diff --git a/include/libcasio/cdefs/integers.h b/include/libcasio/cdefs/integers.h
index 9197922..dbc23c6 100644
--- a/include/libcasio/cdefs/integers.h
+++ b/include/libcasio/cdefs/integers.h
@@ -23,6 +23,13 @@
 CASIO_BEGIN_NAMESPACE
 
 # if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
+#  if defined(__WINDOWS__)
+#   include <BaseTsd.h>
+typedef SSIZE_T ssize_t;
+#  else
+#   include <sys/types.h>
+#  endif
+
 #  include <stdint.h>
 #  include <inttypes.h>
 
diff --git a/include/libcasio/stream.h b/include/libcasio/stream.h
index 77feebb..a27bd7d 100644
--- a/include/libcasio/stream.h
+++ b/include/libcasio/stream.h
@@ -100,7 +100,7 @@ typedef int casio_stream_setattrs_t
 typedef int casio_stream_settm_t
 	OF((void *, const casio_timeouts_t *));
 
-typedef size_t casio_stream_read_t
+typedef ssize_t casio_stream_read_t
 	OF((void *, unsigned char *, size_t));
 typedef int casio_stream_write_t
 	OF((void *, const unsigned char *, size_t));
@@ -359,7 +359,7 @@ CASIO_EXTERN int   CASIO_EXPORT casio_get_lasterr
 
 /* Read and write data from and to a stream. */
 
-CASIO_EXTERN size_t CASIO_EXPORT casio_read
+CASIO_EXTERN ssize_t CASIO_EXPORT casio_read
 	OF((casio_stream_t *casio__stream,
 		void *casio__dest, size_t casio__size));
 CASIO_EXTERN int CASIO_EXPORT casio_write
diff --git a/lib/file/file.h b/lib/file/file.h
index ec14301..30bc53b 100644
--- a/lib/file/file.h
+++ b/lib/file/file.h
@@ -28,14 +28,15 @@
 /* Read from a stream. */
 
 # define  READ(CASIO__TO, CASIO__SZ) /* normal read */ { \
-	int READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
-	if (READ_err == -1) return (errno); }
+	ssize_t READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
+	if (READ_err < 0) return -(READ_err); }
 # define FREAD(CASIO__TO, CASIO__SZ) /* fail-less read */ { \
-	casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
-	err = errno; }
+	ssize_t READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
+	err = READ_err < 0 ? -READ_err : 0; }
 # define GREAD(CASIO__TO, CASIO__SZ) /* read with goto fail */ { \
-	casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
-	if ((err = errno)) \
+	ssize_t READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
+	err = READ_err < 0 ? -READ_err : 0; \
+	if ((err)) \
 		goto fail; }
 
 /* Read using size of the object. */
diff --git a/lib/link/seven/datastream.c b/lib/link/seven/datastream.c
index ea8ef2c..2af1a54 100644
--- a/lib/link/seven/datastream.c
+++ b/lib/link/seven/datastream.c
@@ -51,10 +51,10 @@ typedef struct {
  *	@arg	cookie		the cookie.
  *	@arg	data		the data to read.
  *	@arg	size		the size of the data to read.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-CASIO_LOCAL size_t casio_seven_data_read(seven_data_cookie_t *cookie,
+CASIO_LOCAL ssize_t casio_seven_data_read(seven_data_cookie_t *cookie,
 	unsigned char *data, size_t size)
 {
 	int err; size_t tocopy;
@@ -64,7 +64,7 @@ CASIO_LOCAL size_t casio_seven_data_read(seven_data_cookie_t *cookie,
 
 	/* Check if the stream is faulty. */
 	if (cookie->_faulty)
-		return (casio_error_noread);
+		return -(casio_error_noread);
 
 	/* Empty the current buffer. */
 	tocopy = cookie->_lastsize;
@@ -79,8 +79,7 @@ CASIO_LOCAL size_t casio_seven_data_read(seven_data_cookie_t *cookie,
 
