3BSD/usr/man/man2/vwrite.2

.UC
.TH VWRITE 2
.SH NAME
vwrite \- write (virtually) to file
.SH SYNOPSIS
\fBvwrite(filedes, buffer, nbytes)\fR
.br
\fBchar *buffer;\fR
.SH DESCRIPTION
The
.I vwrite
system call is used in conjunction with
.I vread
to perform efficient updating of large files.
After a call to
.I vread
and updating of the data in the buffer which was given to
.I vread,
a
.I vwrite
of the same buffer to the same
.I filedes
at the same offset in the file
will cause data which has been modified since it was
.I vread
from (or
.I vwritten
to) the file to be returned to the file.
.SH SEE ALSO
vread(2)
.SH DIAGNOSTICS
Returns \-1 on error: bad descriptor, buffer address, count or alignment
as well as on physical I/O errors.
.SH BUGS
The result of
.I vwrite
is defined only when no other
.I vread's
have occured on
.I buffer
since the one matching the
.I vwrite.