dj.1: elaborate on skip/seek behavior, provide another example

docs/dj.1

@@ -4,7 +4,7 @@
 .\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
 .\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
 .\"
-.TH DJ 1 2024-06-29 "Harakit X.X.X"
+.TH DJ 1 2024-07-03 "Harakit X.X.X"
 .SH NAME
 dj \(en disk jockey
 .\"
@@ -34,10 +34,17 @@ respectively. This language is inherited from the
 .BR dd (1p)
 utility and used here to decrease ambiguity.
 
-When seeking or skipping to a byte, writing or reading starts at the byte
-immediately subsequent to the specified byte. Seeks and skips aren\(cqt counted
-in the output statistics because they're guaranteed to succeed (or the utility
-will exit unsuccessfully).
+The offset used when skipping or seeking refers to how many bytes are skipped
+or sought. Running
+.BR dj (1)
+with a skip offset of 1 skips one byte into the input and reads from the second
+byte onwards. A programmer may think of a file as a zero-indexed array of
+bytes; in this analogy, the offset given is the index of the byte at which to
+start reading or writing.
+
+Seeks and skips aren\(cqt counted in the output statistics because they're
+guaranteed to succeed (or the utility will exit unsuccessfully, before it has
+written any data).
 .\"
 .SH OPTIONS
 
@@ -82,6 +89,54 @@ input file is \(lq-\(rq.
 The standard output shall be used as an output if no inputs are specified or if
 the output file is \(lq-\(rq.
 .\"
+.SH EXAMPLES
+
+The following
+.BR sh (1p)
+line:
+
+.RS
+printf 'Hello, world!\(rsn' | dj -c 1 -b 7 -s 7 2>/dev/null
+.RE
+
+Produces the following output:
+
+.RS
+world!
+.RE
+
+The following
+.BR sh (1p)
+lines run sequentially:
+
+.RS
+tr '\(rs0' 0    </dev/zero | dj -c 1 -b 6 -o hello.txt
+
+tr '\(rs0' H    </dev/zero | dj -c 1 -b 1 -o hello.txt
+
+tr '\(rs0' e    </dev/zero | dj -c 1 -b 1 -o hello.txt -S 1
+
+tr '\(rs0' l    </dev/zero | dj -c 1 -b 2 -o hello.txt -S 2
+
+tr '\(rs0' o    </dev/zero | dj -c 1 -b 1 -o hello.txt -S 4
+
+tr '\(rs0' '\(rsn' </dev/zero | dj -c 1 -b 1 -o hello.txt -S 5
+
+dj -i hello.txt
+.RE
+
+Produce the following output:
+
+.RS
+Hello
+.RE
+
+It may be particularly illuminating to print the contents of the example
+.B hello.txt
+after each
+.BR dj (1)
+invocation.
+.\"
 .SH DIAGNOSTICS
 
 On a partial or empty read, a diagnostic message is printed. Then, the program

src/dj.c

@@ -35,15 +35,15 @@ char *program_name = "dj";
  * writing ends of its jockeyed "pipe". User-configurable members are noted
  * with their relevant options. */
 struct Io{
-	int bs;			/* buffer size (-bB) */
+	int bs;       	/* buffer size (-bB) */
 	size_t bufuse;	/* buffer usage */
 	char *buf;		/* buffer */
-	int bytes;		/* bytes processed */
+	size_t bytes;	/* bytes processed */
 	int fd;			/* file descriptor */
 	int fl;			/* file opening flags */
 	char *fn;		/* file name (may be stdin_name or stdout_name) (-io) */
-	int prec;		/* partial records processed */
-	int rec;		/* records processed */
+	size_t prec;	/* partial records processed */
+	size_t rec; 	/* records processed */
 	long seek;		/* bytes to seek/skip (will be 0 after skippage) (-sS) */
 }; 
 
@@ -75,25 +75,6 @@ Io_bufrpad(struct Io *io, int padding){
 	return io;
 }
 
-/* Copies from the buffer in src as much as possible to the free space in the
- * dest buffer, removing the copied units from src and permuting the remaining
- * units in the src buffer to the start of the buffer, modifying both the src
- * and dest bufuse and returning dest. */
-static struct Io*
-Io_bufxapp(struct Io *dest, struct Io *src){
-	int n;
-
-	assert(dest != NULL && src != NULL);
-
-	n = MIN(src->bufuse, dest->bs - dest->bufuse);
-	memcpy(dest->buf + dest->bufuse, src->buf, n);
-	dest->bufuse += n;
-	memmove(src->buf, src->buf + n, src->bs - n);
-	src->bufuse -= n;
-
-	return dest;
-}
-
 /* Copies from the buffer in src to the buffer in dest no more than n units,
  * removing the copied units from src and permuting the remaining units in the
  * src buffer to the start of the buffer, modifying both the src and dest
@@ -216,6 +197,7 @@ int main(int argc, char *argv[]){
 	noerror = 0;
 	for(i = 0; i < 2; ++i){
 		io[i].bs = 1024 /* 1 KiB */; /* GNU dd(1) default; POSIX says 512B */
+		io[i].bufuse = 0;
 		io[i].bytes = 0;
 		io[i].fd = i == 0 ? STDIN_FILENO : STDOUT_FILENO;
 		io[i].fn = i == 0 ? stdin_name   : stdout_name;
@@ -338,7 +320,16 @@ int main(int argc, char *argv[]){
 			int t;
 
 			if(io[1].bs > io[0].bs){
-				Io_bufxapp(&io[1], &io[0]);
+				int n;
+
+				/* copy from ibuf as much as possible to the obuf */
+				memcpy(io[1].buf + io[1].bufuse, io[0].buf,
+				       (n = MIN(io[0].bufuse, io[1].bs - io[1].bufuse)));
+				io[1].bufuse += n;
+				/* permute out the copied units */
+				memmove(io[0].buf, io[0].buf + n, io[0].bs - n);
+				io[0].bufuse -= n;
+
 				if(io[0].bs + io[1].bufuse <= io[1].bs && count != 1)
 					continue; /* we could write more */
 			}else