help with seg fault

This is a discussion on help with seg fault within the C Programming forums, part of the General Programming Boards category; hi all. Can someone explain why U get a segmentation fault in the following block of code Code: int setup(char ...

  1. #1
    Registered User eth0's Avatar
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    help with seg fault

    hi all.

    Can someone explain why U get a segmentation fault in the following block of code

    Code:
    int setup(char *hostname, int ttl, SOCKET *icmpSock, SOCKADDR_IN *dest)
    {
        HOSTENT *hp;
        unsigned int addr;
        int sockRet;
        
        // create socket
        *icmpSock = WSASocket(AF_INET, SOCK_RAW, IPPROTO_ICMP, NULL, 0, 0);
        if (*icmpSock == INVALID_SOCKET) {
           printf("Could not create socket : %d.\n", WSAGetLastError());
           return -1;
        }
        
        // setup for TTL
        sockRet = setsockopt(*icmpSock, IPPROTO_IP, IP_TTL, (const char *)&ttl, sizeof(ttl));
        if (sockRet == SOCKET_ERROR) {
           printf("ttl setsockopt failed : %d. \n", WSAGetLastError());
           return -1;
        }
        
        memset(&dest, 0, sizeof(*dest));
        addr = inet_addr(hostname);
        if (addr == INADDR_NONE) {
           hp = gethostbyname(hostname);
           if (hp != 0) {
              //memcpy(&dest->sin_addr, hp->h_addr, hp->h_length);
              dest->sin_addr = *((struct in_addr *)hp->h_addr);
              dest->sin_family = hp->h_addrtype;
           } else {
              printf("unknown hostname || failed to resolve\n");
              return -1;
           }
        } else {
            dest->sin_addr.s_addr = addr;
            dest->sin_family = AF_INET;
        }
        
        return 0;
    }
    The error is coming from the line dest->sin_addr = *((struct in_addr *)hp->h_addr);
    I get the same error if I try to use memcpy as above it.

    dest is a pointer to an empty structure.

    Thanks.
    Open source isn't a matter of life or death......
    .......its much more important than that!!


    SuSE Linux - GCC 3.4.2
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  2. #2
    and the hat of wrongness Salem's Avatar
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    > memset(&dest, 0, sizeof(*dest));
    Remove the &
    You just made your pointer a NULL pointer, and hosed a bunch of memory following it as well.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    Registered User eth0's Avatar
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    Thanks Salem.

    When you get a sec, can you just quickly explain why the pointer was set null and what happened to the memory.

    Thanks.
    Open source isn't a matter of life or death......
    .......its much more important than that!!


    SuSE Linux - GCC 3.4.2
    XP Pro - Visual Studio 2005 TS, MinGW 3.4.2

  4. #4
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    2.14.6 memset

    Declaration:

    Code:
    void *memset(void *str, int c, size_t n);
    Copies the character c (an unsigned char) to the first n characters of the string pointed to by the argument str.

    The argument str is returned.
    memset(dest, 0, sizeof(*dest));
    The second argument is what is being filled into dest

  5. #5
    Registered User eth0's Avatar
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    I was using this page as a learning reference http://tangentsoft.net/wskfaq/examples/rawping.html

    In their function setup_for_ping they set the address of dest to 0 as I did initially. (I see why thats wrong now. Thanks sand_man.)

    This is quite a popular paper and if theirs was wrong, I would have though someone would have picked up on it.

    So assuming theirs is right, what makes mine different than theirs?
    Open source isn't a matter of life or death......
    .......its much more important than that!!


    SuSE Linux - GCC 3.4.2
    XP Pro - Visual Studio 2005 TS, MinGW 3.4.2

  6. #6
    and the hat of wrongness Salem's Avatar
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    Perhaps because yours is an inaccurate translation of the C++ code (on that page) to the C code (you posted)
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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