man2/truncate.2

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  33 .\"
  34 .\"     @(#)truncate.2  6.9 (Berkeley) 3/10/91
  35 .\"
  36 .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
  37 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
  38 .\" Modified 1998-12-21 by Andries Brouwer <aeb@cwi.nl>
  39 .\" Modified 2002-01-07 by Michael Kerrisk <mtk.manpages@gmail.com>
  40 .\" Modified 2002-04-06 by Andries Brouwer <aeb@cwi.nl>
  41 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  42 .\"
  43 .TH TRUNCATE 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  44 .SH NAME
  45 truncate, ftruncate \- truncate a file to a specified length
  46 .SH SYNOPSIS
  47 .B #include <unistd.h>
  48 .br
  49 .B #include <sys/types.h>
  50 .sp
  51 .BI "int truncate(const char *" path ", off_t " length );
  52 .br
  53 .BI "int ftruncate(int " fd ", off_t " length );
  54 .sp
  55 .in -4n
  56 Feature Test Macro Requirements for glibc (see
  57 .BR feature_test_macros (7)):
  58 .in
  59 .ad l
  60 .PD 0
  61 .sp
  62 .BR truncate ():
  63 .RS 4
  64 _XOPEN_SOURCE\ >=\ 500
  65 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  66 .br
  67     || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
  68     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  69 .RE
  70 .sp
  71 .BR ftruncate ():
  72 .RS 4
  73 _XOPEN_SOURCE\ >=\ 500
  74 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  75     || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE\ >=\ 200112L
  76     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  77 .RE
  78 .PD
  79 .ad b
  80 .SH DESCRIPTION
  81 The
  82 .BR truncate ()
  83 and
  84 .BR ftruncate ()
  85 functions cause the regular file named by
  86 .I path
  87 or referenced by
  88 .I fd
  89 to be truncated to a size of precisely
  90 .I length
  91 bytes.
  92 .LP
  93 If the file previously was larger than this size, the extra data is lost.
  94 If the file previously was shorter, it is extended, and
  95 the extended part reads as null bytes (\(aq\\0\(aq).
  96 .LP
  97 The file offset is not changed.
  98 .LP
  99 If the size changed, then the st_ctime and st_mtime fields
 100 (respectively, time of last status change and
 101 time of last modification; see
 102 .BR stat (2))
 103 for the file are updated,
 104 and the set-user-ID and set-group-ID mode bits may be cleared.
 105 .LP
 106 With
 107 .BR ftruncate (),
 108 the file must be open for writing; with
 109 .BR truncate (),
 110 the file must be writable.
 111 .SH RETURN VALUE
 112 On success, zero is returned.
 113 On error, \-1 is returned, and
 114 .I errno
 115 is set appropriately.
 116 .SH ERRORS
 117 For
 118 .BR truncate ():
 119 .TP
 120 .B EACCES
 121 Search permission is denied for a component of the path prefix,
 122 or the named file is not writable by the user.
 123 (See also
 124 .BR path_resolution (7).)
 125 .TP
 126 .B EFAULT
 127 The argument
 128 .I path
 129 points outside the process's allocated address space.
 130 .TP
 131 .B EFBIG
 132 The argument
 133 .I length
 134 is larger than the maximum file size. (XSI)
 135 .TP
 136 .B EINTR
 137 While blocked waiting to complete,
 138 the call was interrupted by a signal handler; see
 139 .BR fcntl (2)
 140 and
 141 .BR signal (7).
 142 .TP
 143 .B EINVAL
 144 The argument
 145 .I length
 146 is negative or larger than the maximum file size.
 147 .TP
 148 .B EIO
 149 An I/O error occurred updating the inode.
 150 .TP
 151 .B EISDIR
 152 The named file is a directory.
 153 .TP
 154 .B ELOOP
 155 Too many symbolic links were encountered in translating the pathname.
 156 .TP
 157 .B ENAMETOOLONG
 158 A component of a pathname exceeded 255 characters,
 159 or an entire pathname exceeded 1023 characters.