 	/* Check if we have already finished. */
 	if (cookie->_total && cookie->_id == cookie->_total) {
-		errno = casio_error_eof;
-		return (-1);
+		return -(casio_error_eof);
 	}
 
 	/* Receive packets. */
@@ -137,8 +136,7 @@ CASIO_LOCAL size_t casio_seven_data_read(seven_data_cookie_t *cookie,
 fail:
 	/* XXX: tell the distant device we have a problem? */
 	cookie->_faulty = 1;
-	errno = err;
-	return (-1);
+	return -(err);
 }
 
 /**
diff --git a/lib/link/seven/receive.c b/lib/link/seven/receive.c
index 43754ad..410d1e7 100644
--- a/lib/link/seven/receive.c
+++ b/lib/link/seven/receive.c
@@ -175,9 +175,9 @@ CASIO_LOCAL const char *gettermstring(casio_seven_term_t code)
 
 #define buffer handle->casio_link_recv_buffer
 #define COMPLETE_PACKET(N) { \
-	int COMP_PACKET_err = casio_read(handle->casio_link_stream, \
+	ssize_t COMP_PACKET_err = casio_read(handle->casio_link_stream, \
 		&buffer[received], (size_t)N); \
-	received += COMP_PACKET_err; if (COMP_PACKET_err == -1) return (errno); }
+	received += COMP_PACKET_err >= 0 ? COMP_PACKET_err : 0; if (COMP_PACKET_err < 0) return -(COMP_PACKET_err); }
 
 CASIO_LOCAL int casio_seven_decode(casio_link_t *handle, int scralign)
 {
diff --git a/lib/link/seven/scsi.c b/lib/link/seven/scsi.c
index 86ea65a..a8af9b8 100644
--- a/lib/link/seven/scsi.c
+++ b/lib/link/seven/scsi.c
@@ -83,7 +83,7 @@ typedef struct {
  * Read and write from the stream.
  * --- */
 
-CASIO_LOCAL size_t seven_scsi_read(seven_scsi_cookie_t *cookie,
+CASIO_LOCAL ssize_t seven_scsi_read(seven_scsi_cookie_t *cookie,
 	unsigned char *buffer, size_t size)
 {
 	casio_scsi_t scsi; int err;
@@ -129,8 +129,7 @@ CASIO_LOCAL size_t seven_scsi_read(seven_scsi_cookie_t *cookie,
 			scsi.casio_scsi_data_len = 16;
 
 			if ((err = casio_scsi_request(cookie->stream, &scsi))) {
-				errno = err;
-				return (-1);
+				return -(err);
 			}
 
 			mem((ll_info, poll_data, 16));
@@ -182,8 +181,7 @@ CASIO_LOCAL size_t seven_scsi_read(seven_scsi_cookie_t *cookie,
 			scsi.casio_scsi_data_len = avail;
 
 			if ((err = casio_scsi_request(cookie->stream, &scsi))) {
-				errno = err;
-				return (-1);
+				return -(err);
 			}
 		}
 
diff --git a/lib/mcsfile/mcsfile.h b/lib/mcsfile/mcsfile.h
index 8d68e99..46d39f5 100644
--- a/lib/mcsfile/mcsfile.h
+++ b/lib/mcsfile/mcsfile.h
@@ -27,14 +27,15 @@
 /* Read from a stream. */
 
 # define  READ(CASIO__TO, CASIO__SZ) /* normal read */ { \
-	int READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
-	if (READ_err == -1) return (errno); }
+	ssize_t READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
+	if (READ_err < 0) return -(READ_err); }
 # define FREAD(CASIO__TO, CASIO__SZ) /* fail-less read */ { \
-	casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
-	err = errno; }
+	ssize_t READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
+	err = READ_err < 0 ? -READ_err : 0; }
 # define GREAD(CASIO__TO, CASIO__SZ) /* read with goto fail */ { \
-	casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
-	if ((err = errno)) \
+	ssize_t READ_err = casio_read(buffer, (CASIO__TO), (CASIO__SZ)); \
+	err = READ_err < 0 ? -READ_err : 0; \
+	if ((err)) \
 		goto fail; }
 
