字符串 填充前n个指定字符

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

char *myStrnfill(size_t len, char f_char);

int
main(void)
{
    char *r = myStrnfill(5, 'a');
    printf(r);
    free(r);

    return 0;
}

char *
myStrnfill(size_t len, char f_char)
{
    char *str;

    str = (char *)malloc(sizeof(char)*(len+1));
    memset(str, f_char, len);
    str[len] = '\0';

    return str;
}