Ubuntu Manpage: truncate, ftruncate - truncate a file to a specified length

NAME

       truncate, ftruncate - truncate a file to a specified length

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       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

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

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 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.

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

       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

       POSIX.1-2008.

HISTORY

       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

       ftruncate() can also be used to set the size of a POSIX shared memory object; see shm_open(3).

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 versions.