truncate


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():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

       ftruncate():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L

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
       permission 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 Path points outside the process's allocated address space.

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


       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 file system does not  support  extending  a  file
              beyond its current size.

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

       ETXTBSY
              The  file  is  a pure procedure (shared text) 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 descriptor.

       EBADF or EINVAL
              fd is not open for writing.

       EINVAL fd does not reference a regular file.

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

NOTES
       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 file systems.  However, some
       nonnative file systems 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.

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

Linux                             2011-09-08                       TRUNCATE(2)
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