dj.1: clarify skip/seek behavior with regards to statistics output

docs/dj.1

@@ -35,7 +35,9 @@ respectively. This language is inherited from the
 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.
+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).
 .\"
 .SH OPTIONS
 

src/dj.c

@@ -59,11 +59,8 @@ static int write_flags = O_WRONLY | O_CREAT;
 
 #define MIN(a, b) (((a) < (b)) ? (a) : (b))
 
-/* Macro to check if fd is a std* file, e.g. stdin. */
-#define fdisstd(fd) \
-		((fd) == STDIN_FILENO \
-		|| (fd) == STDOUT_FILENO \
-		|| (fd) == STDERR_FILENO)
+/* Macro to check if fd is stdin or stdout */
+#define fdisstd(fd) ((fd) == STDIN_FILENO || (fd) == STDOUT_FILENO)
 
 /* Fills the unused portion of io's buffer with padding, updating io->bufuse.
  * Returns io. */
@@ -132,49 +129,6 @@ Io_fdopen(struct Io *io, char *fn){
 	return fd;
 }
 
-/* Seeks io->seek bytes through *io's file descriptor, subtracting the number
- * of sought bytes from io->seek. This procedure leaves garbage in io->buf. */
-static void
-Io_fdseek(struct Io *io){
-
-	assert(io != NULL);
-	assert(io->fd != STDIN_FILENO  || io->fl == read_flags);
-	assert(io->fd != STDOUT_FILENO || io->fl == write_flags);
-	assert(io->fd != STDERR_FILENO || io->fl == write_flags);
-
-	if(io->seek == 0
-			|| (!fdisstd(io->fd) && lseek(io->fd, io->seek, SEEK_SET) != -1))
-		return;
-
-	if(io->fl == write_flags)
-		memset(io->buf, '\0', io->bs);
-
-	if(io->fl == write_flags){
-		memset(io->buf, '\0', io->bs);
-		/* We're going to cheat and use bufuse as the retval for write(2),
-		 * which is fine because it'll be zeroed as this function returns
-		 * anyway. */
-		do{
-			if((io->bufuse = write(io->fd, io->buf, MIN(io->bs, io->seek)))
-					== 0)
-				/* second chance */
-				io->bufuse = write(io->fd, io->buf, MIN(io->bs, io->seek));
-		}while((io->seek -= io->bufuse) > 0 && io->bufuse != 0);
-	}else if(io->fl == read_flags){
-		do{
-			if((io->bufuse = read(io->fd, io->buf, MIN(io->bs, io->seek)))
-					== 0)
-				/* second chance */
-				io->bufuse = read(io->fd, io->buf, MIN(io->bs, io->seek));
-		}while((io->seek -= io->bufuse) > 0 && io->bufuse != 0);
-	}else
-		assert(0); /* UNREACHABLE */
-
-	io->bufuse = 0;
-
-	return;
-}
-
 /* Reads io->bs bytes from *io's file descriptor into io->buf, storing the
  * number of read bytes in io->bufuse and updating io->bytes. If io->bufuse is
  * 0, errno will probably be set. Returns io. */
@@ -275,7 +229,7 @@ int main(int argc, char *argv[]){
 		int c;
 
 		program_name = argv[0];
-		while((c = getopt(argc, argv, "a:b:B:c:i:hHns:S:o:")) != -1)
+		while((c = getopt(argc, argv, ":a:b:B:c:i:hHns:S:o:")) != -1)
 			switch(c){
 			case 'i': case 'o': i = (c == 'o');
 				if(optarg[0] == '-' && optarg[1] == '\0'){ /* optarg == "-" */
@@ -307,24 +261,61 @@ int main(int argc, char *argv[]){
 			}
 	}
 
+	assert(io->fd != STDIN_FILENO  || io->fl == read_flags);
+	assert(io->fd != STDOUT_FILENO || io->fl == write_flags);
 
 	if(argc > optind){
 		return usage(program_name);
 	}
 
 	for(i = 0; i < 2; ++i){
+		/* buffer allocation */
 		if((io[i].buf = malloc(io[i].bs * (sizeof *(io[i].buf)))) == NULL){
 			fprintf(stderr, "%s: Failed to allocate %d bytes\n",
 				program_name, io[i].bs);
 			return EX_OSERR;
-		}else if(io[i].seek > 0){
-			Io_fdseek(&io[i]);
-			if(io[i].seek > 0){
-				return oserr(io[i].fn);
-			}
 		}
+		/* easy seeking */
+		if(!fdisstd(io[i].fd) && lseek(io[i].fd, io[i].seek, SEEK_SET) != -1)
+			io[i].seek = 0;
 	}
 
+	/* hard skipping */
+	if(io[0].seek > 0){
+		do{
+			if((io[0].bufuse = read(
+						io[0].fd, io[0].buf, MIN(io[0].bs, io[0].seek)))
+					== 0)
+				/* second chance */
+				io->bufuse = read(
+					io[0].fd, io[0].buf, MIN(io[0].bs, io[0].seek));
+		}while((io[0].seek -= io[0].bufuse) > 0 && io[0].bufuse != 0);
+		io[0].bufuse = 0;
+	}
+
+	/* hard seeking */
+	if(io[1].seek > 0){
+		memset(io[1].buf, '\0', io[1].bs);
+		/* We're going to cheat and use bufuse as the retval for write(2),
+		 * which is fine because it'll be zeroed as this function returns
+		 * anyway. */
+		do{
+			if((io[1].bufuse = write(
+						io[1].fd, io[1].buf, MIN(io[1].bs, io[1].seek)))
+					== 0)
+				/* second chance */
+				io[1].bufuse = write(
+					io[1].fd, io[1].buf, MIN(io[1].bs, io[1].seek));
+		}while((io[1].seek -= io[1].bufuse) > 0 && io[1].bufuse != 0);
+		io[1].bufuse = 0;
+	}
+
+	/* Sought bytes aren't counted in the statistics because successful seeking
+	 * is guaranteed here. */
+	for(i = 0; i < 2; ++i)
+		if(io[i].seek > 0)
+			return oserr(io[i].fn);
+
 	do{	/* read */
 		Io_read(&io[0]);
 		if(!noerror && io[0].bufuse == 0)