Measuring a maximum memory block size.

main.c

#if !__STDC__
# error "This code is written in standard C90."
#else
# include <stdio.h> /* printf|size_t */
# include <stdlib.h> /* EXIT_(FAILURE|SUCCESS)|free|realloc|size_t */

static size_t xSize_( void )
{
	auto struct { size_t Size; void *Void; } New, Old = { 0U, 0 };
	auto size_t Mask = ~Old.Size;

	Mask^= Mask >> 1U;
	do Old = 0 == ( New.Void = realloc( Old.Void, New.Size = Old.Size | Mask ) )? Old: New;
	while( Mask>>= 1U );

	switch( Old.Size ) { default: free( Old.Void ); case 0U: return Old.Size; }
}

extern signed int main( void ) { return 0 <= printf( "%#zx\n", xSize_( ) )? EXIT_SUCCESS: EXIT_FAILURE; }

#endif