if defined __GNUC__ define PACKED __attribute__ packed else error Unkn

#if defined __GNUC__
#define PACKED __attribute__((packed))
#else
#error Unknown compiler, modify types.h for it
#endif

...

typedef struct socks4_req_t {
    uint8_t ver;
    uint8_t cmd;
    uint16_t port;
    uint32_t addr;
    char login[1]; // we need at least zero-byte
} PACKED socks4_req;