man/man2/truncate.2

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.\"     @(#)truncate.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 3, 2021
.Dt TRUNCATE 2
.Os BSD 4.2
.Sh NAME
.Nm ftruncate ,
.Nm truncate
.Nd truncate or extend a file to a specified length
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fo ftruncate
.Fa "int fildes"
.Fa "off_t length"
.Fc
.Ft int
.Fo truncate
.Fa "const char *path"
.Fa "off_t length"
.Fc
.Sh DESCRIPTION
.Fn ftruncate
and
.Fn truncate
cause the file named by
.Fa path ,
or referenced by
.Fa fildes ,
to be truncated (or extended) to
.Fa length
bytes in size. If the file size exceeds
.Fa length ,
any extra data is discarded. If the file size is smaller than
.Fa length ,
the file is extended and filled with zeros to the indicated length.
The
.Fn ftruncate
form requires the file to be open for writing.
.Pp
Note:
.Fn ftruncate
and
.Fn truncate
do not modify the current file offset for any open file descriptions associated with 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
.Pp
The
.Fn ftruncate
system call will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EBADF
.Fa fildes
is not a valid descriptor open for writing.
.\" ===========
.It Bq Er EFBIG
The file is a regular file and
.Fa length
is greater than the offset maximum established
in the open file description associated with
.Fa fildes .
.\" ===========
.It Bq Er EINVAL
.Fa fildes
references a socket, not a file.
.\" ===========
.It Bq Er EINVAL
.Fa fildes
is not open for writing.
.\" ===========
.It Bq Er EROFS
The named file resides on a read-only file system.
.El
.Pp
The
.Fn truncate
system call will fail if:
.Bl -tag -width Er
.\" ===========
.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 EFAULT
.Fa Path
points outside the process's allocated address space.
.\" ===========
.It Bq Er EISDIR
The named file is a directory.
.\" ===========
.It Bq Er ELOOP
Too many symbolic links are encountered in translating the pathname.
This is taken to be indicative of a looping symbolic link.
.\" ===========
.It Bq Er ENAMETOOLONG
A component of a pathname exceeds
.Dv {NAME_MAX}
characters, or an entire path name exceeds
.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.
.\" ===========
.It Bq Er EDEADLK
A component of the pathname refers to a
.Dq dataless
directory that requires materialization and the I/O policy of the current
thread or process disallows dataless directory materialization
.Po see
.Xr getiopolicy_np 3
.Pc .
.\" ===========
.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
The
.Fn ftruncate
and
.Fn truncate
system calls will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EFBIG
The length argument was greater than the maximum file size.
.\" ===========
.It Bq Er EINTR
A signal is caught during execution.
.\" ===========
.It Bq Er EINVAL
The
.Fa length
argument is less than 0.
.\" ===========
.It Bq Er EIO
An I/O error occurred while reading from or writing to a file system.
.\" ===========
.It Bq Er EDEADLK
The file is a
.Dq dataless
file that requires materialization and the I/O policy of the current thread
or process disallows dataless file materialization
.Po see
.Xr getiopolicy_np 3
.Pc .
.El
.Sh SEE ALSO
.Xr open 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.
.Sh HISTORY
The
.Fn truncate
and
.Fn ftruncate
function calls appeared in
.Bx 4.2 .