Realloc Invalid Pointer in C [closed]

问题

This is a homework assignment so I don't want to post any code, but I'm pretty stumped with the bug that I have.

Currently I have a array that has been malloced and am copying the pointer to the array. Now, I can memcpy and memmove with this array and that works fine.

However, when I do a realloc with it an invalid pointer error comes up - and I have absolutely no idea why.

Can anyone help?

回答1:

you said 'I copy the pointer to this array into another variable'. The problem is as soon as you do a realloc, the original pointer is no longer valid. I dont see the reason to copy the pointer to a variable?

回答2:

realloc() only works if the pointer you pass it was one returned earlier from malloc(), calloc() or realloc() (or is NULL, in which case it works exactly like malloc()).

You can not call it on arrays. If you have a struct like this:

struct foo {
    char a;
    int x[5];
    double z;
};

then you cannot use realloc() to increase the size of x. You must instead switch to using entirely dynamic allocation: the struct member changes to a pointer:

struct foo {
    char a;
    int *x;
    double z;
};

...and when you create an instance of the struct, you call malloc() for the initial allocation:

struct foo f;

f.x = malloc(5 * sizeof f.x[0]);

Now you can call realloc() on f.x, but the cost is that you must also call free() on f.x when the struct is no longer needed.

回答3:

When realloc or free result in an invalid pointer error or segfault, and you are sure that you are passing a valid pointer obtained from malloc, then a good possibility is that you corrupted your memory: You wrote to the area where malloc stores the book keeping of the memory blocks. In your case this is a memcpy or memmove call.

You can use the valgrind memory error detector to help you find these kind of errors.

来源：https://stackoverflow.com/questions/7869757/realloc-invalid-pointer-in-c

标签

pointers

realloc