truncate(2) - Linux manual page (original) (raw)

truncate(2) System Calls Manual truncate(2)

NAME top

   truncate, ftruncate - truncate a file to a specified length

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <unistd.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 <= 2.19: */ _BSD_SOURCE

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

DESCRIPTION top

   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)](../man7/inode.7.html)) 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 top

   On success, zero is returned.  On error, -1 is returned, and
   _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   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)](../man7/path%5Fresolution.7.html).)

   **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)](../man2/fcntl.2.html) and
          [signal(7)](../man7/signal.7.html).

   **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)](../man2/fcntl.2.html).

   **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**.)

VERSIONS top

   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.

   On some 32-bit architectures, the calling signature for these
   system calls differ, for the reasons described in [syscall(2)](../man2/syscall.2.html).

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001, 4.4BSD, SVr4 (first appeared in 4.2BSD).

   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.

NOTES top

   **ftruncate**() can also be used to set the size of a POSIX shared
   memory object; see [shm_open(3)](../man3/shm%5Fopen.3.html).

BUGS top

   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 top

   [truncate(1)](../man1/truncate.1.html), [open(2)](../man2/open.2.html), [stat(2)](../man2/stat.2.html), [path_resolution(7)](../man7/path%5Fresolution.7.html)

COLOPHON top

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Linux man-pages 6.9.1 2024-06-15 truncate(2)

Pages that refer to this page:truncate(1), fallocate(2), fcntl(2), fsync(2), getrlimit(2), memfd_create(2), memfd_secret(2), mmap(2), syscall(2), syscalls(2), io_uring_prep_ftruncate(3), off_t(3type), shm_open(3), inode(7), inotify(7), landlock(7), shm_overview(7), signal-safety(7), xfs_io(8)