[英]segmentation fault using malloc
I'm new to C, so this may be a silly question to ask: 我是C的新手,所以这可能是一个愚蠢的问题:
What I want to do here is to input the data to the array of pointers to a structure and then print it out. 我在这里要做的是将数据输入到指向结构的指针数组,然后将其打印出来。 But I get a segmentation fault when running into the insert function.
但是,在运行插入功能时遇到了分段错误。
Below is my code 下面是我的代码
common.h 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 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 main.c中
#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;
}
Allocate memory for p->name
before using 使用前为
p->name
分配内存
strcpy(p->name, name);
using malloc
: 使用
malloc
:
p->name = malloc(10); //Or some other size
Remove the cast here: 在此处删除演员表:
p = (Book)malloc(sizeof(struct book));
Why? 为什么? Here is the answer
这是答案
if(b_insert(b, id, name) == -1){
will never be true. if(b_insert(b, id, name) == -1){
永远不会为真。 malloc
to check if it was successful in allocating memory. malloc
的结果以检查它是否成功分配了内存。 scanf
s to see if it was successful in scanning data. scanf
的返回值,以查看扫描数据是否成功。 Add a length modifier to the second scanf
to prevent buffer overflows: 在第二个
scanf
添加一个长度修饰符,以防止缓冲区溢出:
scanf("%9s", name); /* +1 for the NUL-terminator */
You're not allocating space for name: 您没有为名称分配空间:
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;
}
As mentioned before, allocate space for p->name
. 如前所述,为
p->name
分配空间。 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. 您可能还应该使用其他方式读取书名,或者将scanf格式的%ms带有指向char指针的指针,或者将
%9s
带有缓冲区,否则标题“ war or Peace”也将导致段错误。
Here you create a static variable and the space for it is allocated automatically. 在这里,您将创建一个静态变量,并为其自动分配空间。
Book p;
书p;
You can allocate a space manually when you assign it to pointer, in this line it's not pointer but static variable. 您可以在将空间分配给指针时手动分配空间,这行不是指针,而是静态变量。
p = (Book)malloc(sizeof(struct book));
p =(Book)malloc(sizeof(struct book));
What's more if you want to refer to attribute of static variable you should use "." 此外,如果要引用静态变量的属性,则应使用“。” instead of "->".
而不是“->”。 So you have two option.
因此,您有两种选择。 Create a pointer and allocate a space for the structure and then you "->" oraz create static variable.
创建一个指针并为该结构分配一个空间,然后“->” oraz创建静态变量。
p->id = id;
p-> id = id;
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