v7 man page for write

WRITE(2)							System Calls Manual							  WRITE(2)

NAME
       write - write on a file

SYNOPSIS
       write(fildes, buffer, nbytes)
       char *buffer;

DESCRIPTION
       A file descriptor is a word returned from a successful open, creat, dup, or pipe(2) call.

       Buffer  is  the	address  of  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.

       Writes which are multiples of 512 characters long and begin on a 512-byte boundary in the file are more efficient than any others.

SEE ALSO
       creat(2), open(2), pipe(2)

DIAGNOSTICS
       Returns -1 on error: bad descriptor, buffer address, or count; physical I/O errors.

ASSEMBLER
       (write = 4.)
       (file descriptor in r0)
       sys write; buffer; nbytes
       (byte count in r0)

																	  WRITE(2)