第二次recv调用不接收数据，暂停C中的执行

Question

This is my client program that requests files from the server:

#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;
}

This is my server program that accepts request for files from the client:

 #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);   
    } 

The problem is that in the client, my second recv() call, where it is supposed to receive the contents of a file, shows nothing. The programs halts at that point, but the server programs displays that it has sent the file contents. The client doesn't receive it.

Answer 1

The basic problem is that you're not checking the return values to see how much data you actually sent and received. So when the client calls:

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

it will receive up to sizeof(msg) (512) bytes, which is probably both the OK message the server is sending AND the file contents (after the NUL). Which means when it does a second recv call to get the contents, it blocks, because it already read the contents in the first call and there's no more data waiting in the receive buffer.

Answer 2

Your error-checking is haphazard, and consequently you're certainly missing a problem that occurs before the behavior you're observing. I recommend you follow RW Steven's idiom:

int n;
if( (n = recv(new_s, buf, sizeof(buf), 0)) < 0 ) {
    err(EXIT_FAILURE, "recv %d", __LINE__);
}

Test every function call, and handle every error. For simple programs, just call err (3) on error. Do that consistently, and the program's behavior will be much less mysterious (if still occasionally surprising). And don't be afraid of the spacebar! It's easy to hit, and easier still to read.

My other bit of advice, if I may, concerns

int send_file_name,rec_msg;

Names like that are confusing. A name is almost never an integer. For I/O sizes, just use one simple name like n , len , or size . Even if you don't care for yourself, you want to care before publishing your question in an open forum. Otherwise, when people see

send_file_name=send(s,filename,strlen(filename)+1,0);

they may think send is some function other than send (2), or that the person asking the question was careless.

Answer 3

The main problem I see is that neither the client nor the server are handling socket I/O correctly in general. They are not handling the cases where reads and writes transfer fewer bytes then requested, you need to loop the I/O. And the client is reading too many bytes from the server anyway, which is why your second recv() is blocking. And you are relying on a disconnect to indicate the end of the file has been reached, but that does not allow the client to do adequate error checking to know if the full file was actually received or not.

Also, when sending the content of a file, the server is attempting to read the entire file into memory (bad!), not doing adequate error checking on the file I/O, and it is treating the file content as text instead of as binary (don't use strlen() on binary data!).

Try something more like this instead:

#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

int sendstring(int sock, const char *str) {
    if (!str) str = "";
    int len = strlen(str) + 1;

    do {
        int ret = send(sock, str, len, 0);
        if (ret <= 0) return -1;
        str += ret;
        len -= ret;
    }
    while (len > 0);

    return 0;
}

int readbuf(int sock, void *buf, int buflen) {
    char *pbuf = (char*) buf;

    while (buflen > 0) {
        int len = recv(sock, pbuf, buflen, 0);
        if (len <= 0) return -1;
        pbuf += len;
        buflen -= len;
    }

    return 0;
}

int readstring(int sock, char *str, int maxlen) {
    while (maxlen > 0) {
        if (recv(sock, str, 1, 0) <= 0) return -1;
        if (*str == '\0') return 0;
        ++str;
        --maxlen;
    }
    return -2;
}

int readfile(int sock, int fd) {
    int filesize;
    char buf[MAX_LINE];

    if (readbuf(sock, &filesize, sizeof(filesize)) < 0) return -1;
    filesize = ntohl(filesize);

    while (filesize > 0) {
        int len = readbuf(sock, buf, min(sizeof(buf), filesize));
        if (len < 0) return -1;
        if (fwrite(buf, len, 1, fp) != 1) return -2;
        filesize -= len;
    }

    return 0;
}

int main(int argc, char * argv[]) {
    char filename[MAX_LINE], reply[MAX_LINE];

    if (argc != 2) {
        fprintf(stderr, "usage: simplex-talk host\n");
        exit(1);
    }

    char *host = argv[1];

    /* translate host name into peer's IP address */
    struct hostent *hp = gethostbyname(host);
    if (!hp) {
        fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
        exit(1);
    }

    if (hp->h_addrtype != AF_INET) {
        fprintf(stderr, "simplex-talk: unsupported address type %d for host: %s\n", hp->h_addrtype, host);
        exit(1);
    }

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

    printf("Host's remote IP: %s\n", inet_ntoa(&sin.sin_addr));

