I am using Nodejs c++ addon in my nodejs project. JS calls a method defined in c++ with a string as the parameter. I couldn't get the string in c++. Below is my code in c++:
NAN_METHOD(DBNode::Test){
printf("Hello\n");
printf("%s\n", info[0]->ToString());
printf("%d\n", info[1]->ToNumber());
}
Below is my js code:
const test = require('./build/Release/test.node');
test.test('ssss', 99);
Below is the output:
$ node demo.js
Hello
?Ś?C
-272643000
You can see from the above output that the string and integer values are not correctly printed. Is there anything wrong with my code?
Let start from numbers. ToNumber returns value of type Local<Number>. It differs from regular C-like value what printf can digest.
First of all you need unwrap Local. It is v8 pointer-like utility class.
You can do it with overrided * operator. So *(info[1]->ToNumber()) gives us v8 Number child of Value. But this is not the end of story. Now we can pull good-old int from it (*(info[1]->ToNumber())).Int32Value(). Or you can use the fact Handle ancestors override -> operator too and write like info[1]->ToNumber()->Int32Value().
String case is harder. V8 uses utf8 strings and you can use String::Utf8Value utility class to get buffer of char from it. *(String::Utf8Value(info[0]->ToString()))
Usually you do not need it in v8 addons and I suggest you work with v8 objects(like Local, String, Number, etc) in your native code.
below is the solution :--
NAN_METHOD(updateSignalValue) {
Nan::Utf8String lSignal(info[0]);
int len = lSignal.length();
if (len <= 0) {
return Nan::ThrowTypeError("arg must be a non-empty string");
}
std::cout << "\n hello lSignal value is :"<
Regards, Rakesh Kumar Jha
Related
I have a dynamically linked ELF executable on Linux, and I want to swap a function in a library it is linked against. With LD_PRELOAD I can, of course, supply a small library with a replacement for the function that I compile myself. However, what if in the replacement I want to call the original library function? For example, the function may be srand(), and I want to hijack it with my own seed choice but otherwise let srand() do whatever it normally does.
If I were linking to make said executable, I would use the wrap option of the linker but here I only have the compiled binary.
One trivial solution I see is to cut and paste the source code for the original library function into the replacement - but I want to handle the more general case when the source is unavailable. Or, I could hex edit the needed extra code into the binary but that is specific to the binary and also time consuming. Is something more elegant possible than either of these? Such as some magic with the loader?
(Apologies if I were not using the terminology precisely...)
Here's an example of wrapping malloc:
// LD_PRELOAD will cause the process to call this instead of malloc(3)
// report malloc(size) calls
void *malloc(size_t size)
{
// on first call, get a function pointer for malloc(3)
static void *(*real_malloc)(size_t) = NULL;
static int malloc_signal = 0;
if(!real_malloc)
{
// real_malloc = (void *(*)(size_t))dlsym(RTLD_NEXT, "malloc");
*(void **) (&real_malloc) = dlsym(RTLD_NEXT, "malloc");
}
assert(real_malloc);
if (malloc_signal == 0)
{
char *string = getenv("MW_MALLOC_SIGNAL");
if (string != NULL)
{
malloc_signal = 1;
}
}
// call malloc(3)
void *retval = real_malloc(size);
fprintf(stderr, "MW! %f malloc size %zu, address %p\n", get_seconds(), size, retval);
if (malloc_signal == 1)
{
send_signal(SIGUSR1);
}
return retval;
}
The canonical answer is to use dlsym(RTLD_NEXT, ...).
From the man page:
RTLD_NEXT
Find the next occurrence of the desired symbol in the search
order after the current object. This allows one to provide a
wrapper around a function in another shared object, so that,
for example, the definition of a function in a preloaded
shared object (see LD_PRELOAD in ld.so(8)) can find and invoke
the "real" function provided in another shared object (or for
that matter, the "next" definition of the function in cases
where there are multiple layers of preloading).
See also this article.
Just for completeness, regarding editing the function name in the binary - I checked and it works but not without potential hiccups. E.g., in the example I mentioned, one can find the offset of "srand" (e.g., via strings -t x exefile | grep srand) and hex edit the string to "sran0". But names of symbols may be overlapping (to save space), so if the code also calls rand(), then there is only one "srand" string in the binary for both. After the change the unresolved references will then be to sran0 and ran0. Not a showstopper, of course, but something to keep in mind. The dlsym() solution is certainly more flexible.
