求告知为什么出现warning

//发纸牌  花色有梅花，方块，红桃和黑桃，点数有2.3.4.5.6.7.8.9.10.J.Q.K.A。要求根据用户输入的牌数n，随机发n张纸牌。


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

char *str1[4] = {"梅花","黑桃","红桃","方块"};
char *str2[13] = {"2","3","4","5","6","7","8","9","10","J","Q","K","A"};
char *card[13];
void sendcards(int n);
void printcards(int n);

int main(){
    int n;
    printf("请输入发牌张数：");
    scanf("%d",&n);
    sendcards(n);
    printcards(n);
    return 0;
}

void sendcards(int n){
    int sign [4][13] = {0};
    int k, i, j;
    srand(time(NULL));
    for (k = 0; k < n; ){
        i = rand() % 4;
        j = rand() % 13;
        if (sign[i][j]==1){
            continue;
        }else{
            card[k] = (char*)malloc(6);
            strcpy(card[k],str1[i]);
            strcat(card[k],str2[j]);
            sign[i][j] = 1;
            k++;
        }
    }
    return ;
}

void printcards(int n){
    int k;
    for(k = 0; k < n; k++){
        printf("%-10s",card[k]);
    }
    printf("\n");
    return ;
}