# crt_dbg2 Sample: C Run-Time Debugging Hook Functions

The crt_dbg2 sample illustrates several ways to use debugging hook functions with the debug versions of the C run-time library. To add some realism, it has a few elements of an actual application, including two bugs.

Note
This sample is not supported on Intel's Itanium processors.

Security Note
This sample code is provided to illustrate a concept and should not be used in applications or Web sites, as it may not illustrate the safest coding practices.

# Building and Running the Sample

Note
If you are running Visual C++ Express Edition, you might need to install the Platform SDK before running this sample. For information on how to do this, see How to: Use Visual C++ Express Edition with the Microsoft Platform SDK.

#### To build and run this sample

1. Open the solution crt_dbg2.sln.

2. From the Build menu, click Build.

3. From the Debug menu, select Start Without Debugging.

# How the Sample Works

The program stores birthdate information in a linked list of Client blocks. A Client-dump hook function validates the birthday data and reports the contents of the Client blocks. An allocation hook function logs heap operations to a text file, and the report hook function logs selected reports to the same text file.

Note that the allocation hook function explicitly excludes CRT blocks (the memory allocated internally by the C run-time library) from its log. The hook function uses fprintf to write to the log file, and fprintf allocates a CRT block. If CRT blocks were not excluded in this case, an endless loop would overflow the stack: fprintf would cause the hook function to be called, the hook would in turn call fprintf, which would in turn cause the hook to be called again, and so forth.

To be able to report CRT-type blocks in your allocation hook, Windows API functions could be used instead of C run-time functions. Because the Windows APIs do not use the CRT heap, they would not trap the hook in an endless loop.

The debug heap catches two bugs and a data error in the second example. One bug is that the birthday name field is not large enough to hold several of the test names. The field should be larger, and strncpy should be used instead of strcpy. The second bug is that the while loop in the `printRecords` function should not end until the `HeadPtr` itself is equal to null. This bug results not only in an incomplete display of birthdays, but also in a memory leak. Finally, Gauss's birthday should be April 30, not April 32.

# Keywords

This sample uses the following keywords:

_assert; _asserte; _crtcheckmemory; _crtmemcheckpoint; _crtmemdumpallobjectssince; _crtmemdumpstatistics; _crtsetallochook; _crtsetdbgflag; _crtsetdumpclient; _crtsetreportfile; _crtsetreporthook; _crtsetreportmode; _free_dbg; _malloc_dbg; _rpt4; _strdate; _strtime; aboutbox; clear_crt_debug_field; createinstance; createrecord; displaystring; donttouch; exit; fatalerror; fclose; fflush; fopen; fprintf; fputs; get_size; helpstring; id; iid_is; module; myallochook; mydumpclienthook; myreporthook; pointer_default; printf; printrecords; put_size; set_crt_debug_field; strcpy; strstr; uuid