Add mmap.slice and allow for 0-sized fd-only mappings

libs/core/mmap.cpp

@@ -34,10 +34,13 @@ MMap *mmap(String filename, int size, int offset) {
     if (fd < 0)
         return 0;
     
-    void *data = ::mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
+    void *data = NULL;
-    if (data == MAP_FAILED) {
+    if (size > 0) {
-        close(fd);
+        data = ::mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
-        return 0;
+        if (data == MAP_FAILED) {
+            close(fd);
+            return 0;
+        }
     }
 
     auto r = new MMap();
@@ -70,6 +73,18 @@ TNumber getNumber(MMap *buf, NumberFormat format, int offset) {
     return getNumberCore(buf->data + offset, buf->length - offset, format);
 }
 
+/**
+ * Read a range of bytes into a buffer.
+ */
+//%
+Buffer slice(MMap *buf, int offset = 0, int length = -1) {
+    offset = min((int)buf->length, offset);
+    if (length < 0)
+        length = buf->length;
+    length = min(length, buf->length - offset);
+    return mkBuffer(buf->data + offset, length);
+}
+
 /** Returns the length of a Buffer object. */
 //% property
 int length(MMap *s) {

libs/core/shims.d.ts

@@ -20,6 +20,12 @@ declare interface MMap {
     //% shim=MMapMethods::getNumber
     getNumber(format: NumberFormat, offset: int32): number;
 
+    /**
+     * Read a range of bytes into a buffer.
+     */
+    //% offset.defl=0 length.defl=-1 shim=MMapMethods::slice
+    slice(offset?: int32, length?: int32): Buffer;
+
     /** Returns the length of a Buffer object. */
     //% property shim=MMapMethods::length
     length: int32;