如何可视化 C++ 结构/类的布局

Question

What is the best way to visualize the memory layout of a C++ class/struct, compiled by GCC?

I added the GCC switch -fdump-lang-class to my C++ compile options but it didn't output anything to stdout, nor did I notice any files .class files created.

I just want to see the size/offsets of class/data members.

Answer 1

You can use pahole. It is a swiss army tool for this kind of things.

https://manpages.ubuntu.com/manpages/impish/man1/pahole.1.html

For example, say you have this test.cpp file

struct A {
        int i;
        char c;
        double x;
};

void doit( A& a ) {
        a.i = 1;
        a.c = 0;
        a.x = 1.0;
}

Let's compile it into a shared library

$ g++ test.cpp -shared -ggdb -o libtest.so
$ ls -l libtest.so
-rwxrwxr-x 1 awesome awesome 16872 Aug 12 20:15 libtest.so

Then run pahole on the binary (with debug information)

$ pahole libtest.so
struct A {
        int                        i;                    /*     0     4 */
        char                       c;                    /*     4     1 */
        /* XXX 3 bytes hole, try to pack */
        double                     x;                    /*     8     8 */

        /* size: 16, cachelines: 1, members: 3 */
        /* sum members: 13, holes: 1, sum holes: 3 */
        /* last cacheline: 16 bytes */
};

You can also provide a class name with the -C <classname> argument

$ pahole -C A libtest.so
struct A {
        int                        i;                    /*     0     4 */
        char                       c;                    /*     4     1 */
        /* XXX 3 bytes hole, try to pack */
        double                     x;                    /*     8     8 */

        /* size: 16, cachelines: 1, members: 3 */
        /* sum members: 13, holes: 1, sum holes: 3 */
        /* last cacheline: 16 bytes */
};