将ptr分配给char指针

Question

I would like to find out how to get this code to work, my printf in the main function will not print the "test" string.

#include <stdio.h>

int main()
{
char b[10];
test(b);
printf("from main func: %s\n", b);
}

int test(char* buf)
{
char len[] = "test";
char *pt = len;

printf("printing ptr: %s\n", pt);

buf = pt;

printf("from test func: %s\n", buf);

return 1;
}

Answer 1

First of you need to have a prototype for the test function or place it above main.

#include <stdio.h>
#include <string.h>

int test(char *);

int main()
{
char b[10];
test(b);
printf("from main func: %s\n", b);
}

int test(char* buf)
{
char len[] = "test";
char *pt = len;

printf("printing ptr: %s\n", pt);

strcpy(buf, pt);

printf("from test func: %s\n", buf);

return 1;
}

Also you should never try to pass a local variable to a function. It will not work, thus you have to use strcpy(buf, pt); instead of pointing the local variable len to buf .

Answer 2

Instead of assigning pt to buf you need to copy the string to buf :

strcpy(buf, pt);

Here is a safer version of your code:

#include <assert.h>
#include <stdio.h>
#include <string.h>

#define LEN(arr) (sizeof (arr) / sizeof (arr)[0])

void test(char *buf, int bufLen)
{
    char len[] = "test";
    char *pt = len;

    assert(bufLen > strlen(len));

    printf("printing ptr: %s\n", pt);
    strcpy(buf, pt);
    printf("from test func: %s\n", buf);
}


int main()
{
    char b[10];

    test(b, LEN(b));
    printf("from main func: %s\n", b);
    return 0;
}

Answer 3

You cannot use buf = pt; in C. The reason being is that C sees "test" as a list of individual characters. like 't', 'e', 's', 't'. Like an array of integers, you could not have something like...

int myArray[5] = { 0, 1, 2, 3, 4 };

and then have something like...

int newArray[5] = myArray... that just wouldn't work, well, an array of characters is no different. You need to copy each character over to the new array, just like you would with an array of ints. You could write your own function to do this, or use one of the built in functions that are available like strcpy() which will copy them for you.