String compare error in transmission over TCP

Question

I have written a program in c to send a file line by line to a server from a client in c. After the file is transmitted completely, I have provided a line endoffile in the txt file to string compare in the server to identify that the file is over and the next file is being transmitted next. The next file has to be written to another file in the server. But the problem is that the strcmp is never detecting the endoffile in the code and endoffile recieved from the file in client as equal strings amd continues to write the next file from the client to the same file in server.

char enof[]="endoffile";
...
do
{
   rewind(appcrt);
   bytes_recieved = recv(pass_arg.connected, recv_data_c, strlen(recv_data_c), 0);
   recv_data_c[bytes_recieved-1] = '\0';
   fputs(recv_data_c, appcrt);
   if(strcmp(enof,recv_data_c) == 0){break;}
}while(check<count);

The text file:

Necessary data
that is
being transmitted
to be written to the file
endoffile

The code used for reading from the txt file and sending to the server:

while (fgets(line, sizeof(line), crt) != NULL)
{
    send(sock, line, sizeof(line), 0);
}

What change I have to make in the condition so that the problem is resolved and the code exits from the do....while(); loop. Thanks in advance. Operating platform: Linux

Edit1: Edited the do....while() as follows:

do
    {
       rewind(appcrt);
       bytes_recieved = recv(pass_arg.connected, recv_data_c, 100, 0);
       recv_data_c[bytes_recieved] = '\0';
       fputs(recv_data_c, appcrt);
       printf("%s-%s",enof,recv_data_c);
       //if(strcmp(enof,recv_data_c) == 0){break;}
    }while(check<count);

Got the following output in terminal:

endoffile-file1line1
endoffile-file1line2
endoffile-file1line3
endoffile-file1line4
endoffile-file1line5
endoffile-file1line6
endoffile-endoffile
endoffile-file2line1
endoffile-file2line2
endoffile-file2line3
endoffile-file2line4
endoffile-file2line5
endoffile-file2line6
.
.
.

Still no hope.

Answer 1

Even though the client is writing a line at a time to the socket this is not how the server will consume it as data transferred over a socket is just a stream of bytes. The server must read up to the next new line character and then compare. A simple algorithm would read a byte at a time and check if it is the newline character and if is not then append it to a string until a newline character is read:

/* Read next line. */
memset(line, 0, sizeof(line));
size_t line_len = 0;
while (line_len < sizeof(line) - 1 &&
       1 == recv(pass_arg.connected, &line[line_len], 1, 0))
{
    if ('\n' == line[line_len]) break;
    line_len++;
}

Apart from that, there are several problems with the code:

• you need to send what was read from the file as it may be less than the sizeof(line). Change:

  send(sock, line, sizeof(line), 0);
  

  to:

  send(sock, line, strlen(line), 0);
  

  and also check the return value of send() to determine if it was successful.

• the following is incorrect as it would only read at a maximum what was previously read (or if was initialised to an empty string nothing would be read):

  bytes_recieved = recv(pass_arg.connected,
                            recv_data_c, strlen(recv_data_c), 0);
  

  and, again, check the return value especially as the return value is being used to index an array. If recv() fails it returns -1, which will result in out of bounds access on the array causing undefined behaviour.