Alternatives to allocating memory in Mbed TLS (alternative to malloc calloc()/free)

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Alternative ways of allocating memory in Mbed TLS — Mbed TLS documentation

In some cases, such as when there is no operating system on an embedded platform, the heap and calloc()/ free()or is not really available. Mbed TLS still requires some form of dynamic memory allocation to operate the SSL stack. We could assume a maximum size for all structures, but this would consume a lot of memory space. Instead, we chose to have Mbed TLS only use hooks to allocate and deallocate dynamic memory.

Currently you have two options:

1. Provide your own allocation and deallocation functions.

2. Use the buffer allocator function in Mbed TLS.

To enable the memory allocation layer, define MBEDTLS_PLATFORM_Cand MBEDTLS_PLATFORM_MEMORYin mbedtls_config.h. See How to configure Mbed TLS .

If the layer is not enabled, the libc standard sum calloc()is free()used.

Internal components

Internally, there are only two function pointers mbedtls_calloc()and mbedtls_free()calls for each dynamic memory allocation or deallocation in Mbed TLS.

extern void * (*mbedtls_calloc)( size_t n, size_t size );
extern void (*mbedtls_free)( void *ptr );

libcThe prototypes and standard calloc()sums of these functions free(). Without any further calls, libcthese pointers are given names by default.

no libc equivalent 

If your system does not have libcan equivalent, you will get compile errors like the following calloc()or free()not found.

Defines MBEDTLS_PLATFORM_NO_STD_FUNCTIONSin mbedtls_config.hthe file prevent Mbed TLS from understanding these functions.

Provide your own hooks 

If your operating system already provides libcallocator functions, you can set them with:

int mbedtls_platform_set_calloc_free( void * (*calloc_func)( size_t, size_t ),
                                      void (*free_func)( void * ) );

Use the Mbed TLS buffer allocator 

If you want Mbed TLS to allocate all the content in the static buffer, you can define it MBEDTLS_MEMORY_BUFFER_ALLOC_Cin mbedtls_config.hthe file.

Before calling any other Mbed TLS functions, enable the buffer allocator as follows:

unsigned char memory_buf[100000];
mbedtls_memory_buffer_alloc_init( memory_buf, sizeof(memory_buf) );

Safety Warning

A buffer allocator is a simple approach to a dynamic memory allocator. No special heap protection mechanisms are implemented.

Use the buffer allocator elsewhere 

The buffer allocator itself does not depend on any other part of Mbed TLS. So you can use it in your own code base as well.