Socket checksum issue - c

Question

This may be a straightforward solution but basically I am sending a binary file piece-by-piece to another program using TCP and checking to make sure the checksum matches for validity. The problem is the checksum received is never the checksum sent apart from the last part submitted (the remainder).
The code fragment for the sender is:
void* buffer = (void *)malloc(BLOCKSIZE+1);
if (!buffer)
error("Error: malloc error for buffer.\n");
fread(buffer, BLOCKSIZE, 1, fp_read); //read from binary file for the current block.
int checksum = checksum(buffer,BLOCKSIZE);
n = write(sockfd,buffer,BLOCKSIZE);
if (n < 0)
error("ERROR writing to socket");
For the receiver is is:
void* buffer = (void *)malloc(BLOCKSIZE+1);
if (!buffer)
error("Error: malloc error for buffer.\n");
n = read(sockfd,buffer,BLOCKSIZE);
if (n < 0)
error("Error: reading from socket.");
int checksum = checksumv(buffer,BLOCKSIZE);
Anyone see anything wrong with it? The only part having checksums match is the final piece which doesn't completely fill the buffer.
Thanks.
The whole code for the sender is:
FILE* fp_read = fopen("file.jpg", "rb");
if (fp_read == NULL)
error("Cannot open the file for peer piece download.");
fseek(fp_read, 0, SEEK_END);
unsigned long fileLen = ftell(fp_read);
fseek(fp_read, 0, SEEK_SET);
int checksum, loops = fileLen / BLOCKSIZE;
int remainder = fileLen % BLOCKSIZE;
int segment_num = loops+1;
void* buffer4 = (void *)malloc(BLOCKSIZE+1);
if (!buffer4)
error("Error: malloc error for buffer.\n");
int i, sent = 0;
for (i=1; i<=loops; i++)
{
fread(buffer4, BLOCKSIZE, 1, fp_read);
checksum = checksumv(buffer4,BLOCKSIZE);
n = write(sock,buffer4,BLOCKSIZE);
if (n < 0)
error("ERROR writing to socket");
}
if (remainder > 0)
{
//Allocate memory
void* buffer5 = (void *)malloc(remainder+1);
if (!buffer5)
error("Error: malloc error for buffer2.\n");
fread(buffer5, remainder, 1, fp_read);
checksum = checksumv(buffer5,remainder);
n = write(sock,buffer5,remainder);
if (n < 0)
error("ERROR writing to socket");
}

Answer 1

When you are reading data:
n = read(sockfd,buffer,BLOCKSIZE);
if (n < 0)
error("Error: reading from socket.");
int checksum = checksumv(buffer,BLOCKSIZE);
you need to respect the number of bytes that read() says it placed into buffer - it may read fewer than BLOCKSIZE number of bytes before returning.
Also, I don't see where the checksum is sent (or received) - I only see the file data being sent. How are you comparing checksums?
Finally, since TCP is a streaming protocol, you'll need to have some way to indicate to receiver when the file data is finished such as by sending the size ahead of the file data, or having some 'out of band' indication.

Answer 2

You're not accumulating your checksum correctly, but you don't need all this complexity just to copy a file:
byte buffer[8192];
while ((count = fread(buffer, sizeof buffer, 1, fp)) > 0)
{
n = write(sock,buffer4,count);
if (n < 0)
error("ERROR writing to socket");
checksum += checksumv(buffer, count); // or possibly ^=, it depends how your checksum is supposed to accumulate
}