How to build when __builtin_va_arg_pack() is used

For purely educational purposes, I am trying to write examples with the "Constructing Function Calls builtins" (https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Constructing-Calls.html#index-_005f_005fbuiltin_005fva_005farg_005fpack)

I have written an example that incorporates what appears in the documentation:

#include <stdio.h>

extern int myprintf(FILE *f, const char *format, ...);
extern inline __attribute__((__gnu_inline__)) int myprintf(FILE *f, const char *format, ...)
{
    int r = fprintf(f, "myprintf: ");
    if (r < 0)
        return r;
    int s = fprintf(f, format, __builtin_va_arg_pack());
    if (s < 0)
        return s;
    return r + s;
}
int main()
{
    myprintf(stdout, "ciao %d\n", 10);
    return 0;
}

but I cannot build

max@jarvis:~/test$ gcc --version
gcc (Ubuntu 13.2.0-4ubuntu3) 13.2.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

max@jarvis:~/test$ gcc -std=gnu2x -Wall test1.c -o test1
/usr/bin/ld: /tmp/ccgD1BYT.o: in function `main':
test1.c:(.text+0x27): undefined reference to `myprintf'
collect2: error: ld returned 1 exit status

the gcc version is shown above

Where am I going wrong?

Solution

gcc's default optimisation level is -O0 (none). Inlining is not active without optimisation, so:

$ gcc -std=gnu2x -Wall test1.c -o test1

emits a regular external call to myprintf, which has no definition.

GCC Manual: 3.11 Options That Control Optimization:

-fno-inline

Do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing.

[my emphasis]

Compile with:

$ gcc -O1 -std=gnu2x -Wall test1.c -o test1

or higher -On and the program will link and run as you expect.

You can compare the -O0 assembly, where there is no trace of myprintf besides an external call, with the -O1 assembly, where its inlined definition is apparent.

Refer further to sect. 3.11 if you wish to learn which specific inlining options are enabled at each optimisation level.