Merge pull request #48 from adamgreen/gccRetargetSbrk

gcc: Provide _sbrk implementation compatible with RTX

Author: bogdanm

libraries/mbed/common/retarget.cpp

@@ -425,3 +425,30 @@ extern "C" void __iar_argc_argv() {
     mbed_main();
 }
 #endif
+
+// Provide implementation of _sbrk (low-level dynamic memory allocation
+// routine) for GCC_ARM which compares new heap pointer with MSP instead of
+// SP.  This make it compatible with RTX RTOS thread stacks.
+#if defined(TOOLCHAIN_GCC_ARM)
+// Linker defined symbol used by _sbrk to indicate where heap should start.
+extern "C" int __end__;
+
+// Turn off the errno macro and use actual global variable instead.
+#undef errno
+extern "C" int errno;
+
+// Dynamic memory allocation related syscall.
+extern "C" caddr_t _sbrk(int incr) {
+    static unsigned char* heap = (unsigned char*)&__end__;
+    unsigned char*        prev_heap = heap;
+    unsigned char*        new_heap = heap + incr;
+
+    if (new_heap >= (unsigned char*)__get_MSP()) {
+        errno = ENOMEM;
+        return (caddr_t)-1;
+    }
+    
+    heap = new_heap;
+    return (caddr_t) prev_heap;
+}
+#endif