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 2010-11-21 "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 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  71 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  72 .br
  73 || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE\ >=\ 200112L
  74 .RE
  75 .PD
  76 .ad b
  77 .SH DESCRIPTION
  78 The
  79 .BR truncate ()
  80 and
  81 .BR ftruncate ()
  82 functions cause the regular file named by
  83 .I path
  84 or referenced by
  85 .I fd
  86 to be truncated to a size of precisely
  87 .I length
  88 bytes.
  89 .LP
  90 If the file previously was larger than this size, the extra data is lost.
  91 If the file previously was shorter, it is extended, and
  92 the extended part reads as null bytes (\(aq\\0\(aq).
  93 .LP
  94 The file offset is not changed.
  95 .LP
  96 If the size changed, then the st_ctime and st_mtime fields
  97 (respectively, time of last status change and
  98 time of last modification; see
  99 .BR stat (2))
 100 for the file are updated,
 101 and the set-user-ID and set-group-ID permission bits may be cleared.
 102 .LP
 103 With
 104 .BR ftruncate (),
 105 the file must be open for writing; with
 106 .BR truncate (),
 107 the file must be writable.
 108 .SH "RETURN VALUE"
 109 On success, zero is returned.
 110 On error, \-1 is returned, and
 111 .I errno
 112 is set appropriately.
 113 .SH ERRORS
 114 For
 115 .BR truncate ():
 116 .TP
 117 .B EACCES
 118 Search permission is denied for a component of the path prefix,
 119 or the named file is not writable by the user.
 120 (See also
 121 .BR path_resolution (7).)
 122 .TP
 123 .B EFAULT
 124 .I Path
 125 points outside the process's allocated address space.
 126 .TP
 127 .B EFBIG
 128 The argument
 129 .I length
 130 is larger than the maximum file size. (XSI)
 131 .TP
 132 .B EINTR
 133 A signal was caught during execution.
 134 .TP
 135 .B EINVAL
 136 The argument
 137 .I length
 138 is negative or larger than the maximum file size.
 139 .TP
 140 .B EIO
 141 An I/O error occurred updating the inode.
 142 .TP
 143 .B EINTR
 144 While blocked waiting to complete,
 145 the call was interrupted by a signal handler; see
 146 .BR fcntl (2)
 147 and
 148 .BR signal (7).
 149 .TP
 150 .B EISDIR
 151 The named file is a directory.
 152 .TP
 153 .B ELOOP
 154 Too many symbolic links were encountered in translating the pathname.
 155 .TP
 156 .B ENAMETOOLONG
 157 A component of a pathname exceeded 255 characters,
 158 or an entire pathname exceeded 1023 characters.
 159 .TP
 160 .B ENOENT
 161 The named file does not exist.
 162 .TP
 163 .B ENOTDIR
 164 A component of the path prefix is not a directory.
 165 .TP
 166 .B EPERM
 167 .\" This happens for at least MSDOS and VFAT file systems
 168 .\" on kernel 2.6.13
 169 The underlying file system does not support extending
 170 a file beyond its current size.
 171 .TP
 172 .B EROFS
 173 The named file resides on a read-only file system.
 174 .TP
 175 .B ETXTBSY
 176 The file is a pure procedure (shared text) file that is being executed.
 177 .PP
 178 For
 179 .BR ftruncate ()
 180 the same errors apply, but instead of things that can be wrong with
 181 .IR path ,
 182 we now have things that can be wrong with the file descriptor,
 183 .IR fd :
 184 .TP
 185 .B EBADF
 186 .I fd
 187 is not a valid descriptor.
 188 .TP
 189 .BR EBADF " or " EINVAL
 190 .I fd
 191 is not open for writing.
 192 .TP
 193 .B EINVAL
 194 .I fd
 195 does not reference a regular file.
 196 .SH "CONFORMING TO"
 197 4.4BSD, SVr4, POSIX.1-2001 (these calls first appeared in 4.2BSD).
 198 .\" POSIX.1-1996 has
 199 .\" .BR ftruncate ().
 200 .\" POSIX.1-2001 also has
 201 .\" .BR truncate (),
 202 .\" as an XSI extension.
 203 .\" .LP
 204 .\" SVr4 documents additional
 205 .\" .BR truncate ()
 206 .\" error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
 207 .\" .BR ftruncate ()
 208 .\" an additional EAGAIN error condition.
 209 .SH NOTES
 210 The details in DESCRIPTION are for XSI-compliant systems.
 211 For non-XSI-compliant systems, the POSIX standard allows
 212 two behaviors for
 213 .BR ftruncate ()
 214 when
 215 .I length
 216 exceeds the file length
 217 (note that
 218 .BR truncate ()
 219 is not specified at all in such an environment):
 220 either returning an error, or extending the file.
 221 Like most UNIX implementations, Linux follows the XSI requirement
 222 when dealing with native file systems.
 223 However, some nonnative file systems do not permit
 224 .BR truncate ()
 225 and
 226 .BR ftruncate ()
 227 to be used to extend a file beyond its current length:
 228 a notable example on Linux is VFAT.
 229 .\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
 230
 231 The original Linux
 232 .BR truncate ()
 233 and
 234 .BR ftruncate ()
 235 system calls were not designed to handle large file offsets.
 236 Consequently, Linux 2.4 added
 237 .BR truncate64 ()
 238 and
 239 .BR ftruncate64 ()
 240 system calls that handle large files.
 241 However, these details can be ignored by applications using glibc, whose
 242 wrapper functions transparently employ the more recent system calla
 243 where they are available.
 244 .SH BUGS
 245 A header file bug in glibc 2.12 meant that the minimum value of
 246 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=12037
 247 .BR _POSIX_C_SOURCE
 248 required to expose the declaration of
 249 .BR ftruncate ()
 250 was 200809L instead of 200112L.
 251 This has been fixed in later glibc versions.
 252 .SH "SEE ALSO"
 253 .BR open (2),
 254 .BR stat (2),
 255 .BR path_resolution (7)