Question

As I understood the correct programming style tells that if you want to get string (char []) from another function is best to create char * by caller and pass it to string formating function together with created string length. In my case string formating function is “getss”.

void getss(char *ss, int& l)
{

    sprintf (ss,"aaaaaaaaaa%d",1);
    l=11;
}


int _tmain(int argc, _TCHAR* argv[])
{

    char *f = new char [1];
    int l =0;
    getss(f,l);

    cout<<f;
    char d[50] ;
    cin>> d;
    return 0;
}

“getss” formats string and returns it to ss*. I thought that getss is not allowed to got outside string length that was created by caller. By my understanding callers tells length by variable “l” and “getcc” returns back length in case buffer is not filled comleatly but it is not allowed go outside array range defined by caller.

But reality told me that really it is not so important what size of buffer was created by caller. It is ok, if you create size of 1, and getss fills with 11 characters long. In output I will get all characters that “getss” has filled.

So what is reason to pass length variable – you will always get string that is zero terminated and you will find the end according that.

What is the reason to create buffer with specified length if getss can expand it?

How it is done in real world – to get string from another function?

Answer 1

Actually, the caller is the one that has allocated the buffer and knows the maximum size of the string that can fit inside. It passes that size to the function, and the function has to use it to avoid overflowing the passed buffer.

In your example, it means calling snprintf() rather than sprintf():

void getss(char *ss, int& l)
{
    l = snprintf(ss, l, "aaaaaaaaaa%d", 1);
}

In C++, of course, you only have to return an instance of std::string, so that’s mostly a C paradigm. Since C does not support references, the function usually returns the length of the string:

int getss(char *buffer, size_t bufsize)
{
    return snprintf(buffer, bufsize, "aaaaaaaaaa%d", 1);
}