Sending multiple messages through a socket in c - c

Question

So in this program I am trying to send the file content and file name from client to server in two steps. I tried using memset() to empty the buffer so as to use it for storing the filename, however then I realised that the filename is being passed into the buffer along with the content, so there is no point of memset(). So, I need to separate the two transmissions I was thinking of closing the buffer for first process and then opening it for the second transmission again, I am not sure how to proceed.
Here's my code:
Server.c
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
int main(int argc, char *argv[]){
int fd =0, confd = 0;
struct sockaddr_in serv_addr;
char buff[1025];
int num;
fd = socket(AF_INET, SOCK_STREAM, 0);
printf("Socket created\n");
memset(&serv_addr, '0', sizeof(serv_addr));
memset(buff, '0', sizeof(buff));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr.sin_port = htons(5000);
bind(fd, (struct sockaddr*)&serv_addr, sizeof(serv_addr));
listen(fd, 10);
while(1){
confd = accept(fd, (struct sockaddr*)NULL, NULL);
FILE* fp = fopen( "create.txt", "wb");
int b = recv(confd, buff, 1024, 0);
if(fp!=NULL){
while(b>0){
fwrite(buff, 1, b, fp);
b = recv(confd, buff, 1024, 0);
}
}
else{
printf("Error!");
}
printf("Got it :)\n");
//printf("Buff before: %s\n", buff);
memset(buff, 0, sizeof(buff));
num = read(confd, buff, 1024);
//printf("Buff after: %s\n", buff);
if(num<0){
printf("error reading the socket");}
else{
printf("File uploaded: %s\n", buff);
}
//close(confd);
fclose(fp);
}
return 0;
}
Client.c
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
int main(int argc, char *argv[]){
int i=0, k=0,num;
char ip[50];
char upfile[50];
char dwfile[50];
for(i=0; i<argc; i++){
if(strcmp(argv[i], "-i")==0){
strcpy(ip, argv[i+1]);
}
else if(strcmp(argv[i],"-u")==0){
strcpy(upfile, argv[i+1]);
k =2;
}
else if(strcmp(argv[i], "-d")==0){
strcpy( dwfile, argv[i+1]);
k=3; }
/* else(argv[i] = "-l"){
}*/
}
printf("%s %s %s", ip, upfile, dwfile);
int sfd =0, n=0, c;
char rbuff[1024];
struct sockaddr_in serv_addr;
memset(rbuff, '0', sizeof(rbuff));
sfd = socket(AF_INET, SOCK_STREAM, 0);
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(5000);
serv_addr.sin_addr.s_addr = inet_addr(ip);
c = connect(sfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr));
if(c==-1){
perror("Connect");
return 1;
}
if (k==2){
printf("==========This is the upload function");
FILE *fp = fopen(upfile, "rb");
if(fp == NULL){
fprintf(stderr, "oh no!");
return 1;
}
char sendbuffer[100];
int b = fread(sendbuffer, 1, sizeof(sendbuffer),fp);
while(b>0){
send(sfd, sendbuffer, b, 0);
b = fread(sendbuffer, 1, sizeof(sendbuffer),fp);
}
num = write(sfd, upfile, strlen(upfile));
fclose(fp);
printf("Send filename: %s", upfile);
printf("============End upload!");
}
return 0;
}
Can anyone help ?
EDIT1:
Sending the filename before sending the file contents seems to work, but however doing that is printing a long trail of zeroes. For example: File uploaded: tt.txt000000000000000000000....

Answer 1

There are two common solutions: One is to use a separator, a special byte or sequence of bytes, that can't be in the message data and that marks the separator between the two messages. In the case of you sending a binary file it's not really feasible since there are really no safe sequences.
The other common method is to have a special fixed-sized header which contains the length of the data you send. Then you simply send this fixed-sized header first followed by the actual data.
In your case you could use both methods actually, but I really recommend you send the filename first, or it will be hard for the server to open the file and write the data.
You could send the filename with the terminating zero, thereby having the string terminator as a field separator between the fiilename and the file data.
Or you could send both the length of the filename and the file data, followed by the name and the data.

Answer 2

BTW, be sure not to allow a full file name to be sent - you can't trust
the client, and you wouldn't want any client to create an arbitrary
file.