为什么我可以分配局部变量并对其进行读取，但是如果我尝试在结构中分配指针并对其进行读取，则会出现段错误？

Question

I'm playing with a small C program to brush up. I have the following struct...

struct Address {
    int id;
    int set;
    char *name;
    char *email;
};

I am trying to read some data into it. This works:

size_t len = 256 * sizeof(char);

struct Address *thisAddress = malloc(sizeof(struct Address));
char *name = malloc(len);
char *email = malloc(len);

rc = fread(thisAddress, sizeof(struct Address), 1, conn->file);
if(rc != 1) die("Failed to load address.", conn);

rc = fread(name, len, 1, conn->file);
if(rc != 1) die("Failed to load name.", conn);

rc = fread(email, len, 1, conn->file);
if(rc != 1) die("Failed to load email.", conn);

thisAddress->name = name;
thisAddress->email = email;
conn->db->rows[i] = thisAddress;

But this segfaults:

size_t len = 256 * sizeof(char);
struct Address *thisAddress = malloc(sizeof(struct Address));
thisAddress->name = malloc(len);
thisAddress->email = malloc(len);

rc = fread(thisAddress, sizeof(struct Address), 1, conn->file);
if(rc != 1) die("Failed to load address.", conn);

rc = fread(thisAddress->name, len, 1, conn->file);
if(rc != 1) die("Failed to load name.", conn);

rc = fread(thisAddress->email, len, 1, conn->file);
if(rc != 1) die("Failed to load email.", conn);

conn->db->rows[i] = thisAddress;

If you can't see it right away, in the first one I'm malloc'ing the name and edit vars, reading from the file into those vars, then assigning them to the name and email pointers on struct Address. In the second, I'm trying to alloc the pointers on struct Address, and read directly into them.

Why would these two be different? Any insight is helpful, thanks!

Answer 1

First you do

thisAddress->name = malloc(len);
thisAddress->email = malloc(len);

Then you do

rc = fread(thisAddress, sizeof(struct Address), 1, conn->file);

This reading of the structure overwrites the pointers you previously set. So the pointers you use to read the name and email address texts from the file are not the ones you allocated, but the ones you read from the file, which are very unlikely to point to any valid memory, leading to undefined behavior .

If you change the order of the first structure read and the allocations it should work.