include "sys.m";
sys := load Sys Sys->PATH;

read:   fn(fd: ref FD, buf: array of byte, nbytes: int): int;
readn:  fn(fd: ref FD, buf: array of byte, nbytes: int): int;
write:  fn(fd: ref FD, buf: array of byte, nbytes: int): int;

pread:  fn(fd: ref FD, buf: array of byte, nbytes: int,
           offset: big): int;
pwrite: fn(fd: ref FD, buf: array of byte, nbytes: int,
           offset: big): int;

stream: fn(src, dst: ref FD, bufsiz: int): int;

Readn continues to read from fd sequentially into buf, until nbytes have been read, or read returns a non-positive count.

Write writes nbytes bytes of data starting at buf to the file associated with fd at the file offset. The offset is advanced by the number of bytes written. The number of bytes actually written is returned. It should be regarded as an error if this is not the same as requested.

Pread and pwrite take an explicit file offset as a parameter, leaving fd's current offset untouched; they are otherwise identical in behaviour to read and write. They are particulary useful when several processes must access the same fd concurrently and it is inconvenient or undesirable to synchronise their activity to avoid interference.

Stream continually reads data from src, using a buffer of bufsiz bytes, and writes the data to dst. It copies data until a read fails (returning zero bytes or an error) or a write fails. Stream returns the number of bytes actually copied. The implementation may be more efficient than a read/write loop in the application, but is otherwise equivalent to calling read and write directly.