UDP File transfer in C - c

Question

I’m working on a socket programming thing for implementing a file transfer through UDP in C. Both the server and the client codes execute without reporting any errors: when I run the codes, both sockets are created and bound, and I’m prompted to input file name on client side. But after I enter it, nothing else happens. Both sides stay blank, whereas I’m supposed to get a "file name received" output on server side and then a prompt to enter an encryption key. I’m not getting that, and I’m not sure why. I tried several iterations of code and even tried code suggestions online, but I had no luck. What is the problem?
Here’s the server code
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#define SA struct sockaddr
#define PORT 12345
#define SIZE 256
#define EMP "the file does not exist"
char encrypt(char c, char k){
return c ^ k;
}
int sendfl(FILE *fp, char *buff, int l, char k){
int i, len;
if(fp == NULL){
strcpy(buff, EMP);
len = strlen(EMP);
buff[l] = EOF;
for(i = 0; i < len; i++)
buff[i] = encrypt(buff[i], k);
return 1;
}
char c1, c2;
for(i = 0; i < l; i++) {
c1 = fgetc(fp);
c2 = encrypt(c1, k);
buff[i] = c2;
if(c1 == EOF)
return 1;
}
return 0;
}
int main(){
int sockid, len, e, n;
char key;
char flname[SIZE];
char buff[SIZE];
FILE *fp;
struct sockaddr_in servaddr;
len = sizeof(servaddr);
sockid = socket(AF_INET, SOCK_DGRAM, 0);
if(sockid < 0){
perror("failed to create socket");
exit(0);
}
printf("socket created! \n");
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(PORT);
servaddr.sin_addr.s_addr = INADDR_ANY;
e = bind(sockid, (SA*)&servaddr, len);
if(e == -1){
perror("failed to bind");
exit(1);
}
printf("bind successful, \n");
while (1) {
printf("waiting for file name... \n");
bzero(flname, SIZE);
n = recvfrom(sockid, flname, sizeof(flname), 0, (SA*)&servaddr, &len);
printf("file name recieved \t%s\n", flname);
fp = fopen(flname, "r");
if(fp == NULL){
perror("failed to open file");
exit(1);
}
printf("file opened...");
printf("enter encryption key \t");
scanf("%c\n", &key);
while (1) {
if(sendfl(fp, buff, SIZE, key)){
sendto(sockid, buff, SIZE, 0, (SA*)&servaddr, len);
break;
}
sendto(sockid, buff, SIZE, 0, (SA*)&servaddr, len);
bzero(buff, SIZE);
}
if(fp != NULL)
fclose(fp);
}
return 0;
}
Here’s the client code
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#define SA struct sockaddr
#define PORT 12345
#define IPA "127.0.0.1"
#define SIZE 256
#define EMP "the file does not exist"
char decrypt(char c, char k){
return c ^ k;
}
int recfl(char *buff, int l, char k){
int i;
char c1, c2;
for(i = 0; i < l; i++) {
c1 = buff[i];
c2 = decrypt(c1, k);
if(c2 == EOF)
return 1;
else
printf("%c", c2);
}
return 0;
}
int main(){
int sockid, len, n;
char key;
char flname[SIZE];
char buff[SIZE];
FILE *fp;
struct sockaddr_in servaddr;
len = sizeof(servaddr);
sockid = socket(AF_INET, SOCK_DGRAM, 0);
if(sockid < 0){
perror("failed to create socket");
exit(0);
}
printf("\nsocket created!\n");
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(PORT);
servaddr.sin_addr.s_addr = inet_addr(IPA);
while (1) {
bzero(flname, SIZE);
printf("\nenter file name:\t");
scanf("%s\n", flname);
sendto(sockid, flname, sizeof(flname), 0, (SA*)&servaddr, len);
printf("\nfile recieved\n");
printf("\nenter encryption key\t");
scanf("%c\n", &key);
printf("\n--------------------DATA RECIEVED--------------------\n");
while (1) {
bzero(buff, SIZE);
n = recvfrom(sockid, buff, SIZE, 0, (SA*)&servaddr, &len);
if(recfl(buff, SIZE, key)){
break;
}
}
printf("\n-----------------------------------------------------\n");
}
return 0;
}

Answer 1

Foremost you overlooked what the \n does in scanf("%s\n", flname); here's the description from the C standard:
A directive composed of white-space character(s) is executed by reading input up to the
first non-white-space character (which remains unread), or until no more characters can
be read.
So, the waiting for more input is what you perceive as nothing else happens.
Together with the suggestion from chux, you get one step further if you remove the \n.