使用malloc的分段错误

Question

I'm new to C, so this may be a silly question to ask:

What I want to do here is to input the data to the array of pointers to a structure and then print it out. But I get a segmentation fault when running into the insert function.

Below is my code

common.h

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

typedef struct book * Book;

struct book{
    int id;
    char *name;
};

extern int b_insert(Book *b, int id, char *name);
extern int b_print(Book books[], int len);

insert.c

#include "common.h"

int b_insert(Book *b, int id, char *name){
    Book p;
    p = (Book)malloc(sizeof(struct book));
    p->id = id;
    strcpy(p->name, name);

    *b = p;
    printf("success insert book:\n");
    printf("\tID: %d Name: %s\n", (*b)->id, (*b)->name);

    return 0;

}


int b_print(Book books[], int len){
    int i;
    printf("Book List\n");
    for(i=0; i<len; i++){
        printf("books[%d] = ID: %d, Name: %s\n", i, books[i]->id, books[i]->name);
    }
    return 0;

}

main.c

#include "common.h"
#define MAX 2

int main(){
    Book books[MAX];
    Book *b=books;
    int i;
    int id;
    char name[10];




    for(i=0; i<MAX; i++){
        printf("please input new books info\n");
        printf("ID: ");
        scanf("%d", &id);
        printf("Name: ");
        scanf("%s", name);
        if(b_insert(b, id, name) == -1){
            printf("fail to insert\n");
        }
        b++;
    }

    b_print(books, MAX);

    return 0;
}

Answer 1

Main problem:

Allocate memory for p->name before using

strcpy(p->name, name);

using malloc :

p->name = malloc(10); //Or some other size

Other problems:

1. Remove the cast here:

    p = (Book)malloc(sizeof(struct book)); 

   Why? Here is the answer

2. if(b_insert(b, id, name) == -1){ will never be true.
3. Check the result of malloc to check if it was successful in allocating memory.
4. Check the return value of all the scanf s to see if it was successful in scanning data.

5. Add a length modifier to the second scanf to prevent buffer overflows:

    scanf("%9s", name); /* +1 for the NUL-terminator */ 

Answer 2

You're not allocating space for name:

int b_insert(Book *b, int id, char *name){
    Book p; 
    p = malloc(sizeof(struct book));
    if (p != NULL)
    {
       p->name = malloc(strlen(name)+1); // It allocates space where the input name will be copied.

       if (p->name != NULL)
       {
          p->id = id;
          strcpy(p->name, name);

          *b = p;
          printf("success insert book:\n");
          printf("\tID: %d Name: %s\n", (*b)->id, (*b)->name);
       }
       else return -1; // No space to allocate string
    }
    else return -1; // No space to allocate struct

    return 0;
}

Answer 3

As mentioned before, allocate space for p->name . You should probably also use something different to read the book title, either scanf format %ms with a pointer to a char pointer, or %9s with your buffer, otherwise the title "war or peace" will also result in a segfault.

Answer 4

Here you create a static variable and the space for it is allocated automatically.

Book p;

You can allocate a space manually when you assign it to pointer, in this line it's not pointer but static variable.

p = (Book)malloc(sizeof(struct book));

What's more if you want to refer to attribute of static variable you should use "." instead of "->". So you have two option. Create a pointer and allocate a space for the structure and then you "->" oraz create static variable.

p->id = id;