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.\"     @(#)truncate.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: September 10 2015 $
.Dt TRUNCATE 2
.Os
.Sh NAME
.Nm truncate ,
.Nm ftruncate
.Nd truncate or extend a file to a specified length
.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
.Fn truncate
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
.Sh ERRORS
.Fn truncate
and
.Fn ftruncate
will succeed unless:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa length
is a negative value.
.It Bq Er EIO
An I/O error occurred updating the inode.
.El
.Pp
In addition,
.Fn truncate
may return the following errors:
.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 pathname (including the terminating NUL)
exceeded
.Dv PATH_MAX
bytes.
.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 EFAULT
.Fa path
points outside the process's allocated address space.
.El
.Pp
.Fn ftruncate
may return the following errors:
.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.
.It Bq Er EINVAL
The
.Fa fd
is not open for writing.
.El
.Sh SEE ALSO
.Xr open 2
.Sh STANDARDS
The
.Fn truncate
and
.Fn ftruncate
functions conform to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn truncate
and
.Fn ftruncate
system calls first appeared in
.Bx 4.1c .
.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.