c - Segmentation fault while trying to print name by passing ip (getnameinfo) -

i can't handle it, can't see what's wrong code.

trying print name (of site suspect) ip, can't 3 days, it's depressing me.

so, code:

/**  * task1.c -- getnameinfo() usage example  *  * copyright (c) 2014 ovchinnikov alexzander  *  * code licensed under mit-style license.  */  #include <stdio.h> #include <stdlib.h> #include <string.h> #include <errno.h>  #include <sys/types.h> #include <sys/socket.h> #include <netdb.h>  #include <arpa/inet.h>  #include <locale.h>  /**  * main  * @param argv количество параметров командной строки (учитая имя программы)  * @param argv массив строк-параметров командной строки  */ int main(int argc, char *argv[]) {     int result;     struct sockaddr_in *sa;    /* input */     char hbuf[ni_maxhost];      /*      * Инициализация подсистемы locale      */     (void) setlocale(lc_all, "");      /*      * Должен быть один и только один аргумент командной строки      */     if (2 != argc) {         fprintf(stderr, "usage: %s ipv4 address\n", argv[0]);         exit(exit_failure);     }      /*     * Что-то делаем     */     sa->sin_family = af_inet;     sa->sin_port = htons(80);     result = inet_pton(af_inet, argv[1], &(sa->sin_addr));     if (result <= 0){         if (result == 0){                 fprintf(stderr, "not in presentation format");         }else{             perror("inet_pton");             exit(exit_failure);         }     }     memset(&sa->sin_zero, '\0', sizeof(sa->sin_zero));      /*      * Получить имя для адреса в argv[1]      */     if (0 != (result = getnameinfo((struct sockaddr *)sa, sizeof(sa), hbuf, sizeof(hbuf), null, 0, ni_namereqd))){         fprintf(stderr, "getnameinfo() error: %s\n", gai_strerror(result));     }else{         fprintf(stdout, "host:%s\n", hbuf);     }      return 0; } 

when compiling:

gcc -wall -o task1_2 task1_2.c 

, this:

task1_2.c:49:20: warning: ‘sa’ may used uninitialized in function [-wmaybe-uninitialized]  sa->sin_family = af_inet; 

and when running this:

./task1_2 87.240.131.120 

, segmentation fault.

you haven't allocated memory sa

sa = (struct sockaddr_in*) malloc(sizeof(struct sockaddr_in));