Example of creating and using a Windows DLL with MinGW

helper.c

// x86_64-w64-mingw32-gcc -Wall -shared -O2 -s -o helper.dll helper.c

#include <stdio.h>
#include <string.h>

__declspec(dllexport) __stdcall int proc_sql_modify(char *sql, int size) {
	strcpy(sql, "NEW SQL HERE");
	return 0;
}

usage.c

// x86_64-w64-mingw32-gcc -Wall -O2 -s -o usage.exe usage.c

#include <stdio.h>
#include <windows.h>

typedef int (__stdcall *PROC_SQL_MODIFY)(char *sql, int size);

int main() {
	char helper_dll_name[128] = "helper.dll";  // From settings, default NULL
	HMODULE dll_instance = NULL;
	PROC_SQL_MODIFY proc_sql_modify = NULL;
	char sqlstr[1024] = "INSERT INTO table1 (a,b,c) VALUES ('Text', 55, 3.1415)";
	
	if (*helper_dll_name) {
		dll_instance = LoadLibrary(helper_dll_name);
		if (dll_instance)
			proc_sql_modify = (PROC_SQL_MODIFY)GetProcAddress(dll_instance, "proc_sql_modify");
	}
	
	printf("SQL before DLL-modification: %s\n\n", sqlstr);
	
	if (proc_sql_modify)
		proc_sql_modify(sqlstr, sizeof sqlstr);
	
	printf("SQL after DLL-modification: %s\n", sqlstr);

	if (dll_instance)
		FreeLibrary(dll_instance);
}

usage_pascal.pas

type
  TProcSqlModify = function(sql: PAnsiChar; size: Integer): Integer; stdcall;
var
  dll_instance: HMODULE;
  proc_sql_modify: TProcSqlModify;
  sqlstr: array[0..127] of AnsiChar;
begin
  dll_instance := LoadLibrary('helper.dll');
  proc_sql_modify := GetProcAddress(dll_instance, 'proc_sql_modify');
  proc_sql_modify(sqlstr, SizeOf(sqlstr));
  Caption:=sqlstr;
  FreeLibrary(dll_instance);
end;