truncate(2) — Linux manual page


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

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

           _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)) 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 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).)

       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

       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.

              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 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).

STANDARDS         top


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).

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

Linux man-pages (unreleased)     (date)                      truncate(2)

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