAX
 230 is not bounded.
 231 The
 232 .I "resolved_path\ ==\ NULL"
 233 feature, not standardized in POSIX.1-2001,
 234 but standardized in POSIX.1-2008, allows this design problem to be avoided.
 235 .\" .LP
 236 .\" The libc4 and libc5 implementation contained a buffer overflow
 237 .\" (fixed in libc-5.4.13).
 238 .\" Thus, set-user-ID programs like
 239 .\" .BR mount (8)
 240 .\" needed a private version.
 241 .SH SEE ALSO
 242 .BR realpath (1),
 243 .BR readlink (2),
 244 .BR canonicalize_file_name (3),
 245 .BR getcwd (3),
 246 .BR pathconf (3),
 247 .BR sysconf (3)