man2/truncate.2

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  32 .\"     @(#)truncate.2  6.9 (Berkeley) 3/10/91
  33 .\"
  34 .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
  35 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
  36 .\" Modified 1998-12-21 by Andries Brouwer <aeb@cwi.nl>
  37 .\" Modified 2002-01-07 by Michael Kerrisk <mtk.manpages@gmail.com>
  38 .\" Modified 2002-04-06 by Andries Brouwer <aeb@cwi.nl>
  39 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  40 .\"
  41 .TH TRUNCATE 2 2010-09-26 "Linux" "Linux Programmer's Manual"
  42 .SH NAME
  43 truncate, ftruncate \- truncate a file to a specified length
  44 .SH SYNOPSIS
  45 .B #include <unistd.h>
  46 .br
  47 .B #include <sys/types.h>
  48 .sp
  49 .BI "int truncate(const char *" path ", off_t " length );
  50 .br
  51 .BI "int ftruncate(int " fd ", off_t " length );
  52 .sp
  53 .in -4n
  54 Feature Test Macro Requirements for glibc (see
  55 .BR feature_test_macros (7)):
  56 .in
  57 .ad l
  58 .PD 0
  59 .sp
  60 .BR truncate ():
  61 .RS 4
  62 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  63 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  64 .br
  65 || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
  66 .RE
  67 .sp
  68 .BR ftruncate ():
  69 .RS 4
  70 .TP 4
  71 .TP
  72 Since glibc 2.12:
  73 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  74 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED ||
  75 .\" FIXME The next piece seems incorrect; see
  76 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=12037
  77 _POSIX_C_SOURCE\ >=\ 200809L
  78 .TP
  79 Between glibc 2.3.5 and 2.11
  80 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  81 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED ||
  82 _POSIX_C_SOURCE\ >=\ 200112L
  83 .TP
  84 Before glibc 2.3.5:
  85 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  86 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  87 .RE
  88 .PD
  89 .ad b
  90 .SH DESCRIPTION
  91 The
  92 .BR truncate ()
  93 and
  94 .BR ftruncate ()
  95 functions cause the regular file named by
  96 .I path
  97 or referenced by
  98 .I fd
  99 to be truncated to a size of precisely
 100 .I length
 101 bytes.
 102 .LP
 103 If the file previously was larger than this size, the extra data is lost.
 104 If the file previously was shorter, it is extended, and
 105 the extended part reads as null bytes (\(aq\\0\(aq).
 106 .LP
 107 The file offset is not changed.
 108 .LP
 109 If the size changed, then the st_ctime and st_mtime fields
 110 (respectively, time of last status change and
 111 time of last modification; see
 112 .BR stat (2))
 113 for the file are updated,
 114 and the set-user-ID and set-group-ID permission bits may be cleared.
 115 .LP
 116 With
 117 .BR ftruncate (),
 118 the file must be open for writing; with
 119 .BR truncate (),
 120 the file must be writable.
 121 .SH "RETURN VALUE"
 122 On success, zero is returned.
 123 On error, \-1 is returned, and
 124 .I errno
 125 is set appropriately.
 126 .SH ERRORS
 127 For
 128 .BR truncate ():
 129 .TP
 130 .B EACCES
 131 Search permission is denied for a component of the path prefix,
 132 or the named file is not writable by the user.
 133 (See also
 134 .BR path_resolution (7).)
 135 .TP
 136 .B EFAULT
 137 .I Path
 138 points outside the process's allocated address space.
 139 .TP
 140 .B EFBIG
 141 The argument
 142 .I length
 143 is larger than the maximum file size. (XSI)
 144 .TP
 145 .B EINTR
 146 A signal was caught during execution.
 147 .TP
 148 .B EINVAL
 149 The argument
 150 .I length
 151 is negative or larger than the maximum file size.
 152 .TP
 153 .B EIO
 154 An I/O error occurred updating the inode.
 155 .TP
 156 .B EINTR
 157 While blocked waiting to complete,
 158 the call was interrupted by a signal handler; see
 159 .BR fcntl (2)
 160 and
 161 .BR signal (7).
 162 .TP
 163 .B EISDIR
 164 The named file is a directory.
 165 .TP
 166 .B ELOOP
 167 Too many symbolic links were encountered in translating the pathname.
 168 .TP
 169 .B ENAMETOOLONG
 170 A component of a pathname exceeded 255 characters,
 171 or an entire pathname exceeded 1023 characters.
 172 .TP
 173 .B ENOENT
 174 The named file does not exist.
 175 .TP
 176 .B ENOTDIR
 177 A component of the path prefix is not a directory.
 178 .TP
 179 .B EPERM
 180 .\" This happens for at least MSDOS and VFAT file systems
 181 .\" on kernel 2.6.13
 182 The underlying file system does not support extending
 183 a file beyond its current size.
 184 .TP
 185 .B EROFS
 186 The named file resides on a read-only file system.
 187 .TP
 188 .B ETXTBSY
 189 The file is a pure procedure (shared text) file that is being executed.
 190 .PP
 191 For
 192 .BR ftruncate ()
 193 the same errors apply, but instead of things that can be wrong with
 194 .IR path ,
 195 we now have things that can be wrong with the file descriptor,
 196 .IR fd :
 197 .TP
 198 .B EBADF
 199 .I fd
 200 is not a valid descriptor.
 201 .TP
 202 .BR EBADF " or " EINVAL
 203 .I fd
 204 is not open for writing.
 205 .TP
 206 .B EINVAL
 207 .I fd
 208 does not reference a regular file.
 209 .SH "CONFORMING TO"
 210 4.4BSD, SVr4, POSIX.1-2001 (these calls first appeared in 4.2BSD).
 211 .\" POSIX.1-1996 has
 212 .\" .BR ftruncate ().
 213 .\" POSIX.1-2001 also has
 214 .\" .BR truncate (),
 215 .\" as an XSI extension.
 216 .\" .LP
 217 .\" SVr4 documents additional
 218 .\" .BR truncate ()
 219 .\" error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
 220 .\" .BR ftruncate ()
 221 .\" an additional EAGAIN error condition.
 222 .SH NOTES
 223 The above description is for XSI-compliant systems.
 224 For non-XSI-compliant systems, the POSIX standard allows
 225 two behaviors for
 226 .BR ftruncate ()
 227 when
 228 .I length
 229 exceeds the file length
 230 (note that
 231 .BR truncate ()
 232 is not specified at all in such an environment):
 233 either returning an error, or extending the file.
 234 Like most Unix implementations, Linux follows the XSI requirement
 235 when dealing with native file systems.
 236 However, some nonnative file systems do not permit
 237 .BR truncate ()
 238 and
 239 .BR ftruncate ()
 240 to be used to extend a file beyond its current length:
 241 a notable example on Linux is VFAT.
 242 .\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
 243 .SH "SEE ALSO"
 244 .BR open (2),
 245 .BR stat (2),
 246 .BR path_resolution (7)