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Dmalloc Tutorial: 2.5 How the Library Checks Your Program
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2.5 How the Library Checks Your Program

This is one of the newer sections of the library implying that it is incomplete. If you have any questions or issues that you'd like to see handled here, please let me know.

The dmalloc library replaces the heap library calls normally found in your system libraries with its own versions. When you make a call to malloc (for example), you are calling dmalloc's version of the memory allocation function. When you allocate memory with these functions, the dmalloc library keeps track of a number of pieces of debugging information about your pointer including: where it was allocated, exactly how much memory was requested, when the call was made, etc.. This information can then be verified when the pointer is freed or reallocated and the details can be logged on any errors.

Whenever you reallocate or free a memory address, the dmalloc library always performs a number of checks on the pointer to make sure that it is valid and has not been corrupted. You can configure the library to perform additional checks such as detected fence-post writing. The library can also be configured to overwrite memory with non-zeros (only if calloc is not called) when it is allocated and erase the memory when the pointers are freed.

In addition to per-pointer checks, you can configure the library to perform complete heap checks. These complete checks verify all internal heap structures and include walking all of the known allocated pointers to verify each one in turn. You need this level of checking to find random pointers in your program which got corrupted but that won't be freed for a while. To turn on these checks, you will need to enable the check-heap debug token. See section Description of the Debugging Tokens. By default this will cause the heap to be fully checked each and every time dmalloc is called whether it is a malloc, free, realloc, or another dmalloc overloaded function.

Performing a full heap check can take a good bit of CPU and it may be that you will want to run it sporadically. This can be accomplished in a couple different ways including the '-i' interval argument to the dmalloc utility. See section Dmalloc Utility Program. This will cause the check to be run every N-th time. For instance, 'dmalloc -i 3' will cause the heap to be checked before every 3rd call to a memory function. Values of 100 or even 1000 for high memory usage programs are more useful than smaller ones.

You can also cause the program to start doing detailed heap checking after a certain point. For instance, with 'dmalloc -s 1000' option, you can tell the dmalloc library to enable the heap checks after the 1000th memory call. Examine the dmalloc log file produced and use the iteration count if you have LOG_ITERATION_COUNT enabled in your `settings.h' file.

The start option can also have the format `file:line'. For instance, if it is set to `dmalloc_t.c:126', dmalloc will start checking the heap after it sees a dmalloc call from the `dmalloc_t.c' file, line number 126. If you use `dmalloc_t.c:0', with a 0 line number, then dmalloc will start checking the heap after it sees a call from anywhere in the `dmalloc_t.c' file.

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