访问字符串数组时出现奇怪的分段错误

Question

Sorry to post my project code directly. I've been trying to wrap my head around this weird seg fault which occurs at the line for(j=0; j<100 && *nnames[j] != (char *) NULL; j++) . Isn't it legal to access a char** array this (*arr[]) way ?

    char** nnames = getcannames();
    char * new_str ;

    int j =0, len=0;
    ////////////////SEG FAULT HERE //////////////////
    for(j=0; j<100 && *nnames[j] != (char *) NULL; j++){
        len = len + strlen(nnames[j]);

    }
    if((new_str = (char*) malloc(len + 3)) != NULL){
        new_str[0] = '\0';   // ensures the memory is an empty string
        int i=0;    
        //setbuf(client_reply, NULL);
        for(i=0; i<7; i++){ //fix this, garbage values after index 68
            if(*nnames[i] == (char *) NULL) break;

            char *canname = strsave(nnames[i]);


            if( (find_newline = strchr( canname, NEWLINE )) != NULL )
                *find_newline = EOS;
            if(strcmp(canname, "!") != 0){
                strcat(new_str, canname);
                strcat(new_str, "\n");
            }

            //strcat(new_str, "\n\n");  
        }
        strcat(new_str,"\n\0");
        printf("%s", new_str);
        //strcpy( new_str, buf );
        buf = new_str;

    } else {
        perror("malloc failed!\n");
        // exit?
    }


char** getcannames(){
    //INITIALIZE
     char *names[100];
    int i;
    for(i=0; i<100; i++){
        names[i] = strsave("\0");
    }
    int namespos = 0;

     struct sym_list *sp;
     for( sp = Head.s_next;
     sp != (struct sym_list *) NULL;
     sp = sp->s_next )
    {
    if(getcannameindex(names, sp->s_sym.v_value) == -1){
        //strcpy(names[namespos++], sp->s_sym.v_name);
        names[namespos++] = strsave(sp->s_sym.v_value);
    }
    }
    return names;

}

Answer 1

If nnames is a pointer to the first element of an array of pointers of type char * then the valid code will look like

for ( j = 0; j < 100 && nnames[j] != NULL; j++ ){
        len = len + strlen(nnames[j]);

provided that the last element of the array is a null pointer.

The same is valid for statement

if(*nnames[i] == (char *) NULL) break;

that is it has to be rewritten like

if ( nnames[i] == NULL ) break;

Also this function

char** getcannames(){
    //INITIALIZE
     char *names[100];

     //...

     return names;

}

has undefined behaviour because it returns pointer to the first element of a local array that will be destroyed after exiting the function.

Take into account that if function strsave creates dynamically a copy of the string passed to it as the argument

char *canname = strsave(nnames[i]);

then the program has memory leaks because you do not free canname.

And of course you may write like

strcat(new_str,"\n\0");

or even like

strcat(new_str,"\n\0\0\0\0");

but the both statements are equivalent to

strcat(new_str,"\n");