How to know the size (in bytes) of a char array passed to a function?

问题

I am testing the sizeof operator. In two cases in my code, I get the size of the pointer (I think). In the other cases I get how many bytes the arrays occupy. How can I get the size of the array in bytes when I pass it to a function? Isn't the sizeof operator enough? Am I doing something wrong?

#include <stdio.h>

/*Testing the operator sizeof*/

void test (char arrayT[]);
void test2 (char *arrayU);

int main(int argc, char *argv[]){

    char array1[7];
    printf("array1 size is:%d\n", sizeof array1);
    /*array1 size is: 7*/

    char array2[] = { '1', '2', '3', '4', '5', '6', '7'};
    printf("array2 size is:%d\n", sizeof array2);
    /*array2 size is: 7*/

    char array3[] = "123456";
    printf("array3 size is:%d\n", sizeof array3);
    /*array3 size is: 7*/

    unsigned char array4[] = "123456";
    printf("array4 size is:%d\n", sizeof array4);
    /*array4 size is: 7*/

    char arrayX[] = "123456";
    test(arrayX);
    /*arrayT size is: 4*/

    char arrayY[] = "123456";
    test2(&arrayY[0]);
    /*arrayU size is: 4*/

    return 0;
}

void test (char arrayT[]){
    printf("arrayT size is:%d\n", sizeof arrayT);
}

void test2 (char *arrayU){
    printf("arrayU size is:%d\n", sizeof arrayU);
}

回答1:

When an array is passed to a function, what's actually happening is that a pointer to the first element in the array is being passed. Put another way, the array decays into a pointer to the first element.

In these two declarations:

void test (char arrayT[]);
void test2 (char *arrayU);

arrayT and arrayU are of exactly the same type due to this decay, and sizeof will return the same value for both, i.e. the size of a char *.

Contrast the above with this:

char array1[] = "a string";
char *array2;

Where array1 is actually an array of size 9, while array2 is a pointer whose size (on your system) is 4.

Because of this, there is no way to know the length of an array passed to a function. You need to pass in the size as a separate parameter:

void test (char arrayT[], size_t len);

来源：https://stackoverflow.com/questions/33221983/how-to-know-the-size-in-bytes-of-a-char-array-passed-to-a-function

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arrays

pointers

char

sizeof