A truly Cross-Platform socket client written in C language.

cross_socket.c

#include <stdio.h>

// 1/8/2022
// modified version of https://github.com/ApOgEE/mingw-c-socket
// WORKS ON Linux kali 4.19.0-kali4-686-pae #1 SMP Debian 4.19.28-2kali1 (2019-03-18) i686 GNU/Linux

//////////////////////////////////////////////////////////////////////////
// CROSS-PLATFORMING A SOCKET PROGRAM
// See https://stackoverflow.com/questions/28027937/cross-platform-sockets

//////////////////////////////////////////////////////////
// The header and lib files are different
// You'll need to include different header files depending on whether you're using Windows or not:
#ifdef _WIN32
	// FOR WINDOWS
	/* See http://stackoverflow.com/questions/12765743/getaddrinfo-on-win32 */
	#ifndef _WIN32_WINNT
		#define _WIN32_WINNT 0x0501  /* Windows XP. */
	#endif
	/* winsock2.h was designed to replace winsock.h, not extend it. Everything that is defined in winsock.h is also defined in winsock2.h (https://stackoverflow.com/questions/9153911/) */
	#include <winsock2.h>
#else
	// FOR UNIX/POSIX
	/* Assume that any non-Windows platform uses POSIX-style sockets instead. */
	#include <sys/socket.h>
	#include <arpa/inet.h>
	#include <netdb.h>  /* Needed for getaddrinfo() and freeaddrinfo() */
	#include <unistd.h> /* Needed for close() */
	#include <string.h> /* Needed to provide a declaration of memset */
	#include <stdlib.h> /* Needed to provide a declaration of exit */
#endif
//////////////////////////////////////////////////////////


//////////////////////////////////////////////////////////
// WinSock requires initialisation and cleanup.
// The functions below illustrate how you can initialise WinSock v1.1 and clean up afterwards
int sockInit(void){
	#ifdef _WIN32
		WSADATA wsa_data; // winsock2.h
		return WSAStartup(MAKEWORD(1,1), &wsa_data); // winsock2.h
		//WSADATA wsaData; // winsock.h
		//WSAStartup(MAKEWORD(2, 2), &wsaData); // winsock.h
	#else
		return 0;
	#endif
}

int sockQuit(void){
	#ifdef _WIN32
		return WSACleanup();
	#else
		return 0;
	#endif
}
//////////////////////////////////////////////////////////


//////////////////////////////////////////////////////////
// socket connect error variable
#ifdef _WIN32
	// FOR WINDOWS
	#define SOCKET_ERROR_ SOCKET_ERROR
#else
	// FOR UNIX/POSIX
	#define SOCKET_ERROR_ SO_ERROR
#endif
//////////////////////////////////////////////////////////




//////////////////////////////////////////////////////////
// Sockets are closed differently
// The function below illustrates the differences:
/* Note: For POSIX, typedef SOCKET as an int. */
//int sockClose(SOCKET sock){
int sockClose(int sock){
  int status = 0;
  #ifdef _WIN32
	status = shutdown(sock, SD_BOTH);
	if (status == 0) { status = closesocket(sock); }
  #else
	status = shutdown(sock, SHUT_RDWR);
	if (status == 0) { status = close(sock); }
  #endif
  return status;
}
//////////////////////////////////////////////////////////

//////////////////////////////////////////////////////////////////////////


#define PORT 80
#define USERAGENT "ApOgEE MinGW Socket Client 1.0"
char *build_get_query(char *, char *);

int main(int argc, char *argv[]){
	int client_socket;
	char *host;
	struct hostent *hent;
	int iplen = 15; //XXX.XXX.XXX.XXX
	struct sockaddr_in *remote;
	int tmpres;
	char *get;
	char buf[BUFSIZ+1];

	//printf("Socket client example\n");
	
	// create tcp socket 
	sockInit();
	
	client_socket = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
	if (client_socket < 0) {
		printf("client_socket = %d\n",client_socket);
		perror("Can't create TCP socket\n");
		exit(1);
	}  	
	
	// get host ip
	host = "static.licdn.com";
	//printf("Host: %s\n",host);
	char *ip = (char *)malloc(iplen+1);
	memset(ip, 0, iplen+1);
	if((hent = gethostbyname(host)) == NULL)
	{
		perror("Can't get IP");
		exit(1);
	}
	ip = inet_ntoa(*(struct in_addr *)*hent->h_addr_list);
	//printf("The IP: %s\n",ip);
	
	// setup remote socket
	remote = (struct sockaddr_in *)malloc(sizeof(struct sockaddr_in *));
	remote->sin_family = AF_INET;
	//printf("s_addr:%d\n",remote->sin_addr.s_addr);
	remote->sin_addr.s_addr = inet_addr(ip);
	remote->sin_port = htons(PORT);
	//printf("s_addr:%d\n",remote->sin_addr.s_addr);
	
	// have to read 
	// http://msdn.microsoft.com/en-us/library/windows/desktop/ms737625(v=vs.85).aspx
	
	// connect socket
	if(connect(client_socket, (struct sockaddr *)remote, sizeof(struct sockaddr)) == SOCKET_ERROR_){
		perror("Could not connect");
		sockClose(client_socket); // MULTIPLATFORM
		sockQuit(); // FOR WINDOWS
		exit(1);
	}
	
	// prepare query
	get = build_get_query(host,"/sc/h/5q92mjc5c51bjlwaj3rs9aa82");
	//printf("query: \n%s\n",get);

	//Send the query to the server
	int sent = 0;
	while(sent < strlen(get)) { 
		tmpres = send(client_socket, get+sent, strlen(get)-sent, 0);
		if(tmpres == -1) {
			perror("Can't send query");
			exit(1);
		}
		sent += tmpres;
	}

	//now it is time to receive the page
	memset(buf, 0, sizeof(buf));
	int htmlstart = 0;
	char * htmlcontent;
	while((tmpres = recv(client_socket, buf, BUFSIZ, 0)) > 0) {
		if(htmlstart == 0) {
			/* Under certain conditions this will not work.
			 * If the \r\n\r\n part is splitted into two messages
			 * it will fail to detect the beginning of HTML content
			 */
			htmlcontent = strstr(buf, "\r\n\r\n");
			if(htmlcontent != NULL){
				htmlstart = 1;
				htmlcontent += 4;
			}
		} else {
			htmlcontent = buf;
		}
		if(htmlstart) {
			FILE *fp;
			
			fp = fopen("file.svg", "ab+");
			fprintf(fp, htmlcontent);
			//fputs(htmlcontent, fp);
			fclose(fp);
			
			// use only to show downloaded contents
			//fprintf(stdout, htmlcontent);
		}

		memset(buf, 0, tmpres);
	}
	if(tmpres < 0) {
		perror("Error receiving data");
	}
	
	free(get);
	free(remote);
	sockClose(client_socket); // MULTIPLATFORM
	sockQuit(); // FOR WINDOWS
	
	//printf("Program end");
	
	return 0;
}

char *build_get_query(char *host, char *page){
	char *query;
	char *getpage = page;
	char *tpl = "GET /%s HTTP/1.0\r\nHost: %s\r\nUser-Agent: %s\r\n\r\n";
	if(getpage[0] == '/'){
		getpage = getpage + 1;
		//fprintf(stderr,"Removing leading \"/\", converting %s to %s\n", page, getpage);
	}
	// -5 is to consider the %s %s %s in tpl and the ending \0
	query = (char *)malloc(strlen(host)+strlen(getpage)+strlen(USERAGENT)+strlen(tpl)-5);
	sprintf(query, tpl, getpage, host, USERAGENT);
	
	return query;
}