V4/man/man2/write.2

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.th WRITE II 8/5/73
.sh NAME
write \*- write on a file
.sh SYNOPSIS
(write = 4.)
.br
(file descriptor in r0)
.br
.ft B
sys  write; buffer; nbytes
.s3
write(fildes, buffer, nbytes)
.br
char *buffer;
.ft R
.sh DESCRIPTION
A file descriptor is a word returned from a
successful
.it open,
.it creat
or
.it pipe
call.
.s3
.it Buffer
is the address of
.it nbytes
contiguous
bytes which are written on the output file.
The number of characters actually written is returned (in r0).
It should be regarded as an error
if this is not the same as requested.
.s3
Writes which are multiples
of 512 characters long and begin on a 512-byte boundary
are more efficient than any others.
.sh "SEE ALSO"
creat(II), open(II), pipe(II)
.sh DIAGNOSTICS
The error bit
(c-bit) is set on an error: bad descriptor, buffer address, or
count; physical I/O errors.
From C, a returned value of \*-1 indicates an error.