Printing values of type void*

问题

In the below:

int cmp (void* x, void*y) {
    printf("x: %s | y: %s\n", (char*) x, (char*) y);
    return 0;
}

int main(int argc, char *argv[]) {
    char * strings[] = {"xml", "json"};
    qsort(strings, sizeof(strings), sizeof(strings[0]), cmp);
}

How would one see what the values of x,y are? The current approach produces gibberish such as:

x: 2?|?? | y: 2?|??

回答1:

Read the documentation of qsort again. The arguments passed to your comparison function are not the objects to be compared, they are pointers to those objects. So x,y are not the strings themselves (i.e. the pointers to the characters of the strings), but pointers to those pointers.

Thus you want to write

int cmp (void *x, void *y) {
    printf("x: %s | y: %s\n", *(char**) x, *(char**) y);
    return 0;
}

Also see Joshua's answer about the problem with the size arguments to qsort.

Finally, the qsort comparison function is supposed to take pointers to const void, not just void. And it is good practice to keep things const when you do not intend to modify them. In this case, you do not intend to modify the pointer that x points to, nor the characters that *x points to. So it would be even better to write

int cmp (const void *x, const void *y) {
    printf("x: %s | y: %s\n", *(const char* const *) x, *(const char* const *) y);
    return 0;
}

回答2:

sizeof(strings) does not return the number of elements in strings but rather something larger. Your program is undefiend. You want sizeof(strings)/sizeof(strings[0]).

While passing sizeof(argv[0]) for the third argument happens to work, you really should say sizeof(strings[0]) otherwise your code is hard to read.

The gibberish is caused by qsort reading out of range on array strings and passing wild pointers to cmp. Most of the nonsense is unprintable so you get lots of ? characters.

Nate Eldredge caught the other half. a and b arguments to cmp should be cast to char ** not char *.

来源：https://stackoverflow.com/questions/58045052/printing-values-of-type-void

标签

void