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 2009-02-28 "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 .sp
  59 .BR truncate ():
  60 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
  61 .br
  62 .BR ftruncate ():
  63 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 || _POSIX_C_SOURCE\ >=\ 200112L
  64 .ad b
  65 .SH DESCRIPTION
  66 The
  67 .BR truncate ()
  68 and
  69 .BR ftruncate ()
  70 functions cause the regular file named by
  71 .I path
  72 or referenced by
  73 .I fd
  74 to be truncated to a size of precisely
  75 .I length
  76 bytes.
  77 .LP
  78 If the file previously was larger than this size, the extra data is lost.
  79 If the file previously was shorter, it is extended, and
  80 the extended part reads as null bytes (\(aq\\0\(aq).
  81 .LP
  82 The file offset is not changed.
  83 .LP
  84 If the size changed, then the st_ctime and st_mtime fields
  85 (respectively, time of last status change and
  86 time of last modification; see
  87 .BR stat (2))
  88 for the file are updated,
  89 and the set-user-ID and set-group-ID permission bits may be cleared.
  90 .LP
  91 With
  92 .BR ftruncate (),
  93 the file must be open for writing; with
  94 .BR truncate (),
  95 the file must be writable.
  96 .SH "RETURN VALUE"
  97 On success, zero is returned.
  98 On error, \-1 is returned, and
  99 .I errno
 100 is set appropriately.
 101 .SH ERRORS
 102 For
 103 .BR truncate ():
 104 .TP
 105 .B EACCES
 106 Search permission is denied for a component of the path prefix,
 107 or the named file is not writable by the user.
 108 (See also
 109 .BR path_resolution (7).)
 110 .TP
 111 .B EFAULT
 112 .I Path
 113 points outside the process's allocated address space.
 114 .TP
 115 .B EFBIG
 116 The argument
 117 .I length
 118 is larger than the maximum file size. (XSI)
 119 .TP
 120 .B EINTR
 121 A signal was caught during execution.
 122 .TP
 123 .B EINVAL
 124 The argument
 125 .I length
 126 is negative or larger than the maximum file size.
 127 .TP
 128 .B EIO
 129 An I/O error occurred updating the inode.
 130 .TP
 131 .B EINTR
 132 While blocked waiting to complete,
 133 the call was interrupted by a signal handler; see
 134 .BR fcntl (2)
 135 and
 136 .BR signal (7).
 137 .TP
 138 .B EISDIR
 139 The named file is a directory.
 140 .TP
 141 .B ELOOP
 142 Too many symbolic links were encountered in translating the pathname.
 143 .TP
 144 .B ENAMETOOLONG
 145 A component of a pathname exceeded 255 characters,
 146 or an entire pathname exceeded 1023 characters.
 147 .TP
 148 .B ENOENT
 149 The named file does not exist.
 150 .TP
 151 .B ENOTDIR
 152 A component of the path prefix is not a directory.
 153 .TP
 154 .B EPERM
 155 .\" This happens for at least MSDOS and VFAT file systems
 156 .\" on kernel 2.6.13
 157 The underlying file system does not support extending
 158 a file beyond its current size.
 159 .TP
 160 .B EROFS
 161 The named file resides on a read-only file system.
 162 .TP
 163 .B ETXTBSY
 164 The file is a pure procedure (shared text) file that is being executed.
 165 .PP
 166 For
 167 .BR ftruncate ()
 168 the same errors apply, but instead of things that can be wrong with
 169 .IR path ,
 170 we now have things that can be wrong with the file descriptor,
 171 .IR fd :
 172 .TP
 173 .B EBADF
 174 .I fd
 175 is not a valid descriptor.
 176 .TP
 177 .BR EBADF " or " EINVAL
 178 .I fd
 179 is not open for writing.
 180 .TP
 181 .B EINVAL
 182 .I fd
 183 does not reference a regular file.
 184 .SH "CONFORMING TO"
 185 4.4BSD, SVr4, POSIX.1-2001 (these calls first appeared in 4.2BSD).
 186 .\" POSIX.1-1996 has
 187 .\" .BR ftruncate ().
 188 .\" POSIX.1-2001 also has
 189 .\" .BR truncate (),
 190 .\" as an XSI extension.
 191 .\" .LP
 192 .\" SVr4 documents additional
 193 .\" .BR truncate ()
 194 .\" error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
 195 .\" .BR ftruncate ()
 196 .\" an additional EAGAIN error condition.
 197 .SH NOTES
 198 The above description is for XSI-compliant systems.
 199 For non-XSI-compliant systems, the POSIX standard allows
 200 two behaviors for
 201 .BR ftruncate ()
 202 when
 203 .I length
 204 exceeds the file length
 205 (note that
 206 .BR truncate ()
 207 is not specified at all in such an environment):
 208 either returning an error, or extending the file.
 209 Like most Unix implementations, Linux follows the XSI requirement
 210 when dealing with native file systems.
 211 However, some nonnative file systems do not permit
 212 .BR truncate ()
 213 and
 214 .BR ftruncate ()
 215 to be used to extend a file beyond its current length:
 216 a notable example on Linux is VFAT.
 217 .\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
 218 .SH "SEE ALSO"
 219 .BR open (2),
 220 .BR stat (2),
 221 .BR path_resolution (7)