Why does clang give a warning: unterminated ‘#pragma pack (push, …)’ at end of file?

Question

I create a main.cpp in my vscode with clangd enabled, and put the following code in it.

clangd warns the first line with the warning message:

warning: unterminated ‘#pragma pack (push, …)’ at end of file

The whole content of main.cpp:

#pragma pack(push) // warning on this line
#pragma pack(1)
struct A
{
    int   a;
    short b;
    char  c;
};
#pragma pack(pop)

See also: https://releases.llvm.org/13.0.0/tools/clang/docs/DiagnosticsReference.html#wpragma-pack

I think it's a very common usage of #pragma pack(push), I don't understand why the warning is generated.

More strange to me, if I add a semicolon before the first line, the warning disappears.

;                  // Add a semicolon
#pragma pack(push) // The warning disappears
#pragma pack(1)
struct A
{
    int   a;
    short b;
    char  c;
};
#pragma pack(pop)

What's the reason behind?

Answer 1

This is a known bug in clangd, tracked in https://github.com/clangd/clangd/issues/1167.

Please see that issue for an explanation of why this currently happens.

---

A possible workaround discussed in that issue is to add a dummy declaration above the pragma to force clangd to end the file's "preamble" region before the pragma, e.g.:

static_assert(true); // dummy declaration, ends the preamble
#pragma pack(push) // no warning!
#pragma pack(1)
struct A
{
    int   a;
    short b;
    char  c;
};
#pragma pack(pop)

Note, if the file contains #include directives, for performance reasons those should remain in the preamble:

#include <...>  // still in the preamble
static_assert(true); // dummy declaration, ends the preamble
#pragma pack(push) // no warning!
#pragma pack(1)
struct A
{
    int   a;
    short b;
    char  c;
};
#pragma pack(pop)

---

An alternative workaround would be to suppress the warning with a .clangd file that contains

Diagnostics:
    Suppress: pragma-pack