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WRITE(2) System Calls Manual WRITE(2)

write - write on a file

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

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.

creat(2), open(2), pipe(2)

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

(write = 4.)
 
(file descriptor in r0)
 
sys write; buffer; nbytes
 
(byte count in r0)
UNIX-7