c - Setting the first two bytes of a block of memory as a pointer or NULL while still accessing the rest of the block -

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suppose have block of memory such:

void *block = malloc(sizeof(void *) + size);

how set pointer beginning of block while still being able access rest of reserved space? reason, not want assign 'block' pointer or null.

how set first 2 bytes of block null or have point somewhere?

this doesn't make sense unless you're running on 16-bit machine.

based on way you're calling malloc(), you're planning have first n bytes pointer else (where n may 2, 4, or 8 depending on whether you're running on 16-, 32-, or 64-bit architecture). want do?

if is, can create use pointer-to-a-pointer approach (recognizing can't use void* change anything, don't want confuse matters introducing real type):

void** ptr = block;

however, far more elegant define block struct (this may contain syntax errors; haven't run through compiler):

typedef struct {     void* ptr; /* replace void* whatever pointer type */     char[1] data; } my_struct;  my_struct* block = malloc(sizeof(my_struct) + additional); block->ptr = /* */

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