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.\"     @(#)truncate.2	8.1 (Berkeley) 6/4/93
.\"
.Dd July 23, 2014
.Dt TRUNCATE 2
.Os
.Sh NAME
.Nm truncate ,
.Nm ftruncate
.Nd truncate 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
.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
Error return codes common to
.Fn truncate
and
.Fn ftruncate
are:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa length
argument was less than 0.
.It Bq Er EIO
An I/O error occurred updating the inode.
.It Bq Er EISDIR
The named file is a directory.
.It Bq Er ENOSPC
There was no space in the file system to complete the operation.
.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.
.El
.Pp
Error codes specific to
.Fn truncate
are:
.Bl -tag -width Er
.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 EFAULT
.Fa path
points outside the process's allocated address space.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Brq Dv NAME_MAX
characters, or an entire path name exceeded
.Brq Dv PATH_MAX
characters.
.It Bq Er ENOENT
The named file does not exist.
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.El
.Pp
Error codes specific to
.Fn ftruncate
are:
.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 fdiscard 2 ,
.Xr open 2
.Sh STANDARDS
Use of
.Fn truncate
to extend a file is an
.St -p1003.1-2004
extension, and is thus not portable.
Files can be extended in a portable way seeking (using
.Xr lseek 2 )
to the required size and writing a single character with
.Xr write 2 .
.Sh HISTORY
The
.Fn truncate
and
.Fn ftruncate
function calls appeared in
.Bx 4.2 .