关于C中unsigned char *的奇怪问题

unsigned char* someMethod(num)
unsigned short num;
{
    //do some stuff with num and change values of a
    unsigned char * a = (unsigned char*) malloc(4);
    printf("a0 is %xn",a[0]);
    printf("a1 is %xn",a[1]);
    printf("a2 is %xn",a[2]);
    printf("a3 is %xn",a[3]);
    return a;
}

当我调用someMethod(128)时：

unsigned char* s = someMethod(128);
printf("s0 is %xn",s[0]);
printf("s1 is %xn",s[1]);
printf("s2 is %xn",s[2]);
printf("s3 is %xn",s[3]);

它会打印出来

a0 is 30
a1 is 1
a2 is 31
a3 is 30
s0 is 30
s1 is 14
s2 is ffffff9d
s3 is 0

因为我指定了s = someMethod(128),所以对我来说没有任何意义. a和s不应该有相同的值吗？！？
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