In a C library header I have a struct defined like this:

typedef struct
{
  int bar;
} foo;

I would like to change the definition so that the struct is tagged, like this:

typedef struct foo
{
  int bar;
} foo;

Does this change the ABI for C++ user of the library? Or even worse, break old sources in some way?

In a C library header I have a struct defined like this:

typedef struct
{
  int bar;
} foo;

I would like to change the definition so that the struct is tagged, like this:

typedef struct foo
{
  int bar;
} foo;

Does this change the ABI for C++ user of the library? Or even worse, break old sources in some way?

• c++
• c
• struct
• abi

• 3 [C++] Unless you can amend all occurrences this will break the One Definition Rule, which for C++ is "...Each definition consists of the same sequence of tokens..." – Richard Critten Commented Jan 16 at 20:50
• 3 Why do you need to do this? – Barmar Commented Jan 16 at 20:54
• 2 Not exactly your question, but there is the possible perverse case where the C code also has a struct foo { float potato;} ; Then this would make a bad situation totally broken. – Avi Berger Commented Jan 16 at 21:18
• 1 @EricPostpischil Since they asked about ABI compatibility, I think that implies that they're not planning on recompiling all the users. – Barmar Commented Jan 16 at 21:45
• 3 C and C++ have different naming rules here. Please focus on one language. – Lundin Commented Jan 17 at 7:26

1 Answer 1

Updating the struct definition to include a tag does not impact the ABI for C or C++ users of the library. This is because the memory layout, size, and alignment of the struct remain unchanged. Adding a tag is purely a syntactical change, providing a name for the struct without altering its underlying representation or behavior.