Ok, so I'm using C++ to make a library that'd help me to print lines into a console.
So, I want to override " "(quote operators) to create an std::string instead of the string literal, to make it easier for me to append other data types to that string I want to output.
I've seen this done before in the wxWidgets with their wxString, but I have no idea how I can do that myself.
Is that possible and how would I go about doing it?
I've already tried using this code, but with no luck:
class PString{
std::string operator""(const char* text, std::size_t len) {
return std::string(text, len);
}
};
I get this error:
error: expected suffix identifier
std::string operator""(const char* text, std::size_t len) {
^~
which, I'd assume, want me to add a suffix after the "", but I don't want that. I want to only use ""(quotes).
Thanks!
You can't use "" without defining a suffix. "" is a const char* by itself either with a prefix (like L"", u"", U"", u8"", R"()") or followed by suffixes like (""s, ""sv, ...) which can be overloaded.
The way that wxString works is set and implicit constructor wxString::wxString(const char*); so that when you pass "some string" into a function it is essentially the same as wxString("some string").
Overriding operator ""X yields string literals as the other answer.
I am going through RPC tutorial and learn few techniques in rpcgen. I have the idea of adding, multiplying different data types using rpcgen.
But I have not found any clue that how could I declare a function in .x file which will return a string. Actually I am trying to build a procedure which will return a random string(rand string array is in server).
Can any one advise me how to proceed in this issue? It will be helpful if you advise me any tutorial regarding this returning string/pointer issue.
Thank you in advance.
Ok, answering to the original question (more than 2 years old), the first answer is correct but a little tricky.
In your .x file, you define your structure with the string inside, having defined previously the size of the string:
typedef string str_t<255>;
struct my_result {
str_t data;
};
...
Then you invoke rpcgen on your .x file to generate client and server stubs and .xdr file:
$rpcgen -N *file.x*
Now you can compile client and server in addition to any program where you pretend to use the remote functions. To do so, I followed the "repcgen Tutorial" in ORACLE's web page:
https://docs.oracle.com/cd/E19683-01/816-1435/rpcgenpguide-21470/index.html
The tricky part is, although you defined a string of size m (array of m characters) what rpcgen and .xdr file create is a pointer to allocated memmory. Something like this:
.h file
typedef char *str_t;
struct my_result {
int res;
str_t data;
};
typedef struct my_result my_result;
.xdr file
bool_t xdr_str_t (XDR *xdrs, str_t *objp)
{
register int32_t *buf;
if (!xdr_string (xdrs, objp, 255))
return FALSE;
return TRUE;
}
So just take into account when using this structure in your server side that it is not a string of size m, but a char pointer for which you'll have to reserve memory before using it or you'll be prompted the same error than me on execution:
Segmentation fault!
To use it on the server you can write:
static my_result response;
static char text[255];
memset(&response, '\0', sizeof(my_result));
memset(text, '\0', sizeof(text));
response.data = text;
And from there you are ready to use it wisely! :)
According to the XDR protocol specification you can define a string type where m is the length of the string in bytes:
The standard defines a string of n (numbered 0 to n -1) bytes to be the number n encoded as an unsigned integer (as described above), and followed by the n bytes of the string. Each byte must be regarded by the implementation as being 8-bit transparent data. This allows use of arbitrary character set encodings. Byte m of the string always precedes byte m +1 of the string, and byte 0 of the string always follows the string's length. If n is not a multiple of four, then the n bytes are followed by enough (0 to 3) residual zero bytes, r, to make the total byte count a multiple of four.
string object<m>;
You can then define a struct with the string type str_t as one of the variables:
typedef string str_t<255>;
struct my_result {
str_t data;
};
Then in your .x file you can define an RPC in your program which returns a struct of type my_result. Since rpcgen will give you a pointer to this struct (which I have called res) you can print the message with prinf("%s\n", res->data);.
