TRUNCATE(3C) Standard C Library Functions TRUNCATE(3C)

NAME

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(7).

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

chmod(2), fcntl(2), open(2), attributes(7), lf64(7), standards(7)

April 5, 2002 TRUNCATE(3C)