CODEWITHSUNDEEP
c++pointersstruct
Windtalker
Windtalker

Reputation: 796

How to copy a struct from a pointer-to-void?

Say I have a DLL contains a struct, but I don't know the details of this struct. But I have a void pointer which points to address of the struct.

Can anybody tell me how can I get the details of the struct? Such as output the struct to a text file.

Thank you!

Upvotes: 1

Views: 460

Answers (2)

Ruslan Gerasimov
Ruslan Gerasimov

Reputation: 1782

You never know this without structure definition. Also there can be "holes" between the user's variables in the real memory placement because of the alignment and padding.

Say if you have,

struct mystr {
    char x;
    int y;
};

by default such structure most likely will have size 8, and after one byte of char x there will be three bytes of padding (in theory random values), and then 4 bytes of int y, but it depends on compiler and its directives.

Upvotes: 0

mrjoltcola
mrjoltcola

Reputation: 20842

You cannot know the details of the struct without the type definition. Copying a region starting with the void pointer without a type definition will give you the raw binary data, but you wont know where it ends, or which pieces represent which variables. Some of the values could be integer values or they could be pointer addresses. There are all sorts of possibilities.

You should try to obtain the header file.

You might be able to glean some information from the debug / symbol file if you have it (example .pdb files on Windows), or debugging the program with GDB on Linux, this will only work if you have a debug build of the program. Refer to the "whatis" and "ptype" commands in GDB.

Upvotes: 7

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