How to pass variadic parameters?

Question

I am trying to pass some variadic parameters, but I am having no success.

Code:

void Dispositivo::enviarComandoPTZ( AcaoPTZEnum in_acao, DirecaoPTZEnum in_direcao )
{
    std::string direcao = direcaoPTZMap[in_direcao];
    std::string acao = StringUtil::to_string(in_acao);

    char* buf = ByteUtil::hexStringToRawBytes( ComandoUtil::gerarComando( PTZ, m_sessao, acao.c_str(), direcao.c_str(), "4") ); // IT CRASHES HERE

    int len = sizeof(buf);
    m_pSocket->Write( buf, len );
}

std::string ComandoUtil::gerarComando(TipoComandoEnum in_tipo, std::string in_sessao, ...)
{
    va_list parametros;
    va_start ( parametros, in_sessao );

    std::string payload = obterPayload(in_tipo,parametros);

    Requisicao requisicao = Requisicao(in_tipo, payload, in_sessao);

    return requisicao.gerarDados();
}

std::string ComandoUtil::obterPayload( TipoComandoEnum tipo, va_list parametros)
{
    return (wxString::FormatV(_(payloadMap[tipo]),parametros)).ToStdString();
}

payloadMap is a map which holds the strings I need to format.

I am gettin a segmentation fault at the 3rd line of enviarComandoPTZ();

Did you try debugging and seeing what crashes? It might be the `my_fucntion` code that is the problem. — crashmstr, Jul 01 '14 at 12:30
Does your omitted code try to access anything more / other than three C-strings as the optional argumants? No repro because you didn't provide an [MCVE](http://stackoverflow.com/help/mcve). — Deduplicator, Jul 01 '14 at 12:32
Here is the answer: http://stackoverflow.com/questions/1657883/variable-number-of-arguments-in-c — lextiz, Jul 01 '14 at 12:33
It crashes while trying to call my_function, it doesn't even get to the ommited code. It is like it is not accepting the parameters. I get no other error message beside the segmentation fault. — RafaelTSCS, Jul 01 '14 at 12:43
How are you telling the function which variadic types to expect? Are you sure you're accessing the correct number as the correct type (3 of `const char*` in you example)? — Mike Seymour, Jul 01 '14 at 12:43
"What am I doing wrong?" - you're not posting a complete test case. Your code runs with no segmentation fault if I add enough missing code to compile and run it: http://ideone.com/fe1v93. — Mike Seymour, Jul 01 '14 at 12:51
Exactly how do you pass them to that function? Does `some_str` contain the correct format specifiers to match the variadic arguments? Does your real function return a value (your posted code doesn't, which is likely to cause a crash)? — Mike Seymour, Jul 01 '14 at 12:55
I ahve just edited the code so you guys can see exactly what i am doing — RafaelTSCS, Jul 01 '14 at 13:04
`obterPayload` does not expect a `va_list`. No wonder you have trouble. — Deduplicator, Jul 01 '14 at 13:06
I am new to C++, I thought the dots would represent a va_list. But it makes sense. I did this change, but it still doesn't work. It crashes before the call to obterPayload. — RafaelTSCS, Jul 01 '14 at 13:10

borisbn · Answer 1 · 2014-07-01T13:37:54.427

0

Look at this. Maybe your string is greater then 1024 symbols ?

It could be because of ToStdString: your implementation of std::string is not equal to wxWidget's one. Try to call:

return (wxString::FormatV(_(payloadMap[tipo]),parametros)).c_str();

Break this call to steps like this

wxString str = payloadMap[ tipo ];
wxString fmt = _( str ); // BTW, what is this ?
wxString res = wxString::FormatV( fmt, parametros );
return res.c_str();

and run it with debugger

edited Jul 01 '14 at 13:37

answered Jul 01 '14 at 13:12

borisbn

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Thanks for your reply, @borisbn. But if I try to call as you suggested it doesn't compile. wxString::FormatV already returns const *char. I did these changes, but it still crashes on the same call. – RafaelTSCS Jul 01 '14 at 13:26
First: "wxString::FormatV already returns const *char" It can't return `char*` because you can't call `ToStdString` from pointer... only from wxString's object. Second: what is `_` ? Third: look at edit I did to answer – borisbn Jul 01 '14 at 13:37
I meant const char*, sorry. _() is an initializer for wxString. It converts from literal to wxString. [look here](http://wiki.wxwidgets.org/Converting_everything_to_and_from_wxString). I am making some modifications and will post the results. – RafaelTSCS Jul 01 '14 at 13:46
No success :(. It keeps crashing at the call to ComandoUtil::gerarComando(); – RafaelTSCS Jul 01 '14 at 13:58
I was checking the parameters I am trying to pass, and I noticed that the segmentation fault is coming from the m_sessao. This a protected attribute from the class Dispositivo. Idk why this is happening since this method is also in Dispositivo. – RafaelTSCS Jul 01 '14 at 15:56
I've got it. The object (Dispositivo) I was trying to use was null. I am not really a C++ developer, I'm used to Java, so I was expecting the nullpointer to happen before, at the moment I tried to call the function. But it only happens when I try to access the attribute. Thank you for your patience! – RafaelTSCS Jul 02 '14 at 11:03
btw, I changed the string formating to: vsnprintf(&buf[0], buf.size(),payload.c_str(), parametros) < 0 ... performing a check for the buffer size, fo safety. Was getting no luck withthe wxString::FormatV – RafaelTSCS Jul 02 '14 at 12:57

score 0 · Accepted Answer · answered Jul 02 '14 at 11:10

The problem was not really related to the variadic parameters. One of the function parameters (m_sessao) was not accessible because the object Dispositivo was null.

It took me a while to notice this because as I am used to Java, I was expecting any "nullpointerexception" to happen if I tried to access a function of a null object too, and not only after trying to access the attribute; as soon as the function I was calling was into "Dispositivo" and it is not static, I assumed it was not null.

Thank you for those who tried to help. I got some good tips to fix the code.

How to pass variadic parameters?

2 Answers2