ftruncate(), ftruncate64()

Truncate a file

Synopsis:

#include <unistd.h>

int ftruncate( int fildes,
               off_t length );

int ftruncate64( int fildes,
                 off64_t length );

Arguments:

fildes: The descriptor for the file that you want to truncate.
length: The length that you want the file to be, in bytes.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The ftruncate() and ftruncate64() functions cause the file referenced by fildes to have a size of length bytes. The ftruncate64() function is a large-file support version of ftruncate().

Note: In QNX Neutrino 6.6 or later, the large-file support functions and data types appear in the name space only if you define _LARGEFILE64_SOURCE when you compile your code. For more information, see “Classification” in What's in a Function Description?

If the size of the file previously exceeded length, the extra data is discarded (this is similar to using the F_FREESP option with fcntl()). If the size of the file was previously shorter than length, the file size is extended with NUL characters (similar to the F_ALLOCSP option to fcntl()).

These functions don't modify the value of the seek pointer.

Upon successful completion, the ftruncate() function marks the st_ctime and st_mtime fields of the file for update. If the ftruncate() function is unsuccessful, the file is unaffected.

Returns:

Zero for success, or -1 if an error occurred (errno is set).

Errors:

EBADF: The fildes argument isn't a valid file descriptor.
EFBIG: The file is a regular file and length is greater than the offset maximum associated with the file.
EINTR: A signal was caught during the call to ftruncate().
EINVAL: The fildes argument doesn't refer to a file on which this operation is possible, the filedes argument isn't open for writing or the length argument is less than the minimum file size for the specified filesystem.
EIO: An I/O error occurred while reading from or writing to the filesystem.
ENOMEM: There wasn't enough memory to change the size of a shared memory object.
ENOSYS: The ftruncate() function isn't implemented for the filesystem specified by filedes.
ENOTSUP: The ftruncate() function is implemented for the specified filesystem, but the specific operation (F_ALLOCSP or F_FREESP; see fcntl()) isn't supported.
EROFS: The file resides on a read-only filesystem.

Classification:

ftruncate() is POSIX 1003.1; ftruncate64() is Large-file support

Safety:
Cancellation point	No
Interrupt handler	No
Signal handler	Yes
Thread	Yes