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C Run-time ( CRT) topics

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

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C Run-time ( CRT) topics

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  1. C Run-time (CRT) topics Mahmoud SalehWindows 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 <tchar.h> (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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