c - 第二次 recv 调用不接收数据，在 C 中停止执行

Question

这是我从服务器请求文件的客户端程序:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define SERVER_PORT 5959
#define MAX_LINE 512

void setstring(char *str){
    str[MAX_LINE-1]='\0';
}

int main(int argc, char * argv[]){
    FILE *fp;
    struct hostent *hp;
    struct sockaddr_in sin;
    char *host;
    char filename[MAX_LINE],buf[MAX_LINE],reply[MAX_LINE],rec_line[MAX_LINE];
    int s;
    char msg[MAX_LINE];
    int len,new_len,rec_file;
    if (argc==2) {
        host = argv[1];
    }
    else {
        fprintf(stderr, "usage: simplex-talk host\n");
        exit(1);
    }

    /* translate host name into peer's IP address */
    hp = gethostbyname(host);
    if (!hp) {
        fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
        exit(1);
    }
    else
        printf("Client's remote host: %s\n", argv[1]);

    /* build address data structure */  
    bzero((char *)&sin, sizeof(sin));
    sin.sin_family = AF_INET;
    bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
    sin.sin_port = htons(SERVER_PORT);

    /* active open */
    if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
        perror("simplex-talk: socket");
        exit(1);
    }
    else
        printf("Client created socket.\n");

    int send_file_name,rec_msg;
    if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
    {
        perror("simplex-talk: connect");
        close(s);
        exit(1);
    }
    else{
        printf("Client connected.\n");
        /* main loop: get and send lines of text */
    printf("Hello from server\n");
    while(!(strcmp(reply,"bye")==0)){
            printf("Enter the file name:\n");
            scanf("%s",filename);
            setstring(filename);
            send_file_name=send(s,filename,strlen(filename)+1,0);
            if(send_file_name<0)
                fputs("Error sending filename",stdout);
            rec_msg=recv(s,msg,sizeof(msg),0);
            if(strcmp(msg,"File not found")==0)
                printf("File not found\n");
            else{
                printf("%s\n",msg);
                fp=fopen(filename,"w");
                printf("CP1\n");
                if(rec_file=recv(s,rec_line,sizeof(rec_line),0)>0){
                    printf("CP2");
                    printf("String recieved:%s\n",rec_line);
                    if(len=fwrite(rec_line,1,rec_file+1,fp)>0)
                        printf("Recieved file\n");
                    else
                        printf("Error writing to file\n");
                }
                else
                    printf("Not recieved\n");
            }
            printf("Enter 'bye' to terminate requesting files\n");
            scanf("%s",reply);
        }
    }
    return 0;
}

这是我的服务器程序，它接受来自客户端的文件请求:

 #include <stdio.h>
 #include <stdlib.h> 
 #include <string.h>
 #include <strings.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <netdb.h>

 #define SERVER_PORT 5959
 #define MAX_PENDING 5
 #define MAX_LINE 256

 void setstring(char* str){
    str[MAX_LINE-1]='\0';
 }

 int main(){
    FILE *fp;
    struct sockaddr_in sin;
    char buf[MAX_LINE],msg[MAX_LINE],*rec_line;
    int len;
    int s, new_s,count;
    char str[INET_ADDRSTRLEN];
    int error_file,send_msg,read_line,send_file;

    bzero((char *)&sin, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = inet_addr("0.0.0.0");
    sin.sin_port = htons(SERVER_PORT);

    /* setup passive open */
    if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
        perror("simplex-talk: socket");
        exit(1);
    }

    inet_ntop(AF_INET, &(sin.sin_addr), str, INET_ADDRSTRLEN);
    printf("Server is using address %s and port %d.\n", str, SERVER_PORT);

    if ((bind(s, (struct sockaddr *)&sin, sizeof(sin))) < 0) {
        perror("simplex-talk: bind");
        exit(1);
    }
    else
        printf("Server bind done.\n");

    listen(s, MAX_PENDING);
    /* wait for connection, then receive and print text */
    while(1) {
        if ((new_s = accept(s, (struct sockaddr *)&sin, &len)) < 0) {
            perror("simplex-talk: accept");
            exit(1);
        }

        printf("Server Listening.\n");
        printf("Greetings\n");
        int rec_file_name=recv(new_s,buf,sizeof(buf),0);
        if(rec_file_name>0)
            printf("File requested:%s\n",buf);
            fp=fopen(buf,"r");
            if(fp==NULL)
            {
                fputs("File not found\n",stdout);
                strcpy(buf,"File not found");
                if(error_file=send(new_s,buf,strlen(buf)+1,0)>0)
                    fputs("Successfully send error message to client\n",stdout);
            }
            else{
                bzero(buf,MAX_LINE);
                printf("File found :) \n");
                strcpy(buf,"OK");
                if(send_msg=send(new_s,buf,strlen(buf)+1,0)>0)
                    fputs("File found message sent to client\n",stdout);

                fseek(fp,0,SEEK_END);
                int file_size=ftell(fp);
                fseek(fp,0,SEEK_SET);
                printf("File size:%d\n",file_size);
                rec_line=(char *)malloc(sizeof(char)*(file_size));
                read_line=fread(rec_line,1,file_size+1,fp);
                printf("File read: %s\n",rec_line);

                if(send_file=send(new_s,rec_line,strlen(rec_line)+1,0)>0)
                    printf("File string sent to client\n");
            }
        }

        close(new_s);   
    } 

问题是在客户端中，我的第二个 recv() 调用(它应该接收文件的内容)没有显示任何内容。程序在那一点停止，但服务器程序显示它已经发送了文件内容。客户端没有收到。

Answer 1

基本问题是您没有检查返回值以查看您实际发送和接收了多少数据。所以当客户端调用时:

rec_msg=recv(s,msg,sizeof(msg),0);

它将接收最多 sizeof(msg) (512) 个字节，这可能是服务器正在发送的 OK 消息和文件内容(在 NUL 之后).这意味着当它执行第二次 recv 调用以获取内容时，它会阻塞，因为它已经在第一次调用中读取了内容并且接收缓冲区中没有更多数据在等待。