 160 .TP
 161 .B ENOENT
 162 The named file does not exist.
 163 .TP
 164 .B ENOTDIR
 165 A component of the path prefix is not a directory.
 166 .TP
 167 .B EPERM
 168 .\" This happens for at least MSDOS and VFAT filesystems
 169 .\" on kernel 2.6.13
 170 The underlying filesystem does not support extending
 171 a file beyond its current size.
 172 .TP
 173 .B EPERM
 174 The operation was prevented by a file seal; see
 175 .BR fcntl (2).
 176 .TP
 177 .B EROFS
 178 The named file resides on a read-only filesystem.
 179 .TP
 180 .B ETXTBSY
 181 The file is an executable file that is being executed.
 182 .PP
 183 For
 184 .BR ftruncate ()
 185 the same errors apply, but instead of things that can be wrong with
 186 .IR path ,
 187 we now have things that can be wrong with the file descriptor,
 188 .IR fd :
 189 .TP
 190 .B EBADF
 191 .I fd
 192 is not a valid file descriptor.
 193 .TP
 194 .BR EBADF " or " EINVAL
 195 .I fd
 196 is not open for writing.
 197 .TP
 198 .B EINVAL
 199 .I fd
 200 does not reference a regular file.
 201 .TP
 202 .BR EINVAL " or " EBADF
 203 The file descriptor
 204 .I fd
 205 is not open for writing.
 206 POSIX permits, and portable applications should handle,
 207 either error for this case.
 208 (Linux produces
 209 .BR EINVAL .)
 210 .SH CONFORMING TO
 211 POSIX.1-2001, POSIX.1-2008,
 212 4.4BSD, SVr4 (these calls first appeared in 4.2BSD).
 213 .\" POSIX.1-1996 has
 214 .\" .BR ftruncate ().
 215 .\" POSIX.1-2001 also has
 216 .\" .BR truncate (),
 217 .\" as an XSI extension.
 218 .\" .LP
 219 .\" SVr4 documents additional
 220 .\" .BR truncate ()
 221 .\" error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
 222 .\" .BR ftruncate ()
 223 .\" an additional EAGAIN error condition.
 224 .SH NOTES
 225 The details in DESCRIPTION are for XSI-compliant systems.
 226 For non-XSI-compliant systems, the POSIX standard allows
 227 two behaviors for
 228 .BR ftruncate ()
 229 when
 230 .I length
 231 exceeds the file length
 232 (note that
 233 .BR truncate ()
 234 is not specified at all in such an environment):
 235 either returning an error, or extending the file.
 236 Like most UNIX implementations, Linux follows the XSI requirement
 237 when dealing with native filesystems.
 238 However, some nonnative filesystems do not permit
 239 .BR truncate ()
 240 and
 241 .BR ftruncate ()
 242 to be used to extend a file beyond its current length:
 243 a notable example on Linux is VFAT.
 244 .\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
 245
 246 The original Linux
 247 .BR truncate ()
 248 and
 249 .BR ftruncate ()
 250 system calls were not designed to handle large file offsets.
 251 Consequently, Linux 2.4 added
 252 .BR truncate64 ()
 253 and
 254 .BR ftruncate64 ()
 255 system calls that handle large files.
 256 However, these details can be ignored by applications using glibc, whose
 257 wrapper functions transparently employ the more recent system calls
 258 where they are available.
 259
 260 On some 32-bit architectures,
 261 the calling signature for these system calls differ,
 262 for the reasons described in
 263 .BR syscall (2).
 264 .SH BUGS
 265 A header file bug in glibc 2.12 meant that the minimum value of
 266 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=12037
 267 .BR _POSIX_C_SOURCE
 268 required to expose the declaration of
 269 .BR ftruncate ()
 270 was 200809L instead of 200112L.
 271 This has been fixed in later glibc versions.
 272 .SH SEE ALSO
 273 .BR truncate (1),
 274 .BR open (2),
 275 .BR stat (2),
 276 .BR path_resolution (7)