builtin.c: typecheck builtin cfunctions in function_list

[emanuele6]

In C23 (default C standard used by GCC 15), jv (*fptr)(); has become equivalent to jv (*fptr)(void); so we can no longer assign builtin implemenations directly to the fptr member of cfunctions without generating a compile error.

Since there does not seem to be any straight-forward way to tell autoconf to force the compiler to use C99 short of explicitly adding -std=c99 to CFLAGS, it is probably a cleaner solution to just make the code C23 compatible.

A possible solution could have been to just redeclare cfunction.fptr as void*, but then the functions' return type would not have been type checked (e.g. if you tried to add a {printf, "printf", 2}, where printf is a function that does not return jv, the compiler wouldn't have complained.)

We were already not typechecking the arguments of the functions, so e.g.

{binop_plus, "_plus", 3},  /* instead of {f_plus, "_plus, 3},       */
{f_setpath, "setpath", 4}, /* instead of {f_setpath, "setpath", 3}, */

compile without errors despite not having the correct prototype.

So I thought of instead improving the situation by redefining cfunction.fptr as a union of function pointers with the prototypes that the jq bytecode interpreter can call, and use a macro to add the builtin functions to function_list using to the arity argument to assign the implementation function to the appropriate union member.

Now the code won't compile if the wrong arity, or an arity not supported by the bytecode interpreter (>5 = 1input+4arguments), or a prototype not jallable by the bytecode interpreter (e.g. binop_plus that doesn't expect a jq_state* argument).

$ make -j
make  all-recursive
make[1]: Entering directory '/home/emanuele6/git/jq'
make[2]: Entering directory '/home/emanuele6/git/jq'
  CC       src/builtin.lo
src/builtin.c:1831:9: error: initialization of 'jv (*)(jq_state *, jv,  jv,  jv,  jv)' from incompatible pointer type 'jv (*)(jq_state *, jv,  jv,  jv)' [-Wincompatible-pointer-types]
 1831 |   CFUNC(f_setpath, "setpath", 4),
      |         ^~~~~~~~~
src/builtin.c:1794:28: note: in definition of macro 'CFUNC'
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |                            ^~~~
src/builtin.c:1831:9: note: (near initialization for 'function_list[88].fptr.a4')
 1831 |   CFUNC(f_setpath, "setpath", 4),
      |         ^~~~~~~~~
src/builtin.c:1794:28: note: in definition of macro 'CFUNC'
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |                            ^~~~
make[2]: *** [Makefile:1056: src/builtin.lo] Error 1
make[2]: Leaving directory '/home/emanuele6/git/jq'
make[1]: *** [Makefile:1193: all-recursive] Error 1
make[1]: Leaving directory '/home/emanuele6/git/jq'

$ make
mkdir -p src
  GEN      src/version.h
make  all-recursive
make[1]: Entering directory '/home/emanuele6/git/jq'
make[2]: Entering directory '/home/emanuele6/git/jq'
  CC       src/main.o
  CC       src/builtin.lo
src/builtin.c:1794:15: error: 'union <anonymous>' has no member named 'a6'; did you mean 'a1'?
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |               ^
src/builtin.c:1821:3: note: in expansion of macro 'CFUNC'
 1821 |   CFUNC(f_test6, "test6", 6),
      |   ^~~~~
src/builtin.c:1821:9: error: initialization of 'jv (*)(jq_state *, jv)' from incompatible pointer type 'jv (*)(jq_state *, jv,  jv,  jv,  jv,  jv,  jv)' [-Wincompatible-pointer-types]
 1821 |   CFUNC(f_test6, "test6", 6),
      |         ^~~~~~~
src/builtin.c:1794:28: note: in definition of macro 'CFUNC'
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |                            ^~~~
src/builtin.c:1821:9: note: (near initialization for 'function_list[73].fptr.a1')
 1821 |   CFUNC(f_test6, "test6", 6),
      |         ^~~~~~~
src/builtin.c:1794:28: note: in definition of macro 'CFUNC'
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |                            ^~~~
make[2]: *** [Makefile:1056: src/builtin.lo] Error 1
make[2]: Leaving directory '/home/emanuele6/git/jq'
make[1]: *** [Makefile:1193: all-recursive] Error 1
make[1]: Leaving directory '/home/emanuele6/git/jq'
make: *** [Makefile:831: all] Error 2

$ make
make  all-recursive
make[1]: Entering directory '/home/emanuele6/git/jq'
make[2]: Entering directory '/home/emanuele6/git/jq'
  CC       src/builtin.lo
src/builtin.c:1813:9: error: initialization of 'jv (*)(jq_state *, jv,  jv,  jv)' from incompatible pointer type 'jv (*)(jv,  jv)' [-Wincompatible-pointer-types]
 1813 |   CFUNC(binop_plus, "_plus", 3),
      |         ^~~~~~~~~~
src/builtin.c:1794:28: note: in definition of macro 'CFUNC'
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |                            ^~~~
src/builtin.c:1813:9: note: (near initialization for 'function_list[62].fptr.a3')
 1813 |   CFUNC(binop_plus, "_plus", 3),
      |         ^~~~~~~~~~
src/builtin.c:1794:28: note: in definition of macro 'CFUNC'
 1794 |   {.fptr = { .a ## arity = func }, name, arity}
      |                            ^~~~
make[2]: *** [Makefile:1056: src/builtin.lo] Error 1
make[2]: Leaving directory '/home/emanuele6/git/jq'
make[1]: *** [Makefile:1193: all-recursive] Error 1
make[1]: Leaving directory '/home/emanuele6/git/jq'
make: *** [Makefile:831: all] Error 2

Also, the code now compiles with gcc -std=c23.

Fixes #3206

[wader]

Would it make sense to have CI pipeline that builds with highest supported C standard version?