program HELLO_PROG {
version HELLO_VERSION {
my_result abc() = 1;
} = 1;
} = 1000;
I am getting all kinds of errors when passing my array to this function. The function is suppose to have the user enter a name and a score and store them in 2 seperate arrays, one for the names, one for the scores. I believe I have to use pointers but have no idea on how to use them. I don't want the answer, just a push in the right direction. Here is the code:
#include <iostream>
int InputData(int &, char, int);
using namespace std;
int main()
{
char playerName[100][20];
int score[100];
int numPlayers = 0;
InputData(numPlayers, playerName, score);
return 0;
}
int InputData(int &numPlayers, char playerName[][20], int score[])
{
while (numPlayers <= 100)
{
cout << "Enter Player Name (Q to quit): ";
cin.getline(playerName, 100, ā\nā);
if ((playerName[numPlayers] = 'Q') || (playerName[numPlayers] = 'q'))
return 0;
cout << "Enter score for " << playerName[numPlayers] <<": ";
cin >> score[numPlayers];
numPlayers++;
}
}
Ok, I made some more changes and the errors are less, must be getting close, Lol!
This looks like a school assignment and I applaud you for not asking for the answer. There are several ways to do it, but you are already fairly close in the approach that you are using. When you pass an array reference, you do not want to include the length of the array. For example, the parameter int score[100] should be int score[]. The exception, especially in your scenario, is with multidimensional arrays. In this case, you want to use char playerName[][20]. Your function declaration also needs to change to match. Don't forget InputData returns an int. Your declarations and function call are correct; you just need to adjust your function signature.
Keeping the errors aside -
InputData(numPlayers, playerName, score, size);
// ^^^^ size is no where declared
// resulting Undeclared indentifier error
Prototype mentions of taking 3 arguments but calling the function passing 4 parameters.
Hint regarding errors:
An 1D array decays to a pointer pointing to first element in the array while passing to a function.
A 2D array decays to a pointer pointing to the 1D array ( i.e., T[][size] ) while passing to a function.
Return type of main() should be int.
It seems with the given hints you corrected most of the errors. But you forgot to change the prototype. So, change -
int InputData(int &, char, int);
to
int InputData(int &, char[][20], int[]);
Why aren't you using std::string array for player names ? Use it and remove rest of the errors. Good luck.
Our C++ lib works fine with Python2.4 using Swig, returning a C++ char* back to a python str. But this solution hit problem in Python3.0, error is:
Exception=(, UnicodeDecodeError('utf8', b"\xb6\x9d\xa.....",0, 1, 'unexpected code byte')
Our definition is like(working fine in Python 2.4):
void cGetPubModulus(
void* pSslRsa,
char* cMod,
int* nLen );
%include "cstring.i"
%cstring_output_withsize( char* cMod, int* nLen );
Suspect swig is doing a Bytes->Str conversion automatically. In python2.4 it can be implicit but in Python3.0 it's no long allowed.. Anyone got a good idea? thanks
It's rather Python 3 that does that conversion. In Python 2 bytes and str are the same thing, in Python 3 str is unicode, so something somewhere tries to convert it to Unicode with UTF8, but it's not UTF8.
Your Python 3 code needs to return not a Python str, but a Python bytes. This will not work with Python 2, though, so you need preprocessor statements to handle the differences.
I came across a similar problem. I wrote a SWIG typemap for a custom char array (an unsigned char in fact) and it got SEGFAULT when using Python 3. So I debugged the code within the typemap and I realized the problem Lennart states.
My solution to that problem was doing the following in that typemap:
%typemap(in) byte_t[MAX_FONTFACE_LEN] {
if (PyString_Check($input))
{
$1 = (byte_t *)PyString_AsString($input);
}
else if (PyUnicode_Check($input))
{
$1 = (byte_t *)PyUnicode_AsEncodedString($input, "utf-8", "Error ~");
$1 = (byte_t *)PyBytes_AS_STRING($1);
}
else
{
PyErr_SetString(PyExc_TypeError,"Expected a string.");
return NULL;
}
}
That is, I check what kind of string object PyObject is. The functions PyString_AsString() and PyUnicode_AsString() will return > 0 if its input it's an UTF- 8 string or an Unicode string respectively. If it's an Unicode string, we convert that string to bytes in the call PyUnicode_AsEncodedString() and later on we convert those bytes to a char * with the call PyBytes_AS_STRING().
Note that I vaguely use the same variable for storing the unicode string and converting it later to bytes. Despite of being that questionable and maybe, it could derive in another coding-style discussion, the fact is that I solved my problem. I have tested it out with python3 and python2.7 binaries without any problems yet.
And lastly, the last line is for replicating an exception in the python call, to inform about that input wasn't a string, either utf nor unicode.