C如何转换为十六进制整数

im writing a code for converting a number to hexadecimal and im getting a random numbers as result. at first i succeed to convert the number but it was in a reversed order (the first mod needs to be the last number or letter). the code is a part (for cases of %x) from a big project that is kind of implementation of sprintf (so sprintf or printf are not allowed obviously). so the buffer is for composing a string without any placeholders. thank u in advance.

这是我的代码：

int num = *(int*)ptrs[counter];
int tempnum=num;
int mod=0;
int length =0;
for(int i=0;tempnum !=0;i++)
{
    length++;
    tempnum /= 16;
}
int array[length];
for(int i= length; i>0;i--)
{
    mod = num%16;
    num = num/16;
    array[i] = mod;
}
for(int i=0;i<length;i++)
{
    if(array[i]<10)
        *buffer = array[i]+ '0';
    else
        *buffer = array[i] + '87';
    buffer++;
}