closeioproc, iocall, ioclose, iointerrupt, iodial, ioopen, ioproc, ioread, ioreadn, iowrite – slave I/O processes for threaded programs


#include <u.h>
#include <libc.h>
#include <thread.h>
typedef struct Ioproc Ioproc;
Ioproc* ioproc(void);
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);
void      iointerrupt(Ioproc *io);
void      closeioproc(Ioproc *io);
long      iocall(Ioproc *io, long (*op)(va_list *arg), ...);


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.

Ioproc forks a new slave proc and returns a pointer to the Ioproc associated with it. Ioproc uses mallocz and proccreate; if either fails, it calls sysfatal rather than return an error.

Ioopen, ioclose, ioread, ioreadn, iowrite, and iodial execute the similarly named library or system calls (see open(2), read(2), and dial(2)) in the slave process associated with io. It is an error to execute more than one call at a time in an I/O proc.

Iointerrupt interrupts the call currently executing in the I/O proc. If no call is executing, iointerrupt is a no–op.

Closeioproc terminates the I/O proc and frees the associated Ioproc .

Iocall is a primitive that may be used to implement more slave I/O routines. Iocall arranges for op to be called in io's proc, with arg set to the variable parameter list, returning the value that op returns.

Relay messages between two file descriptors, counting the total number of bytes seen:
int tot;
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;
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);

If the two relaythread instances were running in different procs, the common access to tot would be unsafe.

Implement ioread:
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);
ioread(Ioproc *io, int fd, void *a, long n)
return iocall(io, _ioread, fd, a, n);


dial(2), open(2), read(2), thread(2)
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