You probably have to implement some sort of resume message that tells the client/server where it should start reading the file.

The side that should be receiving the file
Code:
FILE *f = fopen("file_to_recv", "rw");
long off;

fseek(f, 0, SEEK_END);
off = ftell(f);

/* tell offset to other side */
The side that should send the file
Code:
FILE *f = fopen("file_to_send", "r");
long off; /* = get offset from other side */

fseek(f, off, SEEK_SET);

/* start reading from file and send it */