The debug malloc library, when enabled with various checking, uses more stack space (i.e. calls more functions, uses more local variables etc.) than the regular libc allocator. This implies that programs that explicitly set the stack size to something smaller than the default may encounter problems such as running out of stack space. This may cause the program to crash. You can prevent this by increasing the stack space allocated to the threads in question.
MALLOC_FILLAREA is used to do fill-area checking. If fill-area checking isn't enabled, the program can't detect certain types of errors. For example, if an application accesses beyond the end of a block, and the real block allocated by the allocator is larger than what was requested, the allocator won't flag an error unless MALLOC_FILLAREA is enabled. By default, this checking isn't enabled.
MALLOC_CKACCESS is used to validate accesses to the str* and mem* family of functions. If this variable isn't enabled, such accesses won't be checked, and errors aren't reported. By default, this checking isn't enabled.
MALLOC_CKCHAIN performs extensive heap checking on every allocation. When you enable this environment variable, allocations can be much slower. Also since full heap checking is performed on every allocation, an error anywhere in the heap could be reported upon entry into the allocator for any operation. For example, a call to free(x) will check block x as well as the complete heap for errors before completing the operation (to free block x). So any error in the heap will be reported in the context of freeing block x, even if the error itself isn't specifically related to this operation.
When the debug library reports errors, it doesn't always exit immediately; instead it continues to perform the operation that causes the error, and corrupts the heap (since the operation that raises the warning is actually an illegal operation). You can control this behavior by using the MALLOC_WARN and MALLOC_FATAL handler described earlier. If specific handlers are not provided, the heap will be corrupted and other errors could result and be reported later because of the first error. The best solution is to focus on the first error and fix it before moving onto other errors. See the description of MALLOC_CKCHAIN for more information on how these errors may end up getting reported.
Although the debug malloc library allocates blocks to the process using the same algorithms as the standard allocator, the library itself requires additional storage to maintain block information, as well as to perform sanity checks. This means that the layout of blocks in memory using the debug allocator is slightly different than with the standard allocator.
If you use certain optimization options such as -O1, -O2, or -O3, the debug malloc library won't work correctly because these options make gcc use builtin versions of some functions, such as strcpy() and strcmp(). Use the -fno-builtin option to prevent this.