How to have a program that uses the pointer ptr for reading and printing stuff about a book struct?

Question

How to have a program that uses the pointer ptr for reading and printing stuff about a book struct without using any other variable. For the access to the stuff of the struct i have to use the -> My program terminates after the first read. Can somebody provide me with a code? Here is my code :

#include<stdio.h>

struct book {
    char title[100];
    char authors;
    int code;
    double prc;
};

int main(void) {
    struct book *ptr;
    printf("Title:\n");
    gets(&ptr->title[100]);
    printf("authors:\n");
    gets(&ptr->authors);
    printf("code:\n");
    scanf("%d",&ptr->code);
    printf("Price:\n");
    scanf("%lf",&ptr->prc);

    printf("T:%s,A:%s,C:%d,P:lf\n",ptr->title,ptr->authors,ptr->code,ptr->prc);
    return 0;
}

I expect the program to read these 4 things about a book and then prints them in the final printf using only the pointer ptr

Answer 1

You have to allocate memory first.

struct book *ptr = malloc(sizeof (*ptr));

Also, never use the function gets. It have been removed. Read here why it is dangerous

Besides, the call to gets is wrong. The argument should be ptr->title. But use this instead:

fgets(ptr->title, 100, stdin);

And always check the return value from scanf to ensure everything went ok.

Answer 2

You have several problems

• you missed to allocate your instance of book

• &ptr->title[100] is invalid

• the use of gets is deprecated since years, use fgets to limit the size and avoid undefined behavior

• you missed to check your scanf return 1 to know if a valid input was done

• you have a memory leak because of the malloc without a free, and in fact you do not need to allocate your book in the heap

• are you sure just a char is enough for authors ? probably you wanted an array

• gets(&ptr->authors); is invalid because authors is just a character

• the format "%s" for authors is invalid because it is just a character

• you missed a % before "lf" to print the price

Probably you do not want the newline in the input string

You wanted something like :

#include<stdio.h>
#include <stdlib.h>
#include <string.h>

struct book {
    char title[100];
    char authors[100];
    int code;
    double prc;
};

int main(void) {
    struct book * ptr = malloc(sizeof(struct book));
    size_t len;

    printf("Title:\n");
    if (fgets(ptr->title, sizeof(ptr->title), stdin) == NULL)
      /* EOF */
      return -1; /* or an other behavior is you want */
    len = strlen(ptr->title);
    if (ptr->title[len - 1] == '\n')
      ptr->title[len - 1] = 0;
    printf("authors:\n");
    if (fgets(ptr->authors, sizeof(ptr->authors), stdin) == NULL)
      /* EOF */
      return -1;/* or an other behavior is you want */
    len = strlen(ptr->authors);
    if (ptr->authors[len - 1] == '\n')
      ptr->authors[len - 1] = 0;
    printf("code:\n");
    if (scanf("%d",&ptr->code) != 1) {
      puts("invalid code");
      return 1; /* or an other behavior is you want */
    }
    printf("Price:\n");
    if (scanf("%lf",&ptr->prc) != 1) {
      puts("invalid price");
      return 1; /* or an other behavior is you want */
    }

    printf("T:%s,A:%s,C:%d,P:%lf\n",ptr->title,ptr->authors,ptr->code,ptr->prc);

    free(ptr);

    return 0;
}

or without allocating the book in the heap :

#include<stdio.h>
#include <string.h>

struct book {
    char title[100];
    char authors[100];
    int code;
    double prc;
};

int main(void) {
    struct book b;
    size_t len;

    printf("Title:\n");
    if (fgets(b.title, sizeof(b.title), stdin) == NULL)
      /* EOF */
      return -1; /* or an other behavior is you want */
    len = strlen(b.title);
    if (b.title[len - 1] == '\n')
      b.title[len - 1] = 0;
    printf("authors:\n");
    if (fgets(b.authors, sizeof(b.authors), stdin) == NULL)
      /* EOF */
      return -1;/* or an other behavior is you want */
    len = strlen(b.authors);
    if (b.authors[len - 1] == '\n')
      b.authors[len - 1] = 0;
    printf("code:\n");
    if (scanf("%d",&b.code) != 1) {
      puts("invalid code");
      return 1; /* or an other behavior is you want */
    }
    printf("Price:\n");
    if (scanf("%lf",&b.prc) != 1) {
      puts("invalid price");
      return 1; /* or an other behavior is you want */
    }

    printf("T:%s,A:%s,C:%d,P:%lf\n",b.title,b.authors,b.code,b.prc);

    return 0;
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -pedantic -Wall -Wextra c.c
pi@raspberrypi:/tmp $ ./a.out
Title:
the mirific title
authors:
me
code:
007
Price:
12.34
T:the mirific title,A:me,C:7,P:12.340000
pi@raspberrypi:/tmp $