V8/usr/man/man2/write.2

.TH WRITE 2 
.SH NAME
write \- write on a file
.SH SYNOPSIS
.nf
.B write(fildes, buffer, nbytes)
.B char *buffer;
.fi
.SH DESCRIPTION
.I Write
writes data on the open file associated with the file descriptor.
.PP
.I Buffer
is the address of
.I nbytes
contiguous
bytes which are written on the output file.
The number of characters actually written is returned.
It should be regarded as an error
if this is not the same as requested.
.PP
If the open file is being read by another process,
data of at most one
.I write
will be delivered to any
.IR read (2)
in the other process.
In particular, if 
.I nbytes
= 0, the 
.I read
will return a count of 0, which is conventionally taken
to mean end of file.
.PP
Writes which are synchronized with file system blocks
are more efficient than others; see
.IR filsys (5).
.PP
On some systems
.I write
clears the set-user-id bit on a file.
This prevents penetration of system security
by a user who
captures a writable set-user-id file that he does not own.
.SH "SEE ALSO"
creat(2), open(2), pipe(2), select(2)
.SH DIAGNOSTICS
Returns
\-1 on error: bad descriptor, buffer address, or
count; physical I/O errors.