ftruncate

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  ex-
       tended 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
       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 be-
              yond 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 de-
       signed 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 5.05 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                             2019-03-06                       TRUNCATE(2)