= read, write, pread, pwrite, stream: read or write file     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; Read  reads  nbytes bytes of data from the offset in the fileassociated with fd into memory at buf.  The  file  offset  isadvanced  by  the number of bytes read.  It is not guaranteedthat all nbytes bytes will be read; for example if  the  filerefers to the console, at most one line will be returned.  Inany event the number of bytes read  is  returned.   A  returnvalue of 0 is conventionally interpreted as end of file. Readn  continues to read from fd sequentially into buf, untilnbytes have been read, or read returns a non-positive count. Write writes nbytes bytes of data starting at buf to the fileassociated  with  fd  at  the  file  offset.   The  offset isadvanced by the number of bytes written.  The number of bytesactually  written  is  returned.  It should be regarded as anerror 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 otherwiseidentical in behaviour to read and write.  They are  particu‐lary  useful  when  several processes must access the same fdconcurrently and it is inconvenient or  undesirable  to  syn‐chronise their activity to avoid interference. Stream  continually  reads  data  from src, using a buffer ofbufsiz bytes, and writes the data to  dst.   It  copies  datauntil  a  read  fails (returning zero bytes or an error) or awrite fails.  Stream returns the  number  of  bytes  actuallycopied.   The  implementation  may  be  more efficient than aread/write loop in the application, but is otherwise  equiva‐lent to calling read and write directly. == see ALSObufio(2), sys-intro(2), sys-dup(2), sys-open(2), read(5)