Mixing C and C++ IO. What extra syncing do I need ?

(Please ignore portability issues, as this is just for fun)

Code:

std::string InputUI(std::string msg="Input",int numchars=50)
{
    std::string command = "kdialog --inputbox \"" + msg + "\"";
    auto f = popen(command.c_str(),"r");
    char* ret = new char[numchars];
    fgets(ret,numchars,f);
    ret[numchars-1]='\0';
    std::string ret_str = std::string(ret);
    delete [] ret;
    return ret_str;
}

But If I call this function twice in a row...and cout the strings returned, they seem to output after the second one is returned, even when I use endl in between.
Other than that, this seems to work fine for me, but have I missed something obvious?

Interesting problem, I don't think this is being caused by the mixing of C-stdio and C++ streams but maybe something to do with how cout chains the output.

With the following calls

Code:

   cout << InputUI("Input", 1000) << " ";
   cout.flush();
   cout << InputUI("Input 2", 1000) << endl;

   cout << InputUI("Input", 1000) << InputUI("Input 2", 1000) << endl;

I got the following results when entering "This is a test.", "Does it work?" for the first two dialogs, and "bye!", "Good" for the second two.

This is a test. Does it work?

and then

Goodbye!

Also shouldn't the following line:

Code:

ret[numchars-1]='\0'

Actually be:

Code:

ret[strlen(ret)-1]='\0'

If you are trying to remove the trailing end of line character.

Jim

The problem with this line:

Code:

cout << InputUI("Input", 1000) << InputUI("Input 2", 1000) << endl;

is that order of evaluation is unspecified, so when InputUI has side effects, the order in which the side effects occur depends on which function call is chosen to be evaluated first.

Shouldn't you be pcloseing the pipe?

Why use new when you can use a vector? This is not thread safe.

Originally Posted by jimblumberg

Actually be:

Code:

ret[strlen(ret)-1]='\0'

If you are trying to remove the trailing end of line character.

Ok.

Originally Posted by oogabooga

Shouldn't you be pcloseing the pipe?

Yes.. I should be.
(Why doesn't it get closed upon the end of the block, though ?)

Originally Posted by Elysia

Why use new when you can use a vector? This is not thread safe.

What happens to the vector's size after being filled, if I pass its address ?

The vector's size does not change. You should pre-allocate it to a proper size. If you were using a C++ function, you could use a push_back_iterator and forget about pre-allocating size.