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 (respectively, time of last
       status change and time of last modification; see inode(7)) 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(3).

       The  details in DESCRIPTION are for XSI-compliant systems.  For non-XSI-compliant systems,
       the POSIX standard allows two behaviors for  ftruncate()  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 versions.

SEE ALSO

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

COLOPHON

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