TRUNCATE(2)		   Linux Programmer's Manual		   TRUNCATE(2)



NAME
       truncate, ftruncate - truncate a file to a specified length

SYNOPSIS
       #include <unistd.h>
       #include <sys/types.h>

       int truncate(const char *path, off_t length);
       int ftruncate(int fd, off_t length);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       truncate():
	   _XOPEN_SOURCE >= 500
	       || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE

       ftruncate():
	   _XOPEN_SOURCE >= 500
	       || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION
       The  truncate()	and ftruncate() functions cause the regular file named
       by path or referenced by fd to be truncated  to	a  size	 of  precisely
       length bytes.

       If  the	file  previously  was larger than this size, the extra data is
       lost.  If the file previously was shorter,  it  is  extended,  and  the
       extended part reads as null bytes ('\0').

       The file offset is not changed.

       If  the	size  changed,	then the st_ctime and st_mtime fields (respec-
       tively, time of last status change and time of last  modification;  see
       stat(2)) for the file are updated, and the set-user-ID and set-group-ID
       mode bits may be cleared.

       With ftruncate(), the file must be open for writing;  with  truncate(),
       the file must be writable.

RETURN VALUE
       On  success,  zero is returned.	On error, -1 is returned, and errno is
       set appropriately.

ERRORS
       For truncate():

       EACCES Search permission is denied for a component of the path  prefix,
	      or  the  named  file  is	not  writable  by the user.  (See also
	      path_resolution(7).)

       EFAULT The argument path points outside the process's allocated address
	      space.

       EFBIG  The argument length is larger than the maximum file size. (XSI)

       EINTR  While blocked waiting to complete, the call was interrupted by a
	      signal handler; see fcntl(2) and signal(7).

       EINVAL The argument length is negative or larger than the maximum  file
	      size.

       EIO    An I/O error occurred updating the inode.

       EISDIR The named file is a directory.

       ELOOP  Too  many	 symbolic  links  were	encountered in translating the
	      pathname.

       ENAMETOOLONG
	      A component of a pathname exceeded 255 characters, or an	entire
	      pathname exceeded 1023 characters.

       ENOENT The named file does not exist.

       ENOTDIR
	      A component of the path prefix is not a directory.

       EPERM  The  underlying  filesystem  does	 not  support extending a file
	      beyond its current size.

       EPERM  The operation was prevented by a file seal; see fcntl(2).

       EROFS  The named file resides on a read-only filesystem.

       ETXTBSY
	      The file is an executable file that is being executed.

       For ftruncate() the same errors apply, but instead of things  that  can
       be  wrong with path, we now have things that can be wrong with the file
       descriptor, fd:

       EBADF  fd is not a valid file descriptor.

       EBADF or EINVAL
	      fd is not open for writing.

       EINVAL fd does not reference a regular file or a	 POSIX	shared	memory
	      object.

       EINVAL or EBADF
	      The  file descriptor fd is not open for writing.	POSIX permits,
	      and portable applications should handle, either error  for  this
	      case.  (Linux produces EINVAL.)

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.4BSD, SVr4 (these calls first appeared in
       4.2BSD).

NOTES
       ftruncate() can also be used to set the size of a POSIX	shared	memory
       object; see shm_open(7).

       The details in DESCRIPTION are for XSI-compliant systems.  For non-XSI-
       compliant systems, the POSIX standard allows two behaviors  for	ftrun-
       cate() when length exceeds the file length (note that truncate() is not
       specified at all in such an environment): either returning an error, or
       extending  the file.  Like most UNIX implementations, Linux follows the
       XSI requirement when dealing with native	 filesystems.	However,  some
       nonnative  filesystems  do  not permit truncate() and ftruncate() to be
       used to extend a file beyond its current length: a notable  example  on
       Linux is VFAT.

       The  original  Linux  truncate()	 and ftruncate() system calls were not
       designed to handle large file offsets.  Consequently, Linux  2.4	 added
       truncate64()  and  ftruncate64()	 system calls that handle large files.
       However, these details can be  ignored  by  applications	 using	glibc,
       whose  wrapper  functions  transparently	 employ the more recent system
       calls where they are available.

       On some 32-bit architectures, the calling signature  for	 these	system
       calls differ, for the reasons described in syscall(2).

BUGS
       A  header  file	bug  in	 glibc	2.12  meant  that the minimum value of
       _POSIX_C_SOURCE required to expose the declaration of  ftruncate()  was
       200809L	instead	 of  200112L.  This has been fixed in later glibc ver-
       sions.

SEE ALSO
       truncate(1), open(2), stat(2), path_resolution(7)

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			   TRUNCATE(2)