sendto() returning -1 in server - sockets

Question

int main()
{
int servsocket,clientsocket;
struct sockaddr_in server,client;
FILE *file;
char filename[100];
char buf[1024];
servsocket=socket(AF_INET,SOCK_DGRAM,0);
server.sin_addr.s_addr=htonl(INADDR_ANY);
server.sin_port=htons(6003);
server.sin_family=AF_INET;
bind(servsocket,(struct sockaddr *) &server,sizeof(server) );
while(1){
int clientsize=0;
printf("Waiting for file requests \n");
recvfrom(servsocket,filename,sizeof(filename),0,(struct sockaddr *)&client,&clientsize);
file=fopen(filename,"r");
int size=0;
do
{
size=fread(buf,1,sizeof(buf),file);
printf("%d bytes read \n",size);
int sentbytes= sendto(servsocket,(const char *)buf,size,0, (struct sockaddr *) &client,sizeof(client));
printf("%d bytes sent ",sentbytes);
}while(size==sizeof(buf));
}
}
I am trying to make a simple program for file transfer using UDP. The problem is that sendto() always returns -1. This is the code for server.

Answer 1

There are quite a few issues with your code. The one you're seeing is that you're not filling in the variable client properly: the clientsize parameter is used for both input and output by the recvfrom system call, so you need to initialise it to the size of the client structure:
int clientsize = sizeof(struct sockaddr_in);
Another issue is that you're not 0-terminating the filename string:
n = recvfrom(...);
filename[n] = '\0';
Finally, you're not testing for errors (bind, recvfrom, sendto, etc.). This will get you into trouble, I promise.