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Anonymous Coward
Asked: 2020-01-14 07:02:44 +0800 CST 2020-01-14 07:02:44 +0800 CST

Is it correct to use the same array as input and output in sprintf?

  • 772

I have found the following code:

char query[255];
sprintf(query, "SELECT NOMBRE, ");
switch (tipo)
{
    case TIPO_1:
        sprintf(query, "%s ID", query);
        break;
    case TIPO_2:
        sprintf(query, "%s APELLIDO", query);
        break;
}

And what bothers me is the use of queryplaying the role of both the read parameter and the write parameter of sprintf.
The code works fine and does what is expected of it in our compiler and in our tests.

Assuming the size of the array is not exceeded. And that whenever query is used as a read parameter, it is the first parameter after the format parameter and the format starts with "%s".
Is it a correct use? Or is there a situation where it could malfunction?

c++

2 Answers

  1. Best Answer

    If we check what the function's documentation in GNU says :

    The behavior of this function is undefined if copying takes place between objects that overlap—for example, if s is also given as an argument to be printed under control of the '%s' conversion. See Copying and Concatenation.

    In your case you are overlapping the input buffer with the output buffer, so the result, as indicated in the first comment, depends only on the implementation you use at any given time.

    In any case, although it works for specific situations, it is something to avoid given the randomness of the result. There are much more robust solutions that only require the use of an additional variable.

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  2. With respect to C standard library functions like sprintf, C++ references the C standard, with some qualifications (but none for sprintf). For its part, the C standard (in §7.19.6.6 in C99, §7.21.6.6 in C11) says that if a copy occurs between overlapping objects, undefined behavior occurs.

    According to both standards, as soon as undefined behavior occurs the result can be unpredictable.

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