c- valgrind (Invalid free() / delete / delete[] / realloc()) Socketing - c

Question

I am writing a simple program using sockets to receive actual date from server.
I am getting this error and i don't know where i am making a mistake.
Client:
/* Make the necessary includes and set up the variables. */
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
int main ()
{
int sockfd;
socklen_t len;
struct sockaddr_in address;
int result;
int id=2, answer, length;
char *s;
/* Create a socket for the client. */
sockfd = socket (AF_INET, SOCK_STREAM, 0);
/* Name the socket, as agreed with the server. */
address.sin_family = AF_INET;
address.sin_addr.s_addr = inet_addr ("127.0.0.1");
address.sin_port = htons (9734);
len = sizeof (address);
/* Now connect our socket to the server's socket. */
result = connect (sockfd, (struct sockaddr *) &address, len);
if (result == -1)
{
perror ("oops: netclient");
exit (1);
}
/* We can now read/write via sockfd. */
write(sockfd, &id, sizeof(id)); /* sending the request id */
read(sockfd, &answer, sizeof(answer)); /* receiving the answer id*/
if(answer==1002){
printf("Odebrano wlasciwa odpowiedz\n");
read(sockfd, &length, sizeof(length)); /* receiving the answer string length*/
s=(char*)malloc(length*sizeof(char)); /* receiving the string with the date */
read(sockfd, s, length);
printf ("Date from server = %s\n", s);
}
free(s);
close (sockfd);
exit (0);
}
Server:
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <signal.h>
#include <unistd.h>
#include <time.h>
#include <string.h>
int main ()
{
int server_sockfd, client_sockfd;
int length;
char *s;
int id;
int answer=1002;
socklen_t server_len, client_len;
time_t rtime;
struct tm *timeinfo;
struct sockaddr_in server_address;
struct sockaddr_in client_address;
server_sockfd = socket (AF_INET, SOCK_STREAM, 0);
server_address.sin_family = AF_INET;
server_address.sin_addr.s_addr = htonl (INADDR_ANY);
server_address.sin_port = htons (9734);
server_len = sizeof (server_address);
bind (server_sockfd, (struct sockaddr *) &server_address, server_len);
/* Create a connection queue and wait for clients. */
listen (server_sockfd, 5);
while (1)
{
printf ("server waiting\n");
/* Accept connection. */
client_len = sizeof (client_address);
client_sockfd = accept (server_sockfd,
(struct sockaddr *) &client_address,
&client_len);
/* We can now read/write to the client on client_sockfd.
The five second delay is just for this demonstration. */
read(client_sockfd, &id, sizeof(int)); /*receive request id */
if(id==2){
write(client_sockfd, &answer, sizeof(int)); /* sending an answer_id*/
time(&rtime);
timeinfo=localtime(&rtime);
s=(char*)malloc(sizeof(asctime(timeinfo))*sizeof(char));
printf("%s\n", asctime(timeinfo));
s=asctime(timeinfo);
printf("Size of s:%lx\n", sizeof(s));
length = htons(strlen(s));
write (client_sockfd, &length, sizeof(length)); /* sending the answer string length to the client */
printf("Date: %s\n", s);
write (client_sockfd, s, length); /* sending string with date to the server */
}
free(s);
close (client_sockfd);
}
}
I am almost certain that there is something wrong with allocing/freeing the space with the string containing actual date but i can't see the actual mistake.
EDIT:
Actually, i didn't know how to solve this problem in this mentioned way but i've came up with other idea.
I just send the result of asctime(timeinfo) over the server without using char* s.
So i don't write the date to char *s.
Program works fine now with no errors but i guess there is a way to do it some other way.
Well, even though, many thanks for the help, was helpful.

Answer 1

In your server, you overwrite s with asctime(...) after allocating it. This leaks the original memory. Also, the return value of asctime is a static buffer that cannot be freed.

Answer 2

Here's an excerpt from your code:
char *s;
if(answer==1002){
printf("Odebrano wlasciwa odpowiedz\n");
read(sockfd, &length, sizeof(length)); /* receiving the answer string length*/
s=(char*)malloc(length*sizeof(char)); /* receiving the string with the date */
printf ("Date from server = %s\n", s);
}
free(s);
Note what happens if 'answer' is not equal to 1002 -- you're calling free() on an uninitialized value. That is likely the cause of the error you are seeing. (This same mistake is present in both programs)

Answer 3

Refering the server code:
Here you allocate memory to s:
s=(char*)malloc(sizeof(asctime(timeinfo))*sizeof(char));
Here you overwrite the pointer with the value receive from asctime() so the original value returned by malloc() is lost introducing a memory leak:
s=asctime(timeinfo);
Here you then try to free what had been received from asctime(), which is a reference to static memory and gherefore cannot be freed.
free(s);