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.\"     @(#)truncate.2	8.1 (Berkeley) 6/4/93
.\"
.Dd August 18, 2002
.Dt TRUNCATE 2
.Os
.Sh NAME
.Nm truncate ,
.Nm ftruncate
.Nd truncate a file to a specified length
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include \*[Lt]unistd.h\*[Gt]
.Ft int
.Fn truncate "const char *path" "off_t length"
.Ft int
.Fn ftruncate "int fd" "off_t length"
.Sh DESCRIPTION
.Fn truncate
causes the file named by
.Fa path
or referenced by
.Fa fd
to have a size of
.Fa length
bytes.
If the file previously was larger than this size, the extra data is discarded.
If it was previously shorter than
.Fa length ,
its size is increased to the specified value and
the extended area appears as if it were zero-filled.
.Pp
With
.Fn ftruncate ,
the file must be open for writing; for
.Fn truncate ,
the process must have write permissions for the file.
.Sh RETURN VALUES
A value of 0 is returned if the call succeeds.
If the call fails a -1 is returned, and the global variable
.Va errno
specifies the error.
.Sh ERRORS
.Fn truncate
succeeds unless:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.It Bq Er ENOENT
The named file does not exist.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix, or
the named file is not writable by the user.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EISDIR
The named file is a directory.
.It Bq Er EROFS
The named file resides on a read-only file system.
.It Bq Er ETXTBSY
The file is a pure procedure (shared text) file that is being executed.
.It Bq Er EIO
An I/O error occurred updating the inode.
.It Bq Er EFAULT
.Fa path
points outside the process's allocated address space.
.El
.Pp
.Fn ftruncate
succeeds unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
is not a valid descriptor.
.It Bq Er EINVAL
The
.Fa fd
references a socket, not a file, or
the
.Fa fd
is not open for writing.
.El
.Sh SEE ALSO
.Xr open 2
.Sh HISTORY
The
.Fn truncate
and
.Fn ftruncate
function calls appeared in
.Bx 4.2 .
.Sh BUGS
These calls should be generalized to allow ranges
of bytes in a file to be discarded.
.Pp
Use of
.Fn truncate
to extend a file is not portable.