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C Run-time (CRT) topics Mahmoud Saleh Windows C++ Team.

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1 C Run-time (CRT) topics Mahmoud Saleh Windows C++ Team

2 Help with memory leaks CRT Debug heap – Debug only, default in debug apps – Allocates slightly larger blocks of memory – Keeps track of memory state (through a linked list) – Uses buffers on both sides of user data NoMansLand: 0xfd New memory: 0xcd Free’d memory: 0xdd

3 Help with memory leaks How to use Debug Heap to find memory leaks – Check state – Dump memory leaks

4 Help with memory leaks Debug heap state and functions are debug only! What about release? – Track allocations Even new operator goes through malloc(), and delete calls free() Break at free and malloc – Other options: _heapwalk Application Verifier (with Debugging Tools for Windows) …

5 Help with unhandled exceptions terminate() set_terminate() Handler must exit; otherwise abort will be called Don’t throw in the handler!

6 Assert and error reporting Asserts (DEBUG only) are very useful in C++ and C Other error reporting types: _CRT_WARN _CRT_ERROR _CRT_ASSERT 3 modes for reporting _CRTDBG_MODE_FILE _CRTDBG_MODE_DEBUG _CRTDBG_MODE_WNDW

7 Assert and error reporting One step further, use a hook _CrtSetReportHook2 _CrtSetReportHookW2 Note: CrtSetReportHook2 and _CrtSetReportHookW2 let you hook or unhook a function, whereas _CrtSetReportHook only lets you hook a function. Make sure to properly terminate the app, if you don’t want the CRT default handlers to kick off

8 CRT entry points Linker defined, based on /SUBSYSTEM Windows apps (/SUBSYSTEM:WINDOWS) – (w)WinMain (called by WinMainCRTStartup) Console apps (/SUBSYSTEM:CONSOLE) – (w)main (called by mainCRTStartup) DllMain (/DLL) – _DllMainCRTStartup initializes the C run-time library then calls DllMain If /DLL or /SUBSYSTEM not specified, linker selects subsystem and entry point based on whether main or WinMain is defined. Or, define your entry point: /ENTRY

9 CRT support for Unicode, SBCS, MBCS Recommended: Use Unicode, unless there’s a specific reason not to. – Windows APIs use Unicode In the CRT, if you want to change the character set: – Project properties: – Or, explicitly define _UNICODE or _MBCS before including (or using /D compiler switch)

10 Optimizing File IO in CRT Buffered vs. low level IO functions – fopen (CRT buffered) – open (low level) Setting buffer size – Default is is 0x1000 (4K) – setvbuf – Sometimes, buffering may result in perf reduction: use with caution!

11 Questions

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