ftruncate(2) — Linux manual page


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

NAME         top

       truncate, ftruncate - truncate a file to a specified length

SYNOPSIS         top

       #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)):

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

           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
               || /* Glibc versions <= 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)) 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 is
       set appropriately.

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).)

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

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

       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

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

       ENOENT The named file does not exist.

              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.

              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

       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         top

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

NOTES         top

       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         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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2019-03-06                      TRUNCATE(2)

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