How can i fix segmentation fault(core dump)?

Question

#include <stdio.h>
#include <string.h>
#include <openssl/sha.h>
#include <dirent.h>
#include <sys/types.h>
#include <pwd.h>
#include <sys/stat.h>
#include <fcntl.h>

char* sha1_hash(char *input_url, char *hashed_url){
        unsigned char hashed_160bits[20];
        char hashed_hex[41];
        int i;

        SHA1(input_url, strlen(hashed_hex), hashed_hex);
        for(i=0; i<sizeof(hashed_160bits);i++)
        sprintf(hashed_hex + i*2, "%02x", hashed_160bits[i]);
        strcpy(hashed_url, hashed_hex);
        return hashed_url;
}

char* getHomeDir(char *home) {
        struct passwd *usr_info = getpwuid(getuid());
        strcpy(home, usr_info->pw_dir);
        return home;
}

int main()
{
        char* token;             //To use strtok             
        char* input_url;
        char* hash_ptr;
        char hash_name[3], *hash_txt;   
        char* home;
        DIR *Pdir;                            
        int fd;                             

        Pdir = opendir(getHomeDir(home)); 
        if(Pdir == NULL)                       
        {
                printf("Open ERROR\n");        
                return 0;
        }
    
        umask(0000);
        mkdir("cache", 0777);
        do
        {
                printf("input URL>");
                scanf("%s",input_url);
                hash_ptr = sha1_hash(input_url, hash_ptr);

                for(int i=0; i<3; i++)
                {
                        hash_name[i] = hash_ptr[i];
                }

                hash_ptr[2] = ' ';         //To use Strtok

                token = strtok(hash_ptr, " ");
                while(token != NULL)
                {
                        token = strtok(NULL, " "); 
                }
                hash_txt = token;

                umask(0000);
                mkdir(hash_name, 0777);
                fd = open(hash_txt, O_CREAT, 0777);

                closedir(Pdir);
                close(fd);
        }while(strcmp(input_url, "bye"||"BYE") != 0);
}

When this program runs. It makes cache directory and get input(URL links) and converts URL Links into 41 range hashed hex. first 3 elements of array are directory's name(stored in cache directory) and left 38 is stored in (cache/00x(eg)) as a file name.

I don't know how to do debug this code with gdb. when i input "gcc -g -o test cache_proxy.c" it don't returns test file. so i can't find the problem line.

Please let me know.

Answer 1

How can i fix segmentation fault(core dump)?

You can fix that by not using uninitialized pointers, for example

        char* input_url;
        …
                scanf("%s",input_url);

Such errors can be avoided by adding -Wall to your compilation command and removing the sources of the warnings you get. Chances are that thereafter you won't need to debug.