 /* Read using size of the object. */
diff --git a/lib/stream/builtin/csum32.c b/lib/stream/builtin/csum32.c
index 3e3a439..de0de11 100644
--- a/lib/stream/builtin/csum32.c
+++ b/lib/stream/builtin/csum32.c
@@ -38,20 +38,20 @@ struct thecookie {
  *	@arg	cookie		the cookie.
  *	@arg	data		the data pointer.
  *	@arg	size		the data size.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-CASIO_LOCAL size_t csum32_read(struct thecookie *cookie,
+CASIO_LOCAL ssize_t csum32_read(struct thecookie *cookie,
 	unsigned char *dest, size_t size)
 {
 	int err;
 
 	/* Make the call. */
 
-	size = casio_read(cookie->_stream, dest, size);
-	if (size == (size_t)-1) {
-		errno = errno;
-		return (-1);
+	ssize_t ssize = casio_read(cookie->_stream, dest, size);
+	if (ssize < 0) {
+		err = -ssize;
+		return -(err);
 	}
 
 	/* Make the checksum. */
diff --git a/lib/stream/builtin/file.c b/lib/stream/builtin/file.c
index b3deb5d..ea3a1ea 100644
--- a/lib/stream/builtin/file.c
+++ b/lib/stream/builtin/file.c
@@ -46,10 +46,10 @@ typedef struct {
  *	@arg	cookie		the cookie.
  *	@arg	data		the data pointer
  *	@arg	size		the data size.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-CASIO_LOCAL size_t casio_file_read(file_cookie_t *cookie,
+CASIO_LOCAL ssize_t casio_file_read(file_cookie_t *cookie,
 	unsigned char *dest, size_t size)
 {
 	size_t recv;
@@ -92,8 +92,7 @@ CASIO_LOCAL size_t casio_file_read(file_cookie_t *cookie,
 			/* End of file. */
 
 			msg((ll_error, "encountered an end of file"));
-			errno = casio_error_eof;
-			return (-1);
+			return -(casio_error_eof);
 
 		case EINTR: /* alarm */
 		case ETIMEDOUT:
@@ -103,21 +102,18 @@ CASIO_LOCAL size_t casio_file_read(file_cookie_t *cookie,
 			/* A timeout has occurred. */
 
 			msg((ll_error, "timeout received"));
-			errno = casio_error_timeout;
-			return (-1);
+			return -(casio_error_timeout);
 
 		case ENODEV:
 		case EPIPE: case ESPIPE:
 			/* A device error has occured. */
 
 			msg((ll_error, "calculator was disconnected"));
-			errno = casio_error_nocalc;
-			return (-1);
+			return -(casio_error_nocalc);
 
 		default:
 			msg((ll_fatal, "errno was %d: %s", errno, strerror(errno)));
-			errno = casio_error_unknown;
-			return (-1);
+			return -(casio_error_unknown);
 	}
 
 	return copiedsize;
diff --git a/lib/stream/builtin/libusb/libusb.h b/lib/stream/builtin/libusb/libusb.h
index 75196b9..0cd1a10 100644
--- a/lib/stream/builtin/libusb/libusb.h
+++ b/lib/stream/builtin/libusb/libusb.h
@@ -58,7 +58,7 @@ CASIO_EXTERN int CASIO_EXPORT casio_libusb_settm
 
