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@gmx.net>
  38 .\" Modified 2002-04-06 by Andries Brouwer <aeb@cwi.nl>
  39 .\" Modified 2004-06-23 by Michael Kerrisk <mtk-manpages@gmx.net>
  40 .\"
  41 .TH TRUNCATE 2 2004-06-23 "Linux 2.6.7" "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 .SH DESCRIPTION
  53 The
  54 .BR truncate ()
  55 and
  56 .BR ftruncate ()
  57 functions cause the regular file named by
  58 .I path
  59 or referenced by
  60 .I fd
  61 to be truncated to a size of precisely
  62 .I length
  63 bytes.
  64 .LP
  65 If the file previously was larger than this size, the extra data is lost.
  66 If the file previously was shorter, it is extended, and
  67 the extended part reads as null bytes ('\\0').
  68 .LP
  69 The file offset is not changed.
  70 .LP
  71 If the size changed, then the st_ctime and st_mtime fields
  72 (respectively, time of last status change and
  73 time of last modification; see
  74 .BR stat (2))
  75 for the file are updated,
  76 and the set-user-ID and set-group-ID permission bits may be cleared.
  77 .LP
  78 With
  79 .BR ftruncate (),
  80 the file must be open for writing; with
  81 .BR truncate (),
  82 the file must be writable.
  83 .SH "RETURN VALUE"
  84 On success, zero is returned.  On error, \-1 is returned, and
  85 .I errno
  86 is set appropriately.
  87 .SH ERRORS
  88 For
  89 .BR truncate ():
  90 .TP
  91 .B EACCES
  92 Search permission is denied for a component of the path prefix,
  93 or the named file is not writable by the user.
  94 (See also
  95 .BR path_resolution (2).)
  96 .TP
  97 .B EFAULT
  98 .I Path
  99 points outside the process's allocated address space.
 100 .TP
 101 .B EFBIG
 102 The argument
 103 .I length
 104 is larger than the maximum file size. (XSI)
 105 .TP
 106 .B EINTR
 107 A signal was caught during execution.
 108 .TP
 109 .B EINVAL
 110 The argument
 111 .I length
 112 is negative or larger than the maximum file size.
 113 .TP
 114 .B EIO
 115 An I/O error occurred updating the inode.
 116 .TP
 117 .B EISDIR
 118 The named file is a directory.
 119 .TP
 120 .B ELOOP
 121 Too many symbolic links were encountered in translating the pathname.
 122 .TP
 123 .B ENAMETOOLONG
 124 A component of a pathname exceeded 255 characters,
 125 or an entire pathname exceeded 1023 characters.
 126 .TP
 127 .B ENOENT
 128 The named file does not exist.
 129 .TP
 130 .B ENOTDIR
 131 A component of the path prefix is not a directory.
 132 .TP
 133 .B EPERM
 134 .\" This happens for at leas MSDOS and VFAT fil systems
 135 .\" on kernel 2.6.13
 136 The underlying file system does not support extending
 137 a file beyond its current size.
 138 .TP
 139 .B EROFS
 140 The named file resides on a read-only file system.
 141 .TP
 142 .B ETXTBSY
 143 The file is a pure procedure (shared text) file that is being executed.
 144 .PP
 145 For
 146 .BR ftruncate ()
 147 the same errors apply, but instead of things that can be wrong with
 148 .IR path ,
 149 we now have things that can be wrong with
 150 .IR fd :
 151 .TP
 152 .B EBADF
 153 The
 154 .I fd
 155 is not a valid descriptor.
 156 .TP
 157 .BR EBADF " or " EINVAL
 158 The
 159 .I fd
 160 is not open for writing.
 161 .TP
 162 .B EINVAL
 163 The
 164 .I fd
 165 does not reference a regular file.
 166 .SH "CONFORMING TO"
 167 4.4BSD, SVr4 (these function calls first appeared in 4.2BSD).
 168 POSIX 1003.1-1996 has
 169 .BR ftruncate ().
 170 POSIX 1003.1-2001 also has
 171 .BR truncate (),
 172 as an XSI extension.
 173 .LP
 174 SVr4 documents additional
 175 .BR truncate ()
 176 error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
 177 .BR ftruncate ()
 178 an additional EAGAIN error condition.
 179 .SH NOTES
 180 The above description is for XSI-compliant systems.
 181 For non-XSI-compliant systems, the POSIX standard allows
 182 two behaviours for
 183 .BR ftruncate ()
 184 when
 185 .I length
 186 exceeds the file length
 187 (note that
 188 .BR truncate ()
 189 is not specified at all in such an environment):
 190 either returning an error, or extending the file.
 191 Like most Unix implementations, Linux follows the XSI requirement
 192 when dealing with native file systems.
 193 However, some non-native file systems do not permit
 194 .BR truncate ()
 195 and
 196 .BR ftruncate ()
 197 to be used to extend a file beyond its current length:
 198 a notable example on Linux is VFAT.
 199 .\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
 200 .SH "SEE ALSO"
 201 .BR open (2),
 202 .BR path_resolution (2),
 203 .BR stat (2)