malloc在Keil C中没有失败

Question

How can i check whether malloc() was fail in Keil C?

unsigned char xdata malloc_mempool [0x100];
void display()
{
    unsigned char xdata *ptr;
    int a;
    init_mempool (&malloc_mempool, sizeof(malloc_mempool));

    ptr = malloc(9000000);
    if(ptr != 0)
    {
        a = 7;
        free(ptr);
    } else {
        a = 9;
    }
}

As stated in http://www.keil.com/support/man/docs/c51/c51_malloc.htm malloc will return null pointer if there is not enough memory to satisfy the allocation request. It is obviously not so mush memory in 8051. But the result of a is always 7.

Answer 1

From Understanding The Memory Organization Of 8051 Microcontroller the 8051 has a maximum 64KB of memory.

So it will be using 2 bytes for pointers. 9000000 decimal is 0x895440 in hex, which is 3 bytes.

Very probably the malloc() function will just ignore the bits it cannot use, so in reality the call you are making is

ptr = malloc(0x5440);

which is

ptr = malloc(21568);