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.)

STANDARDS

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

NOTES

       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

       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.