How to Write in a loop into a binary file in C sockets

Question

I have the following snippet where I read sent bytes usingread() and store the data in a buffer which works well. But how do i make it so that I can continue to write in binary file after every read. My assumption is that it might have something to do with not resetting the buffer. Thanks for the help!

fp = fopen (filename, "wb"); //create a file

while (size > 0){        
    n = read(socket,buffer,size*sizeof(char));
    if (n <=0)
        return -1;
    p += n;
    size -= n;

    fwrite(buffer,sizeof(char),size*sizeof(char),fp); //write the content to the file

}

You shall proof fp if fopen() was successful.

RobertS supports Monica Cellio
– RobertS supports Monica Cellio

2020-03-14 09:46:32 +00:00
Commented Mar 14, 2020 at 9:46 — RobertS supports Monica Cellio
– RobertS supports Monica Cellio, Commented Mar 14, 2020 at 9:46

Barmar · Accepted Answer · 2020-03-14 06:08:10Z

1

You should use n in the call to fwrite(), not size. n is the amount you just read from the socket, size is how many more bytes you're still waiting for.

fwrite(buffer,sizeof(char),n,fp);

You also shouldn't multiply by sizeof there. The second argument is the size of each element, the third argument is the number of elements; fwrite() performs the multiplication internally. It happens to work in your case because sizeof(char) is always 1, so the multiplication didn't change anything.

answered Mar 14, 2020 at 6:08

Barmar

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1 Comment

pat Over a year ago

Yes this was the problem actually! Thank you !

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