Debug versions of heap allocation functions

The C runtime (CRT) library contains special Debug versions of the heap allocation functions. These functions have the same names as the Release versions with _dbg appended to them. This article describes the differences between the Release version of a CRT function and the _dbg version, using malloc and _malloc_dbg as examples.

Behavior in debug builds

When _DEBUG is defined, the CRT maps all malloc calls to _malloc_dbg. Therefore, you don't need to rewrite your code using _malloc_dbg instead of malloc to receive the benefits while debugging.

You might want to call _malloc_dbg explicitly, however. Calling _malloc_dbg explicitly has some added benefits:

  • Tracking _CLIENT_BLOCK type allocations.

  • Storing the source file and line number where the allocation request occurred.

If you don't want to convert your malloc calls to _malloc_dbg, you can obtain the source file information by defining _CRTDBG_MAP_ALLOC, which causes the preprocessor to directly map all calls to malloc to _malloc_dbg instead of relying on a wrapper around malloc.

To track the separate types of allocations in client blocks, you must call _malloc_dbg directly and set the blockType parameter to _CLIENT_BLOCK.

Behavior in non-debug builds

When _DEBUG isn't defined, calls to malloc aren't disturbed, calls to _malloc_dbg are resolved to malloc, the definition of _CRTDBG_MAP_ALLOC is ignored, and source file information pertaining to the allocation request isn't provided. Because malloc doesn't have a block type parameter, requests for _CLIENT_BLOCK types are treated as standard allocations.

See also

CRT debugging techniques