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

    printf("Client created socket.\n");

    if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
    {
        perror("simplex-talk: connect");
        close(s);
        exit(1);
    }

    printf("Client connected.\n");

    /* main loop: get and send lines of text */
    do {
        printf("Enter the file name ('bye' to quit):\n");

        if (scanf("%512s", filename) != 1) {
            printf("Error reading filename\n");
            break;
        }

        if (strcmp(filename, "bye") == 0) {
            sendstring(s, "bye");
            break;
        }

        if (sendstring(s, filename) < 0) {
            printf("Error sending filename\n");
            break;
        }

        if (readstring(s, reply, sizeof(reply)) < 0) {
            printf("Error reading reply\n");
            break;
        }

        if (strcmp(reply, "OK") != 0) {
            printf("%s\n", reply);
            if (strcmp(reply, "bye") == 0) break;
            continue;
        }

        FILE *fp = fopen(filename, "wb");
        if (!fp) {
            printf("Error opening file\n");
            break;
        }

        printf("Receiving file\n");

        int ret = readfile(s, fd);
        fclose(fp);

        if (ret < 0) {
            if (ret == -2)
                printf("Error writing file\n");
            else
                printf("Error reading file\n");

            break;
        }

        printf("Received file\n");
    }
    while (1);

    close(s);
    return 0;
}

#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 512

int sendbuf(int sock, void *buf, int buflen) {
    char *pbuf = (char*) buf;

    while (len > 0) {
        int len = send(sock, pbuf, buflen, 0);
        if (len <= 0) return -1;
        pbuf += len;
        buflen -= len;
    }

    return 0;
}

int sendstring(int sock, const char *str) {
    if (!str) str = "";
    return sendbuf(sock, str, strlen(str) + 1);
}

int sendfile(int sock, int fd) {
    char buf[MAX_LINE];

    struct stat s;
    if (fstat(fd, &s) < 0) return -2;

    int pos = ftell(fp);
    if (pos == -1) return -2;

    int file_size = s.st_size - pos;
    int tmp_file_size = htonl(file_size);
    if (sendbuf(sock, &tmp_file_size, sizeof(tmp_file_size)) < 0) return -1;

    while (file_size > 0) {
        int len = fread(buf, 1, min(sizeof(buf), file_size), fp);
        if (len < 1) return -2;
        if (sendbuf(sock, buf, len) < 0) return -1;
        file_size -= len;
    }

    return 0;
}

int readstring(int sock, char *str, int maxlen) {
    while (maxlen > 0) {
        if (recv(sock, str, 1, 0) <= 0) return -1;
        if (*str == '\0') return 0;
        ++str;
        --maxlen;
    }
    return -2;
}

int main() {
    char msg[MAX_LINE];

    struct sockaddr_in sin;
    bzero((char *)&sin, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = INADDR_ANY;
    sin.sin_port = htons(SERVER_PORT);

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

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

    if (bind(s, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
        perror("simplex-talk: bind");
        close(s);
        exit(1);
    }

    printf("Server bind done.\n");

    if (listen(s, MAX_PENDING) < 0) {
        perror("simplex-talk: listen");
        close(s);
        exit(1);
    }

    printf("Server Listening.\n");

    /* wait for connection, then receive and print text */
    do {
        int len = sizeof(sin);
        int cli_s = accept(s, (struct sockaddr *)&sin, &len);
        if (cli_s < 0) {
            perror("simplex-talk: accept");
            close(s);
            exit(1);
        }

        printf("Client connected\n");

        do {
            if (readstring(cli_s, msg, sizeof(msg)) < 0) {
                printf("Error reading request\n");
                break;
            }

            if (strcmp(msg, "bye") == 0) break;

            printf("File requested: %s\n", msg);

            FILE *fp = fopen(msg, "rb");
            if (!fp)
            {
                printf("Cannot open file\n");
                if (sendstring(cli_s, "Cannot open file") < 0) {
                    printf("Error sending reply\n");
                    break;
                }
                continue;
            }

            printf("File found :) \n");

            if (sendstring(cli_s, "OK") < 0) {
                printf("Error sending reply\n");
                fclose(fp);
                break;
            }

            ret = sendfile(cli_s, fp);
            fclose(fp);

            if (ret < 0) {
                printf("Error sending file\n");
                break;
            }

            printf("File sent to client\n");
        }
        while (1);

        close(cli_s);   
    } 
    while (1);

    close(s);
    return 0;
}