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Segmentation fault SIGSEGV

  • Hey all. I am using ubuntu and I have a library provided by the manufacturer and when I tried to debug it, I got segmentation fault.

    My mentor said he was able to solve it by adding the following in .pro file.
    QMAKE_CXXFLAGS += -std=c++0x -pthread
    LIBS += -pthread

    But when I ran the same project in my PC, I still got the error but it was resolved in his PC.
    Can anyone please tell me where can the problem be?

    This is the function where I am getting the issue. The debugger stops at " int allocsize = strlen(key + strlen(szKey)) + 1;"

    /*! Parses the configuration file

    • \param szFile File name
    • \param szKey Key to search for
    • \param szValue Returned value (if key was found)
    • \return != 0 on success /
      static int GetConfigString(const char
      szFile, const char* szKey, char** szValue)
      int ret = 0;
      FILE* fd = fopen(szFile, "r");

    if(NULL != fd)
    /* File is open /
    buffer = malloc(PARSER_BUFFER_SIZE);

    /* Read file line by line */
    while(NULL != fgets(buffer, PARSER_BUFFER_SIZE, fd))
      char* key;
      /* '#' marks a comment line in the device.conf file */
      if(buffer[0] == '#')
      /* Search for key in the input buffer */
      key = (char*)strcasestr(buffer, szKey);
      if(NULL != key)
        /* We've found the key */
        int   allocsize  = strlen(key + strlen(szKey)) + 1;
        int   valuelen;
        char* tempstring = (char*)malloc(allocsize);
        strcpy(tempstring, key + strlen(szKey));
        valuelen = strlen(tempstring);
        /* strip all trailing whitespaces */
        while( (tempstring[valuelen - 1] == '\n') ||
               (tempstring[valuelen - 1] == '\r') ||
               (tempstring[valuelen - 1] == ' ') )