c - 为什么我在 C 中使用 UDP 套接字接收文件时会出现一些垃圾字节？

Question

我用 C 语言编写了一个客户端/服务器程序，其中客户端将文件发送到服务器。它可以工作，但是一些垃圾字节被添加到接收到的文件中字节的 1400 位置。我尝试使用文本文件，并在发送之前将内存设置为 0。为什么是垃圾字节？

代码如下:

宏.h

#ifndef MACRO_H
#define MACRO_H

#include <stdio.h>
#include <errno.h>
#include <stdint.h>


#define SERV_PORT 49312
#define MAXLINE   1400

#define END_FLAG "================END"

#define abort_on_error(cond, msg) do {                                          \
    if (cond) {                                                                 \
        fprintf(stderr, "%s (errno=%d [%s])\n", msg, errno, strerror(errno));   \
        exit(EXIT_FAILURE);                                                     \
    }                                                                           \
} while (0)

struct pkt {
    // struct sockaddr_in addr;
    // int sockfd;
    uint16_t seq_num;
    uint32_t status;
    uint8_t data[MAXLINE];
};

typedef struct pkt PACK;

#endif // MACRO_H

客户端.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <fcntl.h>

#include "macro.h"

int main(int argc, char **argv)
{
    int sockfd, n, count = 0;
    struct sockaddr_in servaddr;
    // char buf[MAXLINE];
    char *target, *path;
    FILE *file;
    PACK packet;
    long filesize;

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(SERV_PORT);
    inet_pton(AF_INET, argv[1], &servaddr.sin_addr);

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);

    path = argv[2];
    target = argv[3];

    packet.seq_num = 0;
    packet.status = 0;
    memset(packet.data, 0, sizeof(packet.data));
    memcpy(packet.data, target, strlen(target));

    abort_on_error(sendto(sockfd, (struct pkt *) &packet, sizeof(packet), 0, (struct sockaddr *) &servaddr, sizeof(servaddr)) == -1,
                    "sendto() filename error");

    memset(packet.data, 0, sizeof(packet.data));
    abort_on_error(recvfrom(sockfd, &packet, sizeof(packet), 0, NULL, NULL) == -1, "recvfrom() ok error");
    if (!strncmp((const char *) packet.data, "ok", 2)) {
        printf("Filename sent.\n");
    }

    file = fopen(path, "r");
    abort_on_error(file == NULL, "fopen() error");

    fseek(file, 0L, SEEK_END);
    filesize = ftell(file);
    printf("filesize: %ld\n", filesize);
    fseek(file, 0L, SEEK_SET);
    memset(packet.data, 0, sizeof(packet.data));
    sprintf((char *) packet.data, "%ld", filesize);

    abort_on_error(sendto(sockfd, (struct pkt *) &packet, sizeof(packet), 0, (struct sockaddr *) &servaddr, sizeof(servaddr)) == -1,
                        "sendto() data error");

    memset(packet.data, 0, sizeof(packet.data));
    while ((n = fread(packet.data, sizeof(char), MAXLINE, file))) {
        // packet.data[n] = '\0';
        // printf("n: %d\n", n);
        abort_on_error(n == -1, "fread() error");
        abort_on_error(sendto(sockfd, (struct pkt *) &packet, sizeof(packet), 0, (struct sockaddr *) &servaddr, sizeof(servaddr)) == -1,
                        "sendto() data error");
        count = count + n;
        memset(packet.data, 0, sizeof(packet.data));
    }
    printf("Tot Bytes sent: %d\n", count);

    memset(packet.data, 0, sizeof(packet.data));
    memcpy(packet.data, END_FLAG, strlen(END_FLAG));
    abort_on_error(sendto(sockfd, (struct pkt *) &packet, sizeof(packet), 0, (struct sockaddr *) &servaddr, sizeof(servaddr)) == -1,
                    "sendto() end_flag error");

    fclose(file);
    return 0;
}

服务器.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#include "macro.h"

void run(int sockfd, struct sockaddr *cliaddr, socklen_t clilen)
{
    int n = 0, count = 0, status = 0;
    socklen_t len;
    // char buf[MAXLINE];
    FILE *file;
    PACK packet;

    len = clilen;
    n = recvfrom(sockfd, &packet, sizeof(packet), 0, cliaddr, &len);
    abort_on_error(n == -1, "recvfrom() filename error");
    // buf[n] = 0;
    printf("Received from client: [%s]\n", packet.data);

    file = fopen((const char *) packet.data, "w+");
    abort_on_error(file == NULL, "fopen() error");

    packet.seq_num = 0;
    packet.status = 0;
    memset(packet.data, 0, sizeof(packet.data));
    memcpy(packet.data, "ok", strlen("ok"));
    abort_on_error(sendto(sockfd, (struct pkt *) &packet, sizeof(packet), 0, cliaddr, len) == -1,
                    "sendto() ok error");

    memset(packet.data, 0, sizeof(packet.data));
    abort_on_error(recvfrom(sockfd, &packet, sizeof(packet), 0, cliaddr, &len) == -1,
                    "recvfrom() error");
    long filesize = strtol((const char *) packet.data, NULL, 0);
    printf("filesize: %ld\n", filesize);

    while ((n = recvfrom(sockfd, &packet, sizeof(packet), 0, cliaddr, &len))) {
        abort_on_error(n == -1, "recvfrom() data error");
        // packet.data[n] = '\0';
        // printf("%s\n", packet.data);
        if (!(strcmp((const char *) packet.data, END_FLAG))) {
            break;
        }

        int num_packet = filesize / n;
        // printf("num_packet: %d\n", num_packet);

        count += n;
        if(!num_packet){
            fwrite(packet.data, sizeof(char), n-(n-filesize), file);
            printf("byte to write: %ld\n", n-(n-filesize));
        }
        else {
            if(count <= filesize) {
                fwrite(packet.data, sizeof(char), n-8, file);
                printf("byte to write: %d\n", n);
            }
            else if ((count > filesize) && (filesize-num_packet*n-8) && !status) {
                    fwrite(packet.data, sizeof(char), filesize-num_packet*(n-8), file);
                    printf("byte to write: %ld\n", filesize-num_packet*(n-8));
                    status = 1;
            }
        }
    }
    printf("Tot Bytes received: %d\n", count);
    fclose(file);
}

int main(int argc, char **argv)
{
    int sockfd;
    struct sockaddr_in servaddr, cliaddr;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(SERV_PORT);

    bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

    run(sockfd, (struct sockaddr *) &cliaddr, sizeof(cliaddr));

    return 0;
}

对于小于 1400 字节的文件，它可以完美运行!如果文件大于 1400 字节(需要发送更多数据包)，则会添加一些垃圾字节。我找不到原因!

Answer 1

字符串末尾的垃圾字节是丢失字符串 nul 终止问题的致命弱点。

这里:

memcpy(packet.data, target, strlen(target));

或这里:

memcpy(packet.data, "ok", strlen("ok"));

您正在复制不带终止符的字符串。

现在在接收端你无法知道字符串在哪里结束，fprintf 也不知道。

因此，复制字符串时只需使用 strcpy 而不是 memcpy 即可。它会工作并添加 nul 终止符

strcpy(packet.data, target);

和

strcpy(packet.data, "ok");