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.
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 */
};
