man2/chmod.2

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   3 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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  25 .\" Modified by Michael Haardt <michael@moria.de>
  26 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
  27 .\" Modified 1997-01-12 by Michael Haardt
  28 .\"   <michael@cantor.informatik.rwth-aachen.de>: NFS details
  29 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
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  31 .TH CHMOD 2 2010-09-20 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 chmod, fchmod \- change permissions of a file
  34 .SH SYNOPSIS
  35 .B #include <sys/stat.h>
  36 .sp
  37 .BI "int chmod(const char *" path ", mode_t " mode );
  38 .br
  39 .BI "int fchmod(int " fd ", mode_t " mode );
  40 .sp
  41 .in -4n
  42 Feature Test Macro Requirements for glibc (see
  43 .BR feature_test_macros (7)):
  44 .in
  45 .sp
  46 .ad l
  47 .PD 0
  48 .BR fchmod ():
  49 .RS 4
  50 .TP 4
  51 Since glibc 2.12:
  52 _POSIX_C_SOURCE\ >=\ 200809L ||
  53 _BSD_SOURCE ||
  54 _XOPEN_SOURCE\ >=\ 500 ||
  55 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  56 .TP
  57 Before glibc 2.12:
  58 _BSD_SOURCE ||
  59 _XOPEN_SOURCE\ >=\ 500 ||
  60 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  61 .RE
  62 .PD
  63 .ad
  64 .SH DESCRIPTION
  65 These system calls change the permissions of a file.
  66 They differ only in how the file is specified:
  67 .IP * 2
  68 .BR chmod ()
  69 changes the permissions of the file specified whose pathname is given in
  70 .IR path ,
  71 which is dereferenced if it is a symbolic link.
  72 .IP *
  73 .BR fchmod ()
  74 changes the permissions of the file referred to by the open file descriptor
  75 .IR fd .
  76 .PP
  77 The new file permissions are specified in
  78 .IR mode ,
  79 which is a bit mask created by ORing together zero or
  80 more of the following:
  81 .TP 18
  82 .BR S_ISUID "  (04000)"
  83 set-user-ID (set process effective user ID on
  84 .BR execve (2))
  85 .TP
  86 .BR S_ISGID "  (02000)"
  87 set-group-ID (set process effective group ID on
  88 .BR execve (2);
  89 mandatory locking, as described in
  90 .BR fcntl (2);
  91 take a new file's group from parent directory, as described in
  92 .BR chown (2)
  93 and
  94 .BR mkdir (2))
  95 .TP
  96 .BR S_ISVTX "  (01000)"
  97 sticky bit (restricted deletion flag, as described in
  98 .BR unlink (2))
  99 .TP
 100 .BR S_IRUSR "  (00400)"
 101 read by owner
 102 .TP
 103 .BR S_IWUSR "  (00200)"
 104 write by owner
 105 .TP
 106 .BR S_IXUSR "  (00100)"
 107 execute/search by owner ("search" applies for directories,
 108 and means that entries within the directory can be accessed)
 109 .TP
 110 .BR S_IRGRP "  (00040)"
 111 read by group
 112 .TP
 113 .BR S_IWGRP "  (00020)"
 114 write by group
 115 .TP
 116 .BR S_IXGRP "  (00010)"
 117 execute/search by group
 118 .TP
 119 .BR S_IROTH "  (00004)"
 120 read by others
 121 .TP
 122 .BR S_IWOTH "  (00002)"
 123 write by others
 124 .TP
 125 .BR S_IXOTH "  (00001)"
 126 execute/search by others
 127 .PP
 128 The effective UID of the calling process must match the owner of the file,
 129 or the process must be privileged (Linux: it must have the
 130 .B CAP_FOWNER
 131 capability).
 132
 133 If the calling process is not privileged (Linux: does not have the
 134 .B CAP_FSETID
 135 capability), and the group of the file does not match
 136 the effective group ID of the process or one of its
 137 supplementary group IDs, the
 138 .B S_ISGID
 139 bit will be turned off,
 140 but this will not cause an error to be returned.
 141
 142 As a security measure, depending on the file system,
 143 the set-user-ID and set-group-ID execution bits
 144 may be turned off if a file is written.
 145 (On Linux this occurs if the writing process does not have the
 146 .B CAP_FSETID
 147 capability.)
 148 On some file systems, only the superuser can set the sticky bit,
 149 which may have a special meaning.
 150 For the sticky bit, and for set-user-ID and set-group-ID bits on
 151 directories, see
 152 .BR stat (2).
 153
 154 On NFS file systems, restricting the permissions will immediately influence
 155 already open files, because the access control is done on the server, but
 156 open files are maintained by the client.
 157 Widening the permissions may be
 158 delayed for other clients if attribute caching is enabled on them.
 159 .SH "RETURN VALUE"
 160 On success, zero is returned.
 161 On error, \-1 is returned, and
 162 .I errno
 163 is set appropriately.
 164 .SH ERRORS
 165 Depending on the file system, other errors can be returned.
 166 The more general errors for
 167 .BR chmod ()
 168 are listed below:
 169 .TP
 170 .B EACCES
 171 Search permission is denied on a component of the path prefix.
 172 (See also
 173 .BR path_resolution (7).)
 174 .TP
 175 .B EFAULT
 176 .I path
 177 points outside your accessible address space.
 178 .TP
 179 .B EIO
 180 An I/O error occurred.
 181 .TP
 182 .B ELOOP
 183 Too many symbolic links were encountered in resolving
 184 .IR path .
 185 .TP
 186 .B ENAMETOOLONG
 187 .I path
 188 is too long.
 189 .TP
 190 .B ENOENT
 191 The file does not exist.
 192 .TP
 193 .B ENOMEM
 194 Insufficient kernel memory was available.
 195 .TP
 196 .B ENOTDIR
 197 A component of the path prefix is not a directory.
 198 .TP
 199 .B EPERM
 200 The effective UID does not match the owner of the file,
 201 and the process is not privileged (Linux: it does not have the
 202 .B CAP_FOWNER
 203 capability).
 204 .TP
 205 .B EROFS
 206 The named file resides on a read-only file system.
 207 .PP
 208 The general errors for
 209 .BR fchmod ()
 210 are listed below:
 211 .TP
 212 .B EBADF
 213 The file descriptor
 214 .I fd
 215 is not valid.
 216 .TP
 217 .B EIO
 218 See above.
 219 .TP
 220 .B EPERM
 221 See above.
 222 .TP
 223 .B EROFS
 224 See above.
 225 .SH "CONFORMING TO"
 226 4.4BSD, SVr4, POSIX.1-2001.
 227 .SH "SEE ALSO"
 228 .BR chown (2),
 229 .BR execve (2),
 230 .BR fchmodat (2),
 231 .BR open (2),
 232 .BR stat (2),
 233 .BR path_resolution (7)