Append or strcpy segmentation fault

This is a discussion on Append or strcpy segmentation fault within the C++ Programming forums, part of the General Programming Boards category; hi there I was wondering if someone could help me. I'm writing a programme, where I have to return some ...

  1. #1
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    Append or strcpy segmentation fault

    hi there
    I was wondering if someone could help me.
    I'm writing a programme, where I have to return some char array or string.
    The problem is, it works only for the first time.. see
    Code:
    class CliLogic{
        private:
            char orient, state, * name;
            std::string m_buf;
        public:
            CliLogic(){state = 0; m_buf.reserve(1024);}
            void CM( const char * mes);        
    
    };
    Name receives a pointer from another class, so it should be valid (and actually IS valid, tested..). But take next method CM

    Code:
    void CliLogic::CM(const char * mes){
            printf("Start\n");
            m_buf = ""
            m_buf.append(name);
            printf(".");
            m_buf.append(mes);
            printf(".");
            m_buf.append("\r\n");
            printf(".");
            printf("%s\n",m_buf.c_str());        
    }
    On the first run it goes like "Start\n...Name message\r\nStart\n segmentation fault". Valgrind says it touches memory on 0x0, but I didn't figure out the fault. The size of the string (or char array, tested on both) shouldn't be a problem. Name is not bigger than 512 bits and message isn't longer than 5 chars + "\r\n".

    I have tried it with append and with strcpy (which was originally there), both resulting in segfaults.

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Eramol
    Name receives a pointer from another class, so it should be valid (and actually IS valid, tested..).
    So, the CliLogic object does not own the string represented by the name member, right?

    Quote Originally Posted by Eramol
    On the first run it goes like "Start\n...Name message\r\nStart\n segmentation fault". Valgrind says it touches memory on 0x0, but I didn't figure out the fault. The size of the string (or char array, tested on both) shouldn't be a problem. Name is not bigger than 512 bits and message isn't longer than 5 chars + "\r\n".
    You should initialise name to be a null pointer in the constructor, then check that both name and mes are not null pointers.

    It would be better if you posted the smallest and simplest compilable program that demonstrates the segmentation fault. The problem with claiming that something that is not shown "actually IS valid, tested" is that you may have made an oversight, e.g., your tests may show validity of the pointer in a context before it becomes invalid.
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  3. #3
    C++まいる!Cをこわせ! Elysia's Avatar
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    >>m_buf.reserve(1024);
    A string is not a buffer. There is no need to reserve storage.

    >>printf("Start\n");
    Don't use printf. Use std::cout.

    >>m_buf.append(name);
    Or you could just type
    m_buf += name;

    >> m_buf = ""
    >> m_buf.append(name);
    Or you could just do
    m_buf = name;
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  4. #4
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    Quote Originally Posted by Elysia View Post
    A string is not a buffer. There is no need to reserve storage.
    but if you know how much data you'll be putting in it, pre-allocating space can improve performance, especially if you don't append all the data at once. this can become very important if you're dealing with large amounts of data (several megabytes or more).

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