tgp.c

#include <Windows.h>
#include <WinInet.h>

#pragma comment(lib, "Advapi32.lib")
#pragma comment(lib, "WinInet.lib")

int main()
{
    LONG retv;
    LPCTSTR lpSubKey = "Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings";
    REGSAM samDesired = KEY_READ | KEY_WRITE;
    HKEY hkResult;

    LPCTSTR lpValueName = "ProxyEnable";
    DWORD Data, DataSize = sizeof(Data);

    retv = RegOpenKeyEx(HKEY_CURRENT_USER, lpSubKey, 0, samDesired, &hkResult);
    if (retv != ERROR_SUCCESS) {
        printf("Calling RegOpenKeyEx failed.\n");
        return retv;
    }

    retv = RegQueryValueEx(hkResult, lpValueName, 0, NULL, (LPBYTE)&Data, &DataSize);
    if (retv != ERROR_SUCCESS) {
        printf("Calling RegQueryValueEx failed.\n");
        return retv;
    }

    Data = !Data;
    retv = RegSetValueEx(hkResult, lpValueName, 0, REG_DWORD, (LPBYTE)&Data, sizeof(Data));
    if (retv != ERROR_SUCCESS) {
        printf("Calling RegSetValueEx failed.\n");
        return retv;
    }

    if (FALSE == InternetSetOption(0, INTERNET_OPTION_SETTINGS_CHANGED, NULL, 0)) {
        printf("Calling SetInternetOption Failed.\n");
        return GetLastError();
    }
    else {
        if (Data) {
            printf("Enabled\n");
        }
        else {
            printf("Disabled\n");
        }
    }

    return ERROR_SUCCESS;
}