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hjfs: eleminate seek syscalls

reduce syscalls by using pread/pwrite instead
of seek/read/write.

--- a/sys/src/cmd/hjfs/dev.c

+++ b/sys/src/cmd/hjfs/dev.c

@@ -30,20 +30,27 @@

 			b->error = Einval;

 			goto reply;

 		}

-		seek(d->fd, b->off * BLOCK, 0);

 		b->error = nil;

 		if(b->op & BWRITE){

 			memset(buf, 0, sizeof(buf));

 			pack(b, buf);

-			if(write(d->fd, buf, BLOCK) < BLOCK){

+			if(pwrite(d->fd, buf, BLOCK, b->off*BLOCK) < BLOCK){

 				dprint("hjfs: write: %r\n");

 				b->error = Eio;

 			}

 		}else{

-			if(readn(d->fd, buf, BLOCK) < 0){

-				dprint("hjfs: read: %r\n");

+			int n, m;

+

+			for(n = 0; n < BLOCK; n += m){

+				m = pread(d->fd, buf+n, BLOCK-n, b->off*BLOCK+n);

+				if(m < 0)

+					dprint("hjfs: read: %r\n");

+				if(m <= 0)

+					break;

+			}

+			if(n < BLOCK)

 				b->error = Eio;

-			}else

+			else

 				unpack(b, buf);

 		}

 	reply:

-- 

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