How to make a member of a struct dynamic - c

Question

Say I have a struct of something and I want to make its members dynamic. I think the members should be something like this
char* id;
.....
following by a malloc to each member like this
books->id=(char*) malloc(size);
thou it seems when I try it my program crashed so I'm trying to understand what am I doing wrong
This is the part that I think that is relevant to my code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define maxsize 512
typedef struct Books {
char *id;
char title[maxsize];
char author[maxsize];
char pages[maxsize];
char year[maxsize];
char subject[maxsize];
} book;
char* filename;
int libsize=4;
int bookcount=1;
...
int main(int argc, char* argv[]){
if (argc < 1)
return -1;
filename=argv[1];
FILE* fptr;
char tempstring[maxsize],* token;
int i=0,ch;
book *books;
fptr=fopen(filename,"r");
if(fptr==NULL)
return-1;
//this count how many books are in the file
while(ch!= EOF){
ch=fgetc(fptr);
if(ch == '\n')
++bookcount;
}
fclose(fptr);
while(libsize<bookcount){
libsize *= 1.5;
}
books = (book*) malloc(libsize*sizeof(book));
if(books==NULL)
exit(-1);
books->id=(char*)malloc(100);
fptr=fopen(filename,"r");
if(fptr==NULL)
return-1;
//this gets all the books into the book array
for(i=0;i<bookcount;i++){
fgets(tempstring,maxsize,fptr);
token=strtok(tempstring,",");
strcpy(books[i].id,token);
token=strtok(NULL,",");
strcpy(books[i].title,token);
token=strtok(NULL,",");
strcpy(books[i].author,token);
token=strtok(NULL,",");
strcpy(books[i].pages,token);
token=strtok(NULL,",");
strcpy(books[i].year,token);
token=strtok(NULL,",");
strcpy(books[i].subject,token);
}
fclose(fptr);
printf("to add a book press 1\n");
printf("to delete a book press 2\n");
printf("to find a book press 3\n");
printf("to print all books press 4\n");
printf("to save library in a file press 5\n");
printf("to add books from a file press 6\n");
printf("to exit press 0\n");
pick(books);
return 1;
}
void pick(book books[]){
char input;
scanf("%c",&input);
switch (input){
case '1':
addbook(books);
break;
case '2':
delbook(books);
break; //pretty sure break isnt needed but it works so...eh
case '3':
srchbook(books);
break;
case '4':
printbooks(books);
break;
case '5':
printbooksf(books);
break;
case '6':
addbookf(books);
break;
case '0':
free(books);
exit (1);
case '\n':
pick(books);
default:
printf("please enter a valid command\n");
pick(books);
break;
}
}
edit2: added more of the code
edit3: found what was the problem what i did was trying to get to the pointer itself (i think) so the system flipped when i try to did it.
what i should have done was getting to each member array of the struct like that
for(i=0;i<bookcount;i++){
books[i].id=(char*)malloc(charcount);
books[i].title=(char*)malloc(charcount);
books[i].author=(char*)malloc(charcount);
books[i].pages=(char*)malloc(charcount);
books[i].year=(char*)malloc(charcount);
books[i].subject=(char*)malloc(charcount);
}
and not
books->id .....

Answer 1

You only malloc memory for id once. You need to do it for each book:
Like:
// books->id=(char*)malloc(100); Remove this line
fptr=fopen(filename,"r");
if(fptr==NULL)
return-1;
//this gets all the books into the book array
for(i=0;i<bookcount;i++){
fgets(tempstring,maxsize,fptr);
token=strtok(tempstring,",");
books[i].id=(char*)malloc(100); // Add this line
strcpy(books[i].id,token);
BTW
This code seems strange:
while(libsize<bookcount){
libsize *= 1.5;
}
books = (book*) malloc(libsize*sizeof(book));
why not simply do:
books = (book*) malloc(bookcount*sizeof(book));
(note: you don't need to cast malloc)