truncate

, ftruncate

- set a file to a specified length

Synopsis

#include <unistd.h>

int truncate(const char *path, off_t length);

int ftruncate(int fildes, off_t length);

Description

The truncate() function causes the regular file named by path to have a size equal to length bytes.

If the file previously was larger than length, the extra data is discarded. If the file was previously shorter than length, its size is increased, and the extended area appears as if it were zero-filled.

The application must ensure that the process has write permission for the file.

This function does not modify the file offset for any open file descriptions associated with the file.

The ftruncate() function causes the regular file referenced by fildes to be truncated to length. If the size of the file previously exceeded length, the extra data is no longer available to reads on the file. If the file previously was smaller than this size, ftruncate() increases the size of the file with the extended area appearing as if it were zero-filled. The value of the seek pointer is not modified by a call to ftruncate().

The ftruncate() function works only with regular files and shared memory. If fildes refers to a shared memory object, ftruncate() sets the size of the shared memory object to length. If fildes refers to a directory or is not a valid file descriptor open for writing, ftruncate() fails.

If the effect of ftruncate() is to decrease the size of a shared memory object or memory mapped file and whole pages beyond the new end were previously mapped, then the whole pages beyond the new end shall be discarded.

If the effect of ftruncate() is to increase the size of a shared memory object, it is unspecified if the contents of any mapped pages between the old end-of-file and the new are flushed to the underlying object.

These functions do not modify the file offset for any open file descriptions associated with the file. On successful completion, if the file size is changed, these functions will mark for update the st_ctime and st_mtime fields of the file, and if the file is a regular file, the S_ISUID and S_ISGID bits of the file mode are left unchanged.

If the request would cause the file size to exceed the soft file size limit for the process, the request will fail and a SIGXFSZ signal will be generated for the process.

Return Values

Upon successful completion, ftruncate() and truncate() return 0. Otherwise, -1 is returned and errno is set to indicate the error.

Errors

The ftruncate() and truncate() functions will fail if:

EINTR

A signal was caught during execution.

EINVAL

The length argument was less than 0.

EFBIG or EINVAL

The length argument was greater than the maximum file size.

EIO

An I/O error occurred while reading from or writing to a file system.

EROFS

The named file resides on a read-only file system.

The truncate() function will fail if:

EACCES

A component of the path prefix denies search permission, or write permission is denied on the file.

EFAULT

The path argument points outside the process' allocated address space.

EINVAL

The path argument is not an ordinary file.

EISDIR

The named file is a directory.

ELOOP

Too many symbolic links were encountered in resolving path.

EMFILE

The maximum number of file descriptors available to the process has been reached.

ENAMETOOLONG

The length of the specified pathname exceeds {PATH_MAX} bytes, or the length of a component of the pathname exceeds {NAME_MAX} bytes.

ENOENT

A component of path does not name an existing file or path is an empty string.

ENFILE

Additional space could not be allocated for the system file table.

ENOTDIR

A component of the path prefix of path is not a directory.

ENOLINK

The path argument points to a remote machine and the link to that machine is no longer active.

The ftruncate() function will fail if:

EAGAIN

The file exists, mandatory file/record locking is set, and there are outstanding record locks on the file (see chmod(2)).

EBADF or EINVAL

The fildes argument is not a file descriptor open for writing.

EFBIG

The file is a regular file and length is greater than the offset maximum established in the open file description associated with fildes.

EINVAL

The fildes argument references a file that was opened without write permission.

EINVAL

The fildes argument does not correspond to an ordinary file.

ENOLINK

The fildes argument points to a remote machine and the link to that machine is no longer active.

The truncate() function may fail if:

ENAMETOOLONG

Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.

Usage

The truncate() and ftruncate() functions have transitional interfaces for 64-bit file offsets. See lf64(5).

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

chmod(2), fcntl(2), open(2), attributes(5), lf64(5), standards(5)