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.\"     @(#)truncate.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd June 1, 2006
.Dt TRUNCATE 2
.Os
.Sh NAME
.Nm truncate ,
.Nm ftruncate
.Nd truncate or extend a file to a specified length
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn truncate "const char *path" "off_t length"
.Ft int
.Fn ftruncate "int fd" "off_t length"
.Sh DESCRIPTION
The
.Fn truncate
system call
causes the file named by
.Fa path
or referenced by
.Fa fd
to be truncated or extended to
.Fa length
bytes in size.
If the file
was larger than this size, the extra data
is lost.
If the file was smaller than this size,
it will be extended as if by writing bytes
with the value zero.
With
.Fn ftruncate ,
the file must be open for writing.
.Sh RETURN VALUES
.Rv -std
If the file to be modified is not a directory or
a regular file, the
.Fn truncate
call has no effect and returns the value 0.
.Sh ERRORS
The
.Fn truncate
system call
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 255 characters,
or an entire path name exceeded 1023 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.
.It Bq Er EACCES
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
The
.Fa path
argument
points outside the process's allocated address space.
.El
.Pp
The
.Fn ftruncate
system call
succeeds unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument
is not a valid descriptor.
.It Bq Er EINVAL
The
.Fa fd
argument
references a socket, not a file.
.It Bq Er EINVAL
The
.Fa fd
descriptor
is not open for writing.
.El
.Sh SEE ALSO
.Xr open 2
.Sh HISTORY
The
.Fn truncate
system call 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.