Heap Consistency Checking

You can ask malloc to check the consistency of dynamic storage by using the mcheck function. This function is a GNU extension, declared in `malloc.h'.

Function: int mcheck (void (*abortfn) (enum mcheck_status status))

Calling mcheck tells malloc to perform occasional consistency checks. These will catch things such as writing past the end of a block that was allocated with malloc.

The abortfn argument is the function to call when an inconsistency is found. If you supply a null pointer, then mcheck uses a default function which prints a message and calls abort (see section Aborting a Program). The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below.

It is too late to begin allocation checking once you have allocated anything with malloc. So mcheck does nothing in that case. The function returns -1 if you call it too late, and 0 otherwise (when it is successful).

The easiest way to arrange to call mcheck early enough is to use the option `-lmcheck' when you link your program; then you don't need to modify your program source at all.

Function: enum mcheck_status mprobe (void *pointer)

The mprobe function lets you explicitly check for inconsistencies in a particular allocated block. You must have already called mcheck at the beginning of the program, to do its occasional checks; calling mprobe requests an additional consistency check to be done at the time of the call.

The argument pointer must be a pointer returned by malloc or realloc. mprobe returns a value that says what inconsistency, if any, was found. The values are described below.

Data Type: enum mcheck_status

This enumerated type describes what kind of inconsistency was detected in an allocated block, if any. Here are the possible values:

MCHECK_DISABLED

mcheck was not called before the first allocation. No consistency checking can be done.

MCHECK_OK

No inconsistency detected.

MCHECK_HEAD

The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far.

MCHECK_TAIL

The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far.

MCHECK_FREE

The block was already freed.

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