dynamic memory allocation & fragmentation in c & c++

Dynamic Memory Allocation & Fragmentation

in C & C++

Dynamic Memory Allocation & Fragmentation

in C & C++

Colin [email protected]

mailto:[email protected]

2www.mentor.com/embedded


AgendaAgenda

▪ Introduction to memory usage▪ Dynamic memory in C▪ Dynamic memory in C++▪ Issues and problems▪ Memory with an RTOS▪ Real time memory solutions▪ Conclusions

http://www.mentor.com/embedded




AgendaAgenda






C/C++ Memory SpacesC/C++ Memory Spaces

▪ Static memory▪ variables outside of

functions▪ static internal variables

▪ keyword static▪ program sections [FGA]

RA

M a

ddre

ss

Static storage

Variables









▪ Automatic variables▪ stack▪ register▪ keyword auto

RA

M a

ddre

ss

Static storage

Dynamic storage

Variables

Stack









▪ Automatic variables▪ stack▪ register▪ keyword auto

▪ HeapR

AM

add

ress

Static storage

Dynamic storage

Heap

Variables

Stack






▪ With an operating system, there are typically multiple stacks R

AM

add

ress

Static storage

Dynamic storage

Stack

Heap

Variables

Stack

Stack

Stack





AgendaAgenda






Dynamic Memory in CDynamic Memory in C

▪ Management of heap space▪ Key functions

▪ malloc()▪ free()

void *malloc(size_t size); void free(void *pointer);






int my_array[10];my_array[3] = 99;

int *pointer;pointer = malloc(10 * sizeof(int));*(pointer+3) = 99;pointer[3] = 99;...free(pointer);pointer = NULL;






Heap

Size info

Data space

Pointer from malloc()






▪ Variant functions▪ calloc()

▪ simpler parameters▪ initialized to zero

▪ realloc()▪ copies as necessary

void *calloc(size_t nelements, size_t elementSize);void *realloc(void *pointer, size_t size);





AgendaAgenda






Dynamic Memory in C++Dynamic Memory in C++

▪ Same principle as C▪ Library functions available▪ New operators more

flexible and less error prone:

▪ new▪ delete

▪ care needed to use correct deallocator

▪ No reallocator▪ Also STL

p_var = new typename;p_var = new type(initializer);p_array = new type [size];

delete p_var;delete[] p_array;





Dynamic Memory in C++Dynamic Memory in C++

int my_array[10];my_array[3] = 99;

int* pointer;pointer = new int[10];

pointer[3] = 99;

...delete[] pointer;pointer = NULL;





AgendaAgenda






Issues and ProblemsIssues and Problems

▪ Dynamic behavior is troublesome in real time embedded systems

▪ Issues:▪ resource exhaustion▪ non-determinism

▪ Examples:▪ dynamic memory▪ RTOS objects






▪ Stack overflow/underflow▪ push too much data▪ pop when stack empty▪ hard to locate bug

▪ errors may show up much later▪ most likely in another task






▪ malloc() etc.

▪ reentrancy▪ determinism of allocation time▪ memory leaks

▪ mismatch of allocation and deallocation▪ lead to performance degradation over time

▪ allocation failure▪ what to do?▪ not enough memory▪ fragmentation …





Heap [10K]

Memory FragmentationMemory Fragmentation

#define K (1024)...char *p1;

p1 = malloc(3*K);

3K

[7K free]

p1





Heap [10K]


#define K (1024)...char *p1, *p2;

p1 = malloc(3*K);p2 = malloc(4*K);...

Heap [10K]

3K

4K

[3K free]

p1

p2





Heap [10K]


#define K (1024)...char *p1, *p2;


free(p1);

Heap [10K]

[3K free]

4K

[3K free]

p2





Heap [10K]


#define K (1024)...char *p1, *p2;


free(p1);

p1 = malloc(4*K); //fails!

Heap [10K]

[3K free]

4K

[3K free]

p2






▪ Defragmentation not possible with direct pointers

▪ other code using memory address would be broken

▪ Garbage collection performed in other languages

▪ e.g. Java, BASIC▪ no direct pointers▪ not real time





AgendaAgenda






Memory with an RTOSMemory with an RTOS

▪ All OSes provide some memory management facilities

▪ compatible with real time requirements▪ deterministic

▪ May offer re-entrant malloc() facility▪ normally not deterministic

▪ Block/partition memory allocation





Block/partition Memory AllocationBlock/partition Memory Allocation

▪ Partition pool created either statically or dynamically

▪ Fixed size blocks of memory

▪ Specified number of blocks

▪ Resource availability information

▪ Failure modes limited▪ fragmentation not

possible▪ allocation failure

possible▪ task suspend option

▪ memory leaks possible






STATUS NU_Create_Partition_Pool(NU_PARTITION_POOL *pool,CHAR *name, VOID *start_address, UNSIGNED pool_size, UNSIGNED partition_size, OPTION suspend_type)

status = NU_Create_Partition_Pool(&MyPool, “any name”,(VOID *) 0xB000, 2000, 40, NU_FIFO);

▪ descriptor MyPool▪ 40 byte partitions▪ 2000 byte memory area▪ located at 0xB000▪ task suspend in FIFO order






status = NU_Allocate_Partition(&MyPool, &ptr, NU_SUSPEND);

▪ allocate a partition from pool described by MyPool▪ ptr will point to allocated memory▪ task suspend on allocation failure

status = NU_Deallocate_Partition(ptr);





Memory Leak DetectionMemory Leak Detection





AgendaAgenda






StacksStacks

▪ Sizing is a challenge▪ almost impossible to

assess statically▪ best measured

dynamically▪ fill with known value▪ use debugger

trace/coverage

guard word

stack

SP guard word▪ Overflow/underflow

▪ can add protection▪ use guard words or

MMU





Dynamic MemoryDynamic Memory

▪ Define series of partition pools▪ Geometric series of sizes

▪ e.g. 32, 64, 128, 256 bytes

▪ Write malloc() to select best fit▪ can be deterministic

▪ Map free() directly on to deallocate API▪ Failure modes:

▪ no fragmentation▪ allocation failure causes task suspend





AgendaAgenda






ConclusionsConclusions

▪ Dynamic behavior, including memory allocation, is an issue for real time

▪ non-determinism▪ failure

▪ Fragmentation can be eliminated▪ Determinism possible▪ Failure modes can be contained



Questions?Questions?

Colin [email protected]

blogs.mentor.com/colinwalls

mailto:[email protected]

dynamic memory allocation & fragmentation in c & c++

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