Add an advance function to step through the mmaped buffer

simgear/io/CMakeLists.txt

@@ -8,6 +8,7 @@ set(HEADERS
     raw_socket.hxx
     sg_binobj.hxx
     sg_file.hxx
+    sg_mmap.hxx
     sg_netBuffer.hxx
     sg_netChannel.hxx
     sg_netChat.hxx
@@ -29,6 +30,7 @@ set(SOURCES
     raw_socket.cxx
     sg_binobj.cxx
     sg_file.cxx
+    sg_mmap.cxx
     sg_netBuffer.cxx
     sg_netChannel.cxx
     sg_netChat.cxx

simgear/io/sg_mmap.cxx

@@ -180,6 +180,23 @@ int SGMMapFile::read( char *buf, int length ) {
     return result;
 }
 
+const char* SGMMapFile::advance(size_t len) {
+   if (len >= size - offset)
+      return nullptr;
+
+   const char *ptr = buffer + offset;
+   offset += len;
+
+   return ptr;
+}
+
+int SGMMapFile::read( char *buf, int length, int num ) {
+    if (length == 0)
+        return 0;
+
+    size_t size = num*length;
+    return read(buf, size)/length;
+}
 
 // read a line of data, length is max size of input buffer
 int SGMMapFile::readline( char *buf, int length ) {

simgear/io/sg_mmap.hxx

@@ -92,6 +92,9 @@ public:
     // read a block of data of specified size
     int read( char *buf, int length );
 
+    // read a block of data of specified size
+    int read( char *buf, int length, int num );
+
     // read a line of data, length is max size of input buffer
     int readline( char *buf, int length );
 
@@ -99,7 +102,11 @@ public:
     // note: this really defeats the purpose of mmapping a file
     std::string read_all();
 
-    inline const char* get() { return buffer; }
+    // get the pointer to the start of the buffer
+    inline const char *get() { return buffer; }
+
+    // get the pointer at the current offset and increase the offset by len
+    const char* advance(size_t len);
 
     inline size_t get_size() { return size; }