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

       truncate, ftruncate - truncate a file to a specified length

       #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)):

           _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

       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

       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.

       On success, zero is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

       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

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

       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

       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.)

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

       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

       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).

       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.

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

       This page is part of release 5.13 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                              2021-03-22                        TRUNCATE(2)