将char数组复制到struct的数组中的struct的char数组字段？

Question

如果问题太混乱，请原谅我！

我有以下结构：

struct msg { 
    char data[20];
}

struct pkt {
   int seqnum;
   int acknum;
   int checksum;
   char payload[20];
};

我有以下结构数组：

struct msg buffer[1000];

现在，我正在做以下事情：

void A_output(struct msg message)
{
    //printf("Message in A: %s\n", message.data);
    if(inTransit == 0) {
        if(hasBuffer > 0) {
            printf("%s\n", "Sending from buffer...");
            memset(&currentpkt_A->payload, '\0', 20);

            strncpy(currentpkt_A->payload, buffer[buffCount].data, 20);
            currentpkt_A->seqnum = seq_A;
            setChecksum(currentpkt_A);
            printf("Checksum from A: %d\n", currentpkt_A->checksum);
            printf("Payload from A: %s\n", currentpkt_A->payload);
            tolayer3(0, *currentpkt_A);
            inTransit = 1;

            starttimer(0, increment);

            printf("%s\n", "Adding to buffer when sending directly...");
            strncpy(buffer[hasBuffer].data, message.data, 20);
            hasBuffer++;
            buffCount++;
        }
        else {
            printf("Sending directly...\n");
            memset(&currentpkt_A->payload, '\0', 20);

            strncpy(currentpkt_A->payload, message.data, 20);
            currentpkt_A->seqnum = seq_A;
            setChecksum(currentpkt_A);
            printf("Checksum from A: %d\n", currentpkt_A->checksum);
            printf("Payload from A: %s\n", currentpkt_A->payload);
            tolayer3(0, *currentpkt_A);

            starttimer(0, increment);

            inTransit = 1;
        }
    }
    else {
        printf("%s\n", "Adding to buffer when in transit...");
        strncpy(buffer[hasBuffer].data, message.data, 20);
        hasBuffer++;
    }
    printf("Buffer now has: \n");
    for(int i = 0; i<hasBuffer; i++)
        printf("%s\n", buffer[i].data);
}

问题是，当我添加到缓冲区并将其打印出来时，我可以看到以下内容：

在第一个添加缓冲区是：

aaaaaaaaaaaaaaaaaaaa

在第二个添加缓冲区是：

aaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbb

bbbbbbbbbbbbbbbbbbbb

在第三个添加缓冲区是：

aaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbcccccccccccccccccccc

bbbbbbbbbbbbbbbbbbbbcccccccccccccccccccc

cccccccccccccccccccc

也就是说，它被添加到数组的每个单元格的末尾，然后是新单元格。我似乎无法弄清楚为什么会这样。