字符串 复制前n个字符

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

char *myStrndup(const char *str, size_t n);

int
main(void)
{
    char *s = "Hello world\n";
    char *r = myStrndup(s, 5);
    printf(r);
    free(r);

    return 0;
}

char *
myStrndup(const char *str, size_t n)
{
    char *new_str;

    if (str) {
        new_str= (char *)malloc(sizeof(char)*(n+1));
        strncpy(new_str, str, n);
        new_str[n] = '\0';
    } else {
        new_str= NULL;
    }

    return new_str;
}