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read: add -c flag to copy bytes instead of lines

--- a/sys/man/1/cat

+++ b/sys/man/1/cat

@@ -12,8 +12,11 @@

 .B -m

 ] [

 .B -n

-.I nline

+.I nlines

 ] [

+.B -c

+.I nbytes

+] [

 .I file ...

 ]

 .SH DESCRIPTION

@@ -54,8 +57,20 @@

 flag causes it to continue reading and writing multiple lines until end of file;

 .B -n

 causes it to read no more than

-.I nline

+.I nlines

 lines.

+.PP

+With the

+.B -c

+flag,

+.I read

+copies exactly

+.I nbytes

+of characters instead of lines. It is mutually exclusive with

+.B -n

+and

+.B -m

+flag.

 .PP

 .I Read

 always executes a single

--- a/sys/src/cmd/read.c

+++ b/sys/src/cmd/read.c

@@ -3,6 +3,7 @@

 int	multi;

 int	nlines;

+vlong	nchars;

 char	*status = nil;

 int

@@ -53,29 +54,60 @@

 }

 void

+chars(int fd, char *file)

+{

+	char buf[8*1024];

+	vlong m;

+	int n;

+

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

+		n = sizeof(buf);

+		if(n > (nchars - m))

+			n = nchars - m;

+		if((n = read(fd, buf, n)) < 0){

+			fprint(2, "read: error reading %s: %r\n", file);

+			exits("read error");

+		}

+		if(n == 0){

+			if(m == 0)

+				status = "eof";

+			break;

+		}

+		write(1, buf, n);

+	}

+}

+

+void

+usage(void)

+{

+	fprint(2, "usage: read [-m] [-n nlines] [-c nbytes] [files...]\n");

+	exits("usage");

+}

+

+void

 main(int argc, char *argv[])

 {

+	void (*proc)(int, char*);

 	int i, fd;

-	char *s;

+	proc = lines;

 	ARGBEGIN{

+	case 'c':

+		nchars = atoll(EARGF(usage()));

+		proc = chars;

+		break;

+	case 'n':

+		nlines = atoi(EARGF(usage()));

+		break;

 	case 'm':

 		multi = 1;

 		break;

-	case 'n':

-		s = ARGF();

-		if(s){

-			nlines = atoi(s);

-			break;

-		}

-		/* fall through */

 	default:

-		fprint(2, "usage: read [-m] [-n nlines] [files...]\n");

-		exits("usage");

+		usage();

 	}ARGEND

 	if(argc == 0)

-		lines(0, "<stdin>");

+		(*proc)(0, "<stdin>");

 	else

 		for(i=0; i<argc; i++){

 			fd = open(argv[i], OREAD);

@@ -83,7 +115,7 @@

 				fprint(2, "read: can't open %s: %r\n", argv[i]);

 				exits("open");

 			}

-			lines(fd, argv[i]);

+			(*proc)(fd, argv[i]);

 			close(fd);

 		}