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+.TH IOPROC 2
+.SH NAME
+closeioproc,
+iocall,
+ioclose,
+iointerrupt,
+iodial,
+ioopen,
+ioproc,
+ioread,
+ioreadn,
+iowrite \- slave I/O processes for threaded programs
+.SH SYNOPSIS
+.PP
+.de XX
+.ift .sp 0.5
+.ifn .sp
+..
+.EX
+.ta \w'Ioproc* 'u
+#include <u.h>
+#include <libc.h>
+#include <thread.h>
+.sp
+typedef struct Ioproc Ioproc;
+.sp
+Ioproc*	ioproc(void);
+.XX
+int	ioopen(Ioproc *io, char *file, int omode);
+int	ioclose(Ioproc *io, int fd);
+long	ioread(Ioproc *io, int fd, void *a, long n);
+long	ioreadn(Ioproc *io, int fd, void *a, long n);
+long	iowrite(Ioproc *io, int fd, void *a, long n);
+int	iodial(Ioproc *io, char *addr, char *local, char *dir, char *cdfp);
+.XX
+void	iointerrupt(Ioproc *io);
+void	closeioproc(Ioproc *io);
+.XX
+long	iocall(Ioproc *io, long (*op)(va_list *arg), ...);
+.EE
+.SH DESCRIPTION
+.PP
+These routines provide access to I/O in slave procs.
+Since the I/O itself is done in a slave proc, other threads
+in the calling proc can run while the calling thread
+waits for the I/O to complete.
+.PP
+.I Ioproc
+forks a new slave proc and returns a pointer to the
+.B Ioproc
+associated with it.
+.I Ioproc
+uses
+.I mallocz
+and
+.IR proccreate ;
+if either fails, it calls
+.I sysfatal
+rather than return an error.
+.PP
+.IR Ioopen ,
+.IR ioclose ,
+.IR ioread ,
+.IR ioreadn ,
+.IR iowrite ,
+and
+.IR iodial
+are execute the
+similarly named library or system calls
+(see
+.IR open (2),
+.IR read (2),
+and
+.IR dial (2))
+in the slave process associated with
+.IR io .
+It is an error to execute more than one call
+at a time in an I/O proc.
+.PP
+.I Iointerrupt
+interrupts the call currently executing in the I/O proc.
+If no call is executing,
+.IR iointerrupt
+is a no-op.
+.PP
+.I Closeioproc
+terminates the I/O proc and frees the associated
+.B Ioproc .
+.PP
+.I Iocall
+is a primitive that may be used to implement
+more slave I/O routines.
+.I Iocall
+arranges for
+.I op
+to be called in
+.IR io 's
+proc, with
+.I arg
+set to the variable parameter list,
+returning the value that
+.I op
+returns.
+.SH EXAMPLE
+Relay messages between two file descriptors,
+counting the total number of bytes seen:
+.IP
+.EX
+.ta +\w'xxxx'u +\w'xxxx'u +\w'xxxx'u
+int tot;
+
+void
+relaythread(void *v)
+{
+	int *fd, n;
+	char buf[1024];
+	Ioproc *io;
+
+	fd = v;
+	io = ioproc();
+	while((n = ioread(io, fd[0], buf, sizeof buf)) > 0){
+		if(iowrite(io, fd[1], buf, n) != n)
+			sysfatal("iowrite: %r");
+		tot += n;
+	}
+	closeioproc(io);
+}
+
+void
+relay(int fd0, int fd1)
+{
+	int fd[4];
+
+	fd[0] = fd[3] = fd0;
+	fd[1] = fd[2] = fd1;
+	threadcreate(relaythread, fd, 8192);
+	threadcreate(relaythread, fd+2, 8192);
+}
+.EE
+.LP
+If the two
+.I relaythread
+instances were running in different procs, the
+common access to
+.I tot
+would be unsafe.
+.EE
+.PP
+Implement
+.IR ioread :
+.IP
+.EX
+static long
+_ioread(va_list *arg)
+{
+	int fd;
+	void *a;
+	long n;
+
+	fd = va_arg(*arg, int);
+	a = va_arg(*arg, void*);
+	n = va_arg(*arg, long);
+	return read(fd, a, n);
+}
+
+long
+ioread(Ioproc *io, int fd, void *a, long n)
+{
+	return iocall(io, _ioread, fd, a, n);
+}
+.EE
+.SH SOURCE
+.B /sys/src/libthread/io*.c
+.SH SEE ALSO
+.IR dial (2),
+.IR open (2),
+.IR read (2),
+.IR thread (2)
+