How to wrap a function with variable length arguments?
-
09-06-2019 - |
Question
I am looking to do this in C/C++.
I came across Variable Length Arguments but this suggests a solution with Python & C using libffi.
Now, if I want to wrap printf
function with myprintf
What I do is like below:
void myprintf(char* fmt, ...)
{
va_list args;
va_start(args,fmt);
printf(fmt,args);
va_end(args);
}
int _tmain(int argc, _TCHAR* argv[])
{
int a = 9;
int b = 10;
char v = 'C';
myprintf("This is a number: %d and \nthis is a character: %c and \n another number: %d\n",a, v, b);
return 0;
}
But the results are not as expected!
This is a number: 1244780 and
this is a character: h and
another number: 29953463
Any point where did I miss??
Solution
the problem is that you cannot use 'printf' with va_args. You must use vprintf if you are using variable argument lists. vprint, vsprintf, vfprintf, etc. (there are also 'safe' versions in Microsoft's C runtime that will prevent buffer overruns, etc.)
You sample works as follows:
void myprintf(char* fmt, ...)
{
va_list args;
va_start(args,fmt);
vprintf(fmt,args);
va_end(args);
}
int _tmain(int argc, _TCHAR* argv[])
{
int a = 9;
int b = 10;
char v = 'C';
myprintf("This is a number: %d and \nthis is a character: %c and \n another number: %d\n",a, v, b);
return 0;
}
OTHER TIPS
In C++11 this is one possible solution using Variadic templates
:
template<typename... Args>
void myprintf(const char* fmt, Args... args )
{
std::printf( fmt, args... ) ;
}
EDIT
As @rubenvb points out there are trade-offs to consider, for example you will be generating code for each instance which will lead to code bloat.
I am also unsure what you mean by pure
In C++ we use
#include <cstdarg>
#include <cstdio>
class Foo
{ void Write(const char* pMsg, ...);
};
void Foo::Write( const char* pMsg, ...)
{
char buffer[4096];
std::va_list arg;
va_start(arg, pMsg);
std::vsnprintf(buffer, 4096, pMsg, arg);
va_end(arg);
...
}
Actually, there's a way to call a function that has no va_list
version from a wrapper. The idea is to use assembler, do not touch arguments in stack, and temporary replace the function return address.
Example for Visual C x86. call addr_printf
calls printf()
:
__declspec( thread ) static void* _tls_ret;
static void __stdcall saveret(void *retaddr) {
_tls_ret = retaddr;
}
static void* __stdcall _getret() {
return _tls_ret;
}
__declspec(naked)
static void __stdcall restret_and_return_int(int retval) {
__asm {
call _getret
mov [esp], eax ; /* replace current retaddr with saved */
mov eax, [esp+4] ; /* retval */
ret 4
}
}
static void __stdcall _dbg_printf_beg(const char *fmt, va_list args) {
printf("calling printf(\"%s\")\n", fmt);
}
static void __stdcall _dbg_printf_end(int ret) {
printf("printf() returned %d\n", ret);
}
__declspec(naked)
int dbg_printf(const char *fmt, ...)
{
static const void *addr_printf = printf;
/* prolog */
__asm {
push ebp
mov ebp, esp
sub esp, __LOCAL_SIZE
nop
}
{
va_list args;
va_start(args, fmt);
_dbg_printf_beg(fmt, args);
va_end(args);
}
/* epilog */
__asm {
mov esp, ebp
pop ebp
}
__asm {
call saveret
call addr_printf
push eax
push eax
call _dbg_printf_end
call restret_and_return_int
}
}
Are you using C or C++? The next C++ version, C++0x, will support variadic templates which provide a solution to that problem.
Another workaround can be achieved by clever operator overloading to achieve a syntax like this:
void f(varargs va) {
BOOST_FOREACH(varargs::iterator i, va)
cout << *i << " ";
}
f(args = 1, 2, 3, "Hello");
In order to get this to work, the class varargs
has to be implemented to override operator =
that returns a proxy object which, in turn, overrides operator ,
. However, making this variant type safe in current C++ isn't possible as far as I know since it would have to work by type erasure.
How do you mean a pure C/C++ solution?
The rest parameter (...) is supported cross platform in the C runtime.
void myprintf(char* fmt, ...)
{
va_ list args;
va_ start(args,fmt);
printf(fmt,args); ----> This is the fault. vprintf(fmt, args); should have been used.
va_ end(args);
}
If you're just trying to call printf,
there's a printf variant called vprintf that takes
the va_list directly : vprintf(fmt, args);