Use strcpy safely

This is a discussion on Use strcpy safely within the C++ Programming forums, part of the General Programming Boards category; Hi, I was wondering how to use strcpy safely so that I can get reported when overwriting happens? Or other ...

  1. #1
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    Use strcpy safely

    Hi,
    I was wondering how to use strcpy safely so that I can get reported when overwriting happens?
    Or other methods that achieve the same function with security check?
    Also in C++, is there a C++ way to handle it besides using STL string class?

  2. #2
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    If you are not using a smart container like std::string, then the only way to be safe is to ensure that there is enough space before you do the copy. This means using strncpy(), or erroring out if the buffer is too small.

  3. #3
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    Quote Originally Posted by bithub View Post
    std::string
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    Moved to C++

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    Thanks!

    Quote Originally Posted by bithub View Post
    erroring out if the buffer is too small.
    How to detect if the buffer is too small for strcpy()?

  6. #6
    The larch
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    If you have established that you have enough room, why should you use strncpy?

    In C++, you can also use a std::vector<char> to take care of dynamic memory management for an otherwise C-style string (and knowing at any time how much memory you have for the buffer).

    How to detect if the buffer is too small for strcpy()?
    Unless you know how much memory is allocated for the target, there is no way. std::vector<char> helps in storing the size of the buffer for you.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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    How to detect if the buffer is too small for strcpy()?
    Usually this is pretty easy:
    Code:
    void do_copy(char* str)
    {
        char buffer[BUFFER_SIZE];
        size_t size = strlen(str);
        if(size >= BUFFER_SIZE)
        {
            /* Error, buffer is too small */
        }
        else
        {
            strcpy(buffer, str);
        }
        // ...
    }

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    Quote Originally Posted by lehe View Post
    Hi,
    I was wondering how to use strcpy safely so that I can get reported when overwriting happens?
    Or other methods that achieve the same function with security check?
    Also in C++, is there a C++ way to handle it besides using STL string class?
    Is there a compelling reason to not use string or vector<char>?

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    If you're in C++, use std::string, as others have mentioned. If you absolutely must use strcpy(), just don't pass it a buffer that can't hold what you're going to copy into it. And don't forget about strdup(). Otherwise, it's generally:
    Code:
    // Determine how much space I need.
    // Allocate a buffer of that size + room for null terminator
    // Check for buffer allocation errors
    // Do the copy.
    long time; /* know C? */
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    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by lehe View Post
    Hi,
    I was wondering how to use strcpy safely so that I can get reported when overwriting happens?
    Or other methods that achieve the same function with security check?
    Also in C++, is there a C++ way to handle it besides using STL string class?
    So you want to take the "ambulance at the bottom of the cliff" approach?
    Why not go so far as to prevent buffer overrun from even happening, by using std::string like you're supposed to?!
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  11. #11
    Guest Sebastiani's Avatar
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    ...or at least strncpy...
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  12. #12
    The larch
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    Why exactly is strncpy such a safety feature? OK, it allows you to copy stuff without finding out the length of the source string at first, but then you run the risk that not the entire string is copied and it can be left unterminated if you are not careful. Eventually you'll need to put a lot of work into it to get the same result as with strcpy if your intention is to copy the whole string. It would seem that strncpy has a somewhat different purpose (something like substr )? (I've never felt any need to use strncpy, because if the string manipulation gets this complicated, I'd definitely use std::string)
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  13. #13
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by anon
    Why exactly is strncpy such a safety feature? OK, it allows you to copy stuff without finding out the length of the source string at first, but then you run the risk that not the entire string is copied and it can be left unterminated if you are not careful. Eventually you'll need to put a lot of work into it to get the same result as with strcpy if your intention is to copy the whole string.
    You are not being paranoid enough: assume that the contents of the source string cannot be trusted, i.e., it might not be null terminated. strcpy() would then be unable to avoid buffer overrun, whereas strncpy() can.
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    You are not being paranoid enough: assume that the contents of the source string cannot be trusted, i.e., it might not be null terminated. strcpy() would then be unable to avoid buffer overrun, whereas strncpy() can.
    Of course that only saves you from overrunning the destination buffer. If the source cannot be trusted (ie not null terminated), then there's still a good chance you will overrun the source buffer.

  15. #15
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by bithub
    If the source cannot be trusted (ie not null terminated), then there's still a good chance you will overrun the source buffer.
    That is why I stated "contents of the source", since it may be a reasonable assumption that the size of the source buffer is known and trusted even when the contents of the source are entirely untrusted.
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