Extract numbers and characters from buffer as one whole string

Question

I write a C program that listens to port 443 and receives SSL packets :

#include <errno.h>
#include <ctype.h>
#include <limits.h>
#include <string.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/stat.h>
#include <stdio.h>

#define MAX_SIZE 10000

void* message_processing(int sockfd)
{
    char* buff = calloc(MAX_SIZE + 1, sizeof(char));

    /*struct handshake *pkt;       
    pkt = calloc (1, sizeof (struct handshake));        */

    while (1) {

        int len_of_read_data = read(sockfd, buff, MAX_SIZE);

        if (len_of_read_data > 0) {

            FILE* file = fopen("logfile", "a");

            fprintf(file, "ssl type : %d\n", (int)buff[0]);
            fprintf(file, "ssl version : %d.%d\n", (int)buff[1], (int)buff[2]);
            fprintf(file, "ssl length : %d%d\n", (int)buff[3], (int)buff[4]);
            fprintf(file, "handshake type  : %d\n", (int)buff[5]);
            fprintf(file, "handshake len  : %d%d%d\n", (int)buff[6], (int)buff[7], (int)buff[8]);
            fprintf(file, "ssl version  : %d.%d\n", (int)buff[9], (int)buff[10]);

            fprintf(file, "random  : %c\n", buff[11]);
            fprintf(file, "random  : %c\n", buff[12]);
            fprintf(file, "random  : %c\n", buff[13]);
            fprintf(file, "random  : %c\n", buff[14]);
            fprintf(file, "random  : %c\n", buff[15]);
            fprintf(file, "random  : %c\n", buff[16]);
            fprintf(file, "random  : %c\n", buff[17]);
            fprintf(file, "random  : %c\n", buff[18]);

            fclose(file);
        }
    }
}

int main()
{
    struct sockaddr_in serv_addr;
    int sock_descriptor = socket(AF_INET, SOCK_STREAM, 0);

    int trueval = 1;
    setsockopt(sock_descriptor, SOL_SOCKET, SO_REUSEPORT | SO_REUSEADDR, (char*)&trueval, sizeof(trueval));

    bzero((char*)&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(443);

    bind(sock_descriptor, (struct sockaddr*)&serv_addr, sizeof(serv_addr));
    listen(sock_descriptor, 50);

    while (1) {

        struct sockaddr_in cli_addr;
        socklen_t clilen;
        clilen = sizeof(cli_addr);

        int client_socket = accept(sock_descriptor, (struct sockaddr*)&cli_addr, &clilen);

        message_processing(client_socket);

    }
}

As you can see, the program reads data from port 443 and stores them in buff variable. I extract SSL fields from buff according to SSL packet format and print them in logfile .

The problem is with buff[11] to buff[43] . This section is a random string that contains numbers and characters. I want to print buff[11] - buff[43] as one string. So I first print the bytes one by one in order to see contents (and then concatenate them into one string ), but some bytes are unreadable :

random  : �
random  : 
random  : )
random  : 
random  : M
random  : 
random  : #
random  : 

I'm new in C and I have not any idea about the solution. Can anybody guide me how can I extract this section of buff as one string that contains numbers and characters?

Answer 1

Your problem may have to with that you do not get the proper ints from the buffer, where the string follows these ints.

Instead of:

        fprintf(file, "ssl type : %d\n", (int)buff[0]);
        fprintf(file, "ssl version : %d.%d\n", (int)buff[1], (int)buff[2]);

Use:

        int *pInt=(int *)buff;
        fprintf(file, "ssl type : %d\n", pInt[0]);
        fprintf(file, "ssl version : %d.%d\n", pInt[1], pInt[2]);

Etc.

Following pInt[10] is the string. That would be:

        char *s= (char *)&pInt[11];

Note that (int)buff[1] makes an int from the second char of the buffer. But you already made (int)buff[0] an int, so to address the second int in the buffer, you need to add the size of an int, (int)buff[0+sizeof(int)] to get the second int form the buffer. The easier method is to use a pointer to an int, as I showed above.