File Transfer Using TCP on Linux

Question

I am trying to transfer a TCP file to Linux. After the connection is established, the server should send "send.txt" to the client, and the client will receive the file and save it as "receive.txt". Then the connection is interrupted.

The correct input and output should be:

Server Terminal:

$./server & [server] obtain socket descriptor successfully. [server] bind tcp port 5000 in addr 0.0.0.0 successfully. [server] listening the port 5000 successfully. [server] server has got connect from 127.0.0.1. [server] send send.txt to the client…ok! [server] connection closed. 

Client terminal:

 $./client [client] connected to server at port 5000…ok! [client] receive file sent by server to receive.txt…ok! [client] connection lost. 

And both the server and the client must exit after the process.

But now I have

 $ ./server & [server] obtain socket descriptor successfully. [server] bind tcp port 5000 in addr 0.0.0.0 sucessfully. [server] listening the port 5000 sucessfully. [server] server has got connect from 127.0.0.1. [server] send send.txt to the client...ok! [server] connection closed. /*Here the server doesn't exit*/ $ ./client [client] connected to server at port 5000...ok! /*Here the client doesn't exit*/ 

An EMPTY "receive.txt" is also created.

My code was first written to pass simple strings, and it worked correctly. Therefore, I assume the problem is the file transfer.

My code is as follows:

server.c

 #include <stdlib.h> #include <stdio.h> #include <errno.h> #include <string.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/wait.h> #include <sys/socket.h> #define PORT 5000 // The port which is communicate with server #define BACKLOG 10 #define LENGTH 512 // Buffer length int main () { int sockfd; // Socket file descriptor int nsockfd; // New Socket file descriptor int num; int sin_size; // to store struct size struct sockaddr_in addr_local; struct sockaddr_in addr_remote; /* Get the Socket file descriptor */ if( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1 ) { printf ("ERROR: Failed to obtain Socket Descriptor.\n"); return (0); } else printf ("[server] obtain socket descriptor successfully.\n"); /* Fill the local socket address struct */ addr_local.sin_family = AF_INET; // Protocol Family addr_local.sin_port = htons(PORT); // Port number addr_local.sin_addr.s_addr = INADDR_ANY; // AutoFill local address bzero(&(addr_local.sin_zero), 8); // Flush the rest of struct /* Bind a special Port */ if( bind(sockfd, (struct sockaddr*)&addr_local, sizeof(struct sockaddr)) == -1 ) { printf ("ERROR: Failed to bind Port %d.\n",PORT); return (0); } else printf("[server] bind tcp port %d in addr 0.0.0.0 sucessfully.\n",PORT); /* Listen remote connect/calling */ if(listen(sockfd,BACKLOG) == -1) { printf ("ERROR: Failed to listen Port %d.\n", PORT); return (0); } else printf ("[server] listening the port %d sucessfully.\n", PORT); int success = 0; while(success == 0) { sin_size = sizeof(struct sockaddr_in); /* Wait a connection, and obtain a new socket file despriptor for single connection */ if ((nsockfd = accept(sockfd, (struct sockaddr *)&addr_remote, &sin_size)) == -1) printf ("ERROR: Obtain new Socket Despcritor error.\n"); else printf ("[server] server has got connect from %s.\n", inet_ntoa(addr_remote.sin_addr)); /* Child process */ if(!fork()) { char* f_name = "send.txt"; char sdbuf[LENGTH]; // Send buffer printf("[server] send %s to the client...", f_name); FILE *fp = fopen(f_name, "r"); if(fp == NULL) { printf("ERROR: File %s not found.\n", f_name); exit(1); } bzero(sdbuf, LENGTH); int f_block_sz; while((f_block_sz = fread(sdbuf, sizeof(char), LENGTH, fp))>0) { if(send(nsockfd, sdbuf, f_block_sz, 0) < 0) { printf("ERROR: Failed to send file %s.\n", f_name); break; } bzero(sdbuf, LENGTH); } printf("ok!\n"); success = 1; close(nsockfd); printf("[server] connection closed.\n"); while(waitpid(-1, NULL, WNOHANG) > 0); } } } 

client.c

 #include <stdio.h> #include <stdlib.h> #include <errno.h> #include <string.h> #include <netdb.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #define PORT 5000 #define LENGTH 512 // Buffer length int main(int argc, char *argv[]) { int sockfd; // Socket file descriptor char revbuf[LENGTH]; // Receiver buffer struct sockaddr_in remote_addr; /* Get the Socket file descriptor */ if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { printf("ERROR: Failed to obtain Socket Descriptor!\n"); return (0); } /* Fill the socket address struct */ remote_addr.sin_family = AF_INET; remote_addr.sin_port = htons(PORT); inet_pton(AF_INET, "127.0.0.1", &remote_addr.sin_addr); bzero(&(remote_addr.sin_zero), 8); /* Try to connect the remote */ if (connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr)) == -1) { printf ("ERROR: Failed to connect to the host!\n"); return (0); } else printf("[client] connected to server at port %d...ok!\n", PORT); //printf ("OK: Have connected to %s\n",argv[1]); printf("[client] receive file sent by server to receive.txt..."); char* f_name = "receive.txt"; FILE *fp = fopen(f_name, "a"); if(fp == NULL) printf("File %s cannot be opened.\n", f_name); else { bzero(revbuf, LENGTH); int f_block_sz = 0; int success = 0; while(success == 0) { while(f_block_sz = recv(sockfd, revbuf, LENGTH, 0)) { if(f_block_sz < 0) { printf("Receive file error.\n"); break; } int write_sz = fwrite(revbuf, sizeof(char), f_block_sz, fp); if(write_sz < f_block_sz) { printf("File write failed.\n"); break; } bzero(revbuf, LENGTH); } printf("ok!\n"); success = 1; fclose(fp); } } close (sockfd); printf("[client] connection lost.\n"); return (0); } 

Many thanks!

Answer 1

You need to add the code to the f_block_sz code. recv() has several positive return values:

• <0 - Error, error number in errno ( errno.h )
• 0 - Connection closed
• >0 - read data, number of bytes

You need to handle the second case. Add this else case:

 else if(f_block_sz) { break; } 

So that the loop is broken when the server closes the connection and your code prints you [client] connection lost and exits.

Answer 2

You have one more problem that your server program is multiprocessor and fork at each incoming connection. The parent remains alive to accept new connections, and the child process processes the connection. Exiting the child process is not enough to force the parent to exit as well.

If you only want to handle one connection, then do not use fork.

If you want to continue your current setup, you will need to modify the child process to use _exit instead of exit and force the parent process to process the SIGCHLD signal (received when the child process exited).

t

 #include <signal.h> void terminate(int signal) { // close sockfd -- you will need to make it global // or have terminate alter some global variable that main can monitor to // detect when it is meant to exit exit(0); // don't exit if you choose the second option } int main() { signal(SIGCHLD, terminate); ... }