int gethostbyname_r(const char *name,
        struct hostent *ret, char *buf, size_t buflen,
        struct hostent **result, int *h_errnop);

為了避開非線程安全的gethostbyname，想用這貨，用起來類似這樣：

int host2addr(const char *host, struct in_addr *addr) {
    struct hostent he, *result;
    int herr, ret, bufsz = 512;
    char *buff = NULL;
    do {
        char *new_buff = (char *)realloc(buff, bufsz);
        if (new_buff == NULL) {
            free(buff);
            return ENOMEM;
        }   
        buff = new_buff;
        ret = gethostbyname_r(host, &he, buff, bufsz, &result, &herr);
        bufsz *= 2;
    } while (ret == ERANGE);
    if (ret == 0 && result != NULL) 
        *addr = *(struct in_addr *)he.h_addr;
    else if (result != &he) 
        ret = herr;
    free(buff);
    return ret;
}

基本上跟GNU官方文檔里的例子一致（GNU的還少了個free的樣子）。

但是對hostent還是不太放心：

struct hostent {
   char  *h_name;            /* official name of host */
   char **h_aliases;         /* alias list */
   int    h_addrtype;        /* host address type */
   int    h_length;          /* length of address */
   char **h_addr_list;       /* list of addresses */
}
#define h_addr h_addr_list[0] /* for backward compatibility */

這里面有h_name，h_aliases，h_addr_list ... 我又翻看了下eglibc-2.15的gethostbyname的源碼，邏輯基本上跟我上面這段一樣。但是，這些東西沒釋放的話，真的沒問題嗎？