如何在 C 中创建 char* 字符串的 char* substring？

Question

I want to create a substring, which I see online everywhere and it makes sense. However, is there any way to, instead of outputting to a regular array of characters, output the substring as a char* array?

This is the idea of my code:

char *str = "ABCDEF";
char *subStr = calloc(3, sizeof(char));
memcpy(subStr, &str[3], 3);
fprintf(log, "Substring: %s", subStr);

I am hoping this will print out DEF. Let me know what you guys think I should do, or if this will work. Thanks!

Answer 1

If you need just to output a substring then you can write

fprintf(log, "Substring: %.*s", 3, str + 3);

Here is a demonstrative program.

#include <stdio.h>

int main(void) 
{
    char *str = "ABCDEF";
    FILE *log = stdout;
    
    fprintf(log, "Substring: %.*s", 3, str + 3);
    
    return 0;
}

The program output is

Substring: DEF

Answer 2

Your code does not create C substring as you allocate only 3 element char array, but you also need the 4th one for the null terminating character.

char *str = "ABCDEF";
char *subStr = calloc(4, sizeof(char));
memcpy(subStr, &str[3], 3);

or less expensive

char *str = "ABCDEF";
char *subStr = malloc(4);
memcpy(subStr, &str[3], 3);
substr[3] = 0;

You should also check if the result of the allocation was successful,

char *str = "ABCDEF";
char *subStr = calloc(4, sizeof(char));
if(subStr) memcpy(subStr, &str[3], 3);

Answer 3

You have to terminate the string by adding terminating null-character.

const char *str = "ABCDEF"; /* use const char* for constant string */
char *subStr = calloc(3 + 1, sizeof(char)); /* allocate one more element */
memcpy(subStr, &str[3], 3);
fprintf(log, "Substring: %s", subStr);

calloc() will zero-clear the buffer, so no explicit terminating null-character is written here.