Call a C function that accepts a const pointer to struct from Nim - ffi

Question

I am using GNU argparse from Nim using its foreign function interface.
In <argp.h> there is function argp_parse which is declared as follows
extern error_t argp_parse (const struct argp *__restrict __argp,
int __argc, char **__restrict __argv,
unsigned __flags, int *__restrict __arg_index,
void *__restrict __input);
the tricky part are the const and __restrict before the first parameter, in Nim I wrote this code
type
ArgpChild {.importc: "struct argp_child"} = object
argp : ptr[Argp]
flags : cint
header : cstring
group : cint
Argp* {.importc: "struct argp", header: "<argp.h>"} = object
options : ptr[ArgpOption]
parser : pointer #proc (key : cint, arg: cstring, state : ptr[ArgpState]) : Error
args_doc : cstring
doc : cstring
children : ArgpChild
help_filter : pointer #proc(key : int, text : cstring, input: pointer) : cstring
argp_domain : cstring
ArgpOption* {.importc: "struct argp_option"} = object
name : cstring
key : cint
arg : cstring
flags : cint
doc : cstring
group : cint
ArgpState {.importc: "struct argp_state"} = object
input : pointer
Error* = enum
ARGP_KEY_ARG = 0,
ARGP_ERR_UNKNOWN = 7,
ARGP_KEY_END = 0x1000001
proc argp_parse*(argp : Argp, argc : cint, argv : cstringArray, flags: cuint, arg_index: ptr[cint], input : pointer) : Error {.importc: "argp_parse".}
the problem is that the Nim compiler (with c backend) generates this line for argp_parse function (note the missing const and __restrict keywords before the first parameter)
N_CDECL(Error177010, argp_parse)(struct argp* argp0, int argc0, NCSTRING* argv0, unsigned int flags0, int* argindex0, void* input0);
then, when gcc handles that file, it complains yelling "conflicting types for ‘argp_parse’", I guess this is because it finds two definitions of the same functions with different types in the first parameter.
Is there any way to force the Nim compiler to add the const (and maybe __restrict) keyword before a function parameter when using the c backend?

Answer 1

Nim will support a restrict pragma in the future that will allow you to specify such parameters. This is evident from the latest todo.txt in the Nim's repository.
In the meantime, there is one possible work-around for this. Instead of using the importc pragma, you can use the emit pragma:
proc argp_parse*(argp: Argp,
argc: cint,
argv: cstringArray,
flags: cuint,
arg_index: ptr[cint],
input: pointer): Error =
{.emit: "argp_parse(`argp`, `argc`, `argv`, `flags`, `arg_index`, `input`);".}
This will create a regular non-imported proc that just forwards the call the argp_parse function.

Answer 2

The correct fix is simply to add header:
when defined case_bad: # error: conflicting types for 'strlen'
proc c_strlen(a: cstring): csize_t {.importc: "strlen".}
else: # ok
proc c_strlen(a: cstring): csize_t {.importc: "strlen", header: "<string.h>".}
echo c_strlen("abc")