 /* Character device callbacks. */
 
-CASIO_EXTERN size_t CASIO_EXPORT casio_libusb_read
+CASIO_EXTERN ssize_t CASIO_EXPORT casio_libusb_read
 	OF((cookie_libusb_t *casio__cookie,
 		unsigned char *casio__dest, size_t casio__size));
 CASIO_EXTERN int CASIO_EXPORT casio_libusb_write
diff --git a/lib/stream/builtin/libusb/read.c b/lib/stream/builtin/libusb/read.c
index 9859394..e249b32 100644
--- a/lib/stream/builtin/libusb/read.c
+++ b/lib/stream/builtin/libusb/read.c
@@ -26,10 +26,10 @@
  *	@arg	vcookie		the cookie (voided)
  *	@arg	data		the data pointer.
  *	@arg	size		the data size.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-size_t CASIO_EXPORT casio_libusb_read(cookie_libusb_t *cookie,
+ssize_t CASIO_EXPORT casio_libusb_read(cookie_libusb_t *cookie,
 	unsigned char *dest, size_t size)
 {
 	int libusberr;
@@ -66,18 +66,15 @@ size_t CASIO_EXPORT casio_libusb_read(cookie_libusb_t *cookie,
 		case LIBUSB_ERROR_NO_DEVICE:
 		case LIBUSB_ERROR_IO:
 			msg((ll_error, "The calculator is not here anymore :("));
-			errno = casio_error_nocalc;
-			return (-1);
+			return -(casio_error_nocalc);
 
 		case LIBUSB_ERROR_TIMEOUT:
-			errno = casio_error_timeout;
-			return (-1);
+			return -(casio_error_timeout);
 
 		default:
 			msg((ll_fatal, "libusb error was %d: %s", libusberr,
 				libusb_strerror(libusberr)));
-			errno = casio_error_unknown;
-			return (-1);
+			return -(casio_error_unknown);
 		}
 
 		/* Get the current size to copy. */
diff --git a/lib/stream/builtin/limited.c b/lib/stream/builtin/limited.c
index d07a84d..3ef47f8 100644
--- a/lib/stream/builtin/limited.c
+++ b/lib/stream/builtin/limited.c
@@ -37,10 +37,10 @@ typedef struct {
  *	@arg	vcookie		the cookie (uncasted).
  *	@arg	data		the data pointer.
  *	@arg	size		the data size.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-CASIO_LOCAL size_t casio_limited_read(void *vcookie, unsigned char *dest,
+CASIO_LOCAL ssize_t casio_limited_read(void *vcookie, unsigned char *dest,
 	size_t size)
 {
 	int err; limited_cookie_t *cookie = (void*)vcookie;
@@ -52,21 +52,19 @@ CASIO_LOCAL size_t casio_limited_read(void *vcookie, unsigned char *dest,
 		left = cookie->_left;
 		cookie->_left = 0;
 		if ((err = casio_skip(cookie->_stream, left))) {
-			errno = err;
-			return (-1);
+			return -(err);
 		}
 
 		/* Once the skip is done successfully, we return that we
 		 * have an EOF. */
-		errno = casio_error_eof;
-		return (-1);
+		return -(casio_error_eof);
 	}
 
-	size = casio_read(cookie->_stream, dest, size);
-	if (size == (size_t)-1) {
+	ssize_t ssize = casio_read(cookie->_stream, dest, size);
+	if (ssize < 0) {
 		cookie->_left = 0; /* XXX: depends on the error? */
-		errno = errno;
-		return (-1);
+		err = -ssize;
+		return -(err);
 	}
 	cookie->_left -= size;
 	return size;
diff --git a/lib/stream/builtin/memory.c b/lib/stream/builtin/memory.c
index 33fa8e6..d7a3166 100644
--- a/lib/stream/builtin/memory.c
+++ b/lib/stream/builtin/memory.c
@@ -35,21 +35,19 @@ typedef struct {
  *	@arg	vcookie		the cookie (uncasted)
  *	@arg	data		the data pointer.
  *	@arg	size		the data size.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-CASIO_LOCAL size_t casio_memory_read(void *vcookie, unsigned char *dest, size_t size)
+CASIO_LOCAL ssize_t casio_memory_read(void *vcookie, unsigned char *dest, size_t size)
 {
 	memory_cookie_t *cookie = (void*)vcookie;
 
 	if (((size_t)-1 - cookie->_offset) < size) { /* overflow */
-		errno = casio_error_read;
-		return (-1);
+		return -(casio_error_read);
 	}
 	if (cookie->_offset + (casio_off_t)size > cookie->_size) {
 		cookie->_offset = cookie->_size - 1;
-		errno = casio_error_eof;
-		return (-1);
+		return -(casio_error_eof);
 	}
 
