CCS: qsort is work wrong, can you tell me why?

lawrence

Tool/software: Code Composer Studio

I add test code in tivxDmpacSdeCreate() fuction, after qsort() I print the result, but is not right, the right order is 10, 7, 5, 3, 1.

Could you help me to find out why qsort() is wrong?

Thanks & Regards,

over 5 years ago

0 Keith Barkley over 5 years ago

Guru 42085 points

The compare function needs to return:

Taking two pointers as arguments (both converted to const void*). The function defines the order of the elements by returning (in a stable and transitive manner):

return value	meaning
`<0`	The element pointed to by `p1` goes before the element pointed to by `p2`
`0`	The element pointed to by `p1` is equivalent to the element pointed to by `p2`
`>0`	The element pointed to by `p1` goes after the element pointed to by `p2`

Your compare function never returns a negative number. Read the docs!

0 Chester Gillon over 5 years ago

Guru 92251 points

The documentation for the qsort comparison function is:

The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second. If two members compare as equal, their order in the sorted array is undefined.

Whereas your compare_res function returns:

1 if a < b
0 if a >= b

Try the following compare_res function:

int compare_res(const void *a, const void *const b)
{
    const uint32_t *pa = a;
    const uint32_t *pb = b;

    if (*pa < *pb)
    {
        return -1;
    }
    else if (*pa == *pb)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}

0 Keith Barkley over 5 years ago in reply to Chester Gillon

Guru 42085 points

Chester:

Why did you make pa and pb const?

0 Chester Gillon over 5 years ago in reply to Keith Barkley

Guru 92251 points

Keith Barkley said:
Why did you make pa and pb const?

Keith, it is avoid a warning because the data to the qsort comparison function is const-qualified.

With the TI ARM v20.2.1 compiler the following is warning free:

int compare_res_const(const void *a, const void *b)
{
    const uint32_t *pa = a;
    const uint32_t *pb = b;

    if (*pa < *pb)
    {
        return -1;
    }
    else if (*pa == *pb)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}

Whereas the following produces warnings:

int compare_res_no_const(const void *a, const void *b)
{
    uint32_t *pa = a;
    uint32_t *pb = b;

    if (*pa < *pb)
    {
        return -1;
    }
    else if (*pa == *pb)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}

"../main.c", line 62: warning #145-D: a value of type "const void *" cannot be used to initialize an entity of type "uint32_t *"
      uint32_t *pa = a;
                     ^
 
"../main.c", line 63: warning #145-D: a value of type "const void *" cannot be used to initialize an entity of type "uint32_t *"
      uint32_t *pb = b;
                     ^

0 Keith Barkley over 5 years ago in reply to Chester Gillon

Guru 42085 points

Ah, thanks.

I would just use uint32_t x = *a;

And skip the pointers all together.

0 lawrence over 5 years ago in reply to Keith Barkley

Prodigy 100 points

Thanks.

Because if two members compare as equal, their order in the sorted array is undefined, on different platforms have different result, it is my code's bug.

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CCS: qsort is work wrong, can you tell me why?