fdaman.txt - brief usage information for FDA (Free Debug Allocator)
Copyright (C) 1998 Thomas Helvey <tomh@inxpress.net>
1. Base Functionality
Basic use of the fda tools is as simple as including the header
and source file with your source defining DEBUG and using capitalized
versions of malloc(), calloc(), realloc(), and free().
The fda allocation functions verify all your arguments and will call
assert() if something is wrong. FDA trashes all allocated memory
in a predictable manner and applies prefix and postfix bounds checking
signatures to every allocation. When a pointer is freed it's validated,
and checked for overflows and underflows. The fda Realloc function
does not allow malloc or free through realloc and forces reallocation
if the required memory is larger than the current allocation.
In both the DEBUG and non-debug versions of fda, if a memory allocation
fails once, a low memory callback function is called to allow you to
release some memory and allow malloc to succeed, the default version
prints a warning message to stderr. If the low memory callback returns
the allocation is attempted again, if the second allocation fails a
no memory callback function is called to allow you to clean up before
terminating the application, if you allow the no memory callback to
return the results are undefined. (a NULL ptr will be returned from the
allocation call) The default no memory handler prints an error message
to stderr and calls abort(). The DEBUG version will assert if you allow
the no memory function to return.
Both the low memory and no memory callbacks have the signature of:
void handler(void)
The debug version of fda will not allow you to do the following:
Allocate a zero length chunk of memory.
Free a non-allocated pointer.
Free a pointer twice.
Free a pointer at the wrong offset.
Use realloc to free memory. (realloc(p, 0))
Use realloc to malloc memory. (realloc(0, s))
Overwrite the bounds of the memory you allocated. (checked on free)
The base functions for fda are:
void* malloc(size_t)
void* realloc(void*, size_t)
void* calloc(size_t, size_t)
void free(void*)
char* strdup(const char*)
void set_lowmem_handler(void (*fn)(void))
void set_nomem_handler(void (*fn)(void))
FDA uses a hash table to lookup pointers. The size of the hash table can
be changed at compile time by using -DFDA_HASH_TABLE_SIZE <prime>
where prime is a prime number somewhere around the number of allocations
expected. The default hash table size is 16339 which should be large enough
to hold most medium sized applications. FDA allocates memory for it's
debugging records so if your application uses a lot of memory you
may want to make sure you have enough memory available to use the debug
version. FDA allocates 20 bytes to store each allocation and 20 bytes
to store location information (file and line info). This overhead only
applies if you have DEBUG or _DEBUG defined.
2. Extended functionality
FDA provides a few handy functions for validating pointers and
checking for overruns before they occur when debugging.
The first function fda_sizeof(ptr) returns the size of the buffer
pointed to by ptr, this allows you to verify that your pointer
is what it says it is. fda_sizeof() will call assert() if the
pointer you pass it is not at the start of an allocation.
The second function valid_ptr(ptr, size) verifies that ptr lies within
allocated memory and there are at least size bytes available in the buffer.
You can pass valid_ptr() a pointer to any location in allocated memory.
valid_ptr() calls assert if the pointer points outside of allocated memory
or the remaining size is less than the size specified.
valid_ptr() is more efficient if the pointer argument is the value
returned from malloc because it's a simple hash table lookup, a more
exhaustive algorithm is used if it's not found in the hash table.
FDA extended functions:
size_t fda_sizeof(const void*)
int valid_ptr(const void*, size_t)
Typical usage for the fda extended functions:
Note: the assert macro compiles to nothing if NDEBUG is defined.
Verify a foo_ptr:
assert(sizeof(struct foo) == fda_sizeof(foo_ptr));
assert(valid_ptr(foo_ptr, sizeof(struct foo)));
Check for room to write:
assert(valid_ptr(buf, len));
3. Leak checking and block validation
FDA has several functions for leak checking, and reference marking.
fda_clear_refs() iterates through all of the allocated blocks of
memory and clears the referenced flag.
fda_set_ref() marks a single allocation(block) as being referenced or
in use by somebody.
fda_assert_refs() iterates through all the allocated blocks of
memory and calls assert() if it finds one that's not referenced.
Typical usage of the block validation functions:
fda_clear_refs(); /* clear all block references */
for each allocation do
fda_set_ref(allocation); /* mark allocation in use */
done
fda_assert_refs(); /* this will assert if a leak is found */
4. Reporting functions:
FDA has 4 functions for reporting various aspects of allocation
status.
fda_get_byte_count() tells you the current total number of bytes
your application has allocated. (this does not include debugging records)
This will give you an idea of how much memory your application is
using at any given time.
fda_get_block_count() returns the total count of current allocations.
fda_enum_locations() calls a user supplied enumeration function with
file, line, count information, this allows you to see your file by
file allocation density. ;) fda_enum_locations() returns the total
number of locations that have memory allocated.
fda_enum_leaks() calls a user supplied enumeration function with
file, line, size, and ptr for every block not marked as referenced.
One use for fda_enum_leaks() is checking for leaks when your program
exits:
void enum_fn(const char* file, int line, size_t size, void* ptr)
{
printf("Memory leak: %s: %d - %d bytes at (%p)\n", file, line, size, ptr);
}
int main(void)
{
...
#if defined(DEBUG)
/* check for memory leaks before exiting */
fda_clear_refs();
fda_enum_leaks(enum_fn);
#endif
return 0; /* return from main */
}
The test file fdatest.c gives examples of usage for most of the functions
available with FDA.
Please let me know if you encounter any problems with FDA.
So far FDA has been built and tested on linux and Windows NT.
If you find that it works with or without change on other platforms
please let me know so I can make the appropriate changes to the
code and add it to the list of tested platforms.