 	memcpy(dest, &cookie->_memory[cookie->_offset], size);
diff --git a/lib/stream/builtin/streams/read.c b/lib/stream/builtin/streams/read.c
index 2f61377..e51f4f8 100644
--- a/lib/stream/builtin/streams/read.c
+++ b/lib/stream/builtin/streams/read.c
@@ -26,10 +26,10 @@
  *	@arg	cookie		the cookie.
  *	@arg	data		the data pointer.
  *	@arg	size		the data size.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-size_t CASIO_EXPORT casio_streams_read(streams_cookie_t *cookie,
+ssize_t CASIO_EXPORT casio_streams_read(streams_cookie_t *cookie,
 	unsigned char *dest, size_t size)
 {
 	int fd = cookie->_readfd;
@@ -65,12 +65,11 @@ size_t CASIO_EXPORT casio_streams_read(streams_cookie_t *cookie,
 		if (recv < 0) switch (errno) {
 			case 0: continue;
 			case ENODEV: case EIO:
-				return (casio_error_nocalc);
+				return -(casio_error_nocalc);
 			default:
 				msg((ll_fatal, "error was %d: %s",
 					errno, strerror(errno)));
-				errno = casio_error_unknown;
-				return (-1);
+				return -(casio_error_unknown);
 		}
 
 		/* Get the current size to copy. */
diff --git a/lib/stream/builtin/streams/streams.h b/lib/stream/builtin/streams/streams.h
index d884a1c..9f077f1 100644
--- a/lib/stream/builtin/streams/streams.h
+++ b/lib/stream/builtin/streams/streams.h
@@ -51,7 +51,7 @@ CASIO_EXTERN int CASIO_EXPORT casio_streams_settm
 
 /* Character device callbacks. */
 
-CASIO_EXTERN size_t CASIO_EXPORT casio_streams_read
+CASIO_EXTERN ssize_t CASIO_EXPORT casio_streams_read
 	OF((streams_cookie_t *casio__cookie,
 		unsigned char *casio__dest, size_t casio__size));
 CASIO_EXTERN int CASIO_EXPORT casio_streams_write
diff --git a/lib/stream/builtin/windows/read.c b/lib/stream/builtin/windows/read.c
index af56a3e..872ccfb 100644
--- a/lib/stream/builtin/windows/read.c
+++ b/lib/stream/builtin/windows/read.c
@@ -20,14 +20,14 @@
 #ifndef LIBCASIO_DISABLED_WINDOWS
 
 /**
- *	casio_windows_read:
+ *	casio_windows_read: TODO
  *	Read from an MS-Windows stream.
  *
  *	@arg	cookie		the cookie.
- *	@return				the error code (0 if ok).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-int CASIO_EXPORT casio_windows_read(win_cookie_t *cookie,
+ssize_t CASIO_EXPORT casio_windows_read(win_cookie_t *cookie,
 	unsigned char *dest, size_t size)
 {
 	DWORD werr, wsuccess, recv; size_t tocopy;
diff --git a/lib/stream/builtin/windows/windows.h b/lib/stream/builtin/windows/windows.h
index 34402a9..05b5eea 100644
--- a/lib/stream/builtin/windows/windows.h
+++ b/lib/stream/builtin/windows/windows.h
@@ -51,7 +51,7 @@ CASIO_EXTERN int CASIO_EXPORT casio_windows_settm
 
 /* Character device callbacks. */
 
-CASIO_EXTERN int CASIO_EXPORT casio_windows_read
+CASIO_EXTERN ssize_t CASIO_EXPORT casio_windows_read
 	OF((win_cookie_t *cookie, unsigned char *dest, size_t size));
 CASIO_EXTERN int CASIO_EXPORT casio_windows_write
 	OF((win_cookie_t *cookie, const unsigned char *data, size_t size));
diff --git a/lib/stream/read.c b/lib/stream/read.c
index c49a9d8..d6b3118 100644
--- a/lib/stream/read.c
+++ b/lib/stream/read.c
@@ -25,27 +25,27 @@
  *	@arg	stream		the stream to read from.
  *	@arg	dest		the destination buffer.
  *	@arg	size		the amount of bytes to read.
- *	@return				the size written and -1 if error (error code is in errno).
+ *	@return				the size if > 0, or if < 0 the error code is -[returned value].
  */
 
-size_t CASIO_EXPORT casio_read(casio_stream_t *stream, void *dest, size_t size)
+ssize_t CASIO_EXPORT casio_read(casio_stream_t *stream, void *dest, size_t size)
 {
 	int err = casio_error_ok;
+	ssize_t ssize = 0;
 
 	/* check if we can read */
 	failure(~stream->casio_stream_mode & CASIO_OPENMODE_READ, casio_error_read)
 
 	/* read */
 	if (size == 0) {
-		errno = err;
 		return (0);
 	}
-	size = (*getcb(stream, read))(stream->casio_stream_cookie, dest, size);
+	ssize = (*getcb(stream, read))(stream->casio_stream_cookie, dest, size);
 	
-	if (size == (size_t)-1) {
-		err = errno;
+	if (ssize < 0) {
+		err = -ssize;
 		if (err == casio_error_eof) {
-			msg((ll_info, "Stream reading get to the end (EOF)"));
+			msg((ll_info, "Stream reading is at the end (EOF)"));
 			goto fail;
 		}
 		msg((ll_error, "Stream reading failure: %s", casio_strerror(err)));
@@ -53,11 +53,10 @@ size_t CASIO_EXPORT casio_read(casio_stream_t *stream, void *dest, size_t size)
 	}
 
 	/* move the cursor and return */
-	if(size != 1) {
+	if(ssize >= 0) {
 		stream->casio_stream_offset += size;
 	}
 fail:
 	stream->casio_stream_lasterr = err;
-	errno = err;
-	return size;
+	return ssize;
 }
diff --git a/lib/stream/seek.c b/lib/stream/seek.c
index c688d0d..293cb0a 100644
--- a/lib/stream/seek.c
+++ b/lib/stream/seek.c
@@ -63,9 +63,9 @@ int CASIO_EXPORT casio_seek(casio_stream_t *stream, casio_off_t offset,
 				size_t rd = min(to_skip, 128);
 
 				/* Read that much from the stream. */
-				size_t readed = casio_read(stream, buf, rd);
-				if (readed == (size_t)-1) {
-					err = errno;
+				ssize_t readed = casio_read(stream, buf, rd);
+				if (readed < 0) {
+					err = -readed;
 					goto fail;
 				}
 
diff --git a/src/p7/main.c b/src/p7/main.c
index 8b3797f..1a987c5 100644
--- a/src/p7/main.c
+++ b/src/p7/main.c
@@ -394,13 +394,17 @@ int main(int ac, char **av)
 				err = casio_error_unknown;
 				break;
 			}
-			size_t size;
+			ssize_t size;
 			do
 			{
 				size = casio_read(fileStream, buffer, sizeof(buffer));
-				if(size == (size_t)-1)
-					err = errno;
-				if(err == casio_error_eof)
+				if(size < 0) {
+					err = -size;
+					if(err == casio_error_eof)
+						break;
+					else 
+						goto fail;
+				}
 				fwrite(buffer, 1, size, file);
 			} while (err == 0);
 			
diff --git a/src/p7os/procs/fxremote_flash.c b/src/p7os/procs/fxremote_flash.c
index 7fdc13c..fd703a3 100644
--- a/src/p7os/procs/fxremote_flash.c
+++ b/src/p7os/procs/fxremote_flash.c
@@ -83,11 +83,12 @@ static int send_sector(casio_link_t *handle, casio_stream_t *buffer,
 			/* Prepare the block. */
 			block.destination = casio_be32toh(buf);
 			block.length      = casio_be32toh(len);
-			len = casio_read(buffer, block.data, len);
-			if (len == (size_t)-1) {
+			ssize_t slen = casio_read(buffer, block.data, len);
+			if (slen < 0) {
 				osdisp_interrupt(&osdisp_cookie);
 				return (casio_error_read);
 			}
+			len = slen;
 
 			/* Pass the block. */
 			buf += len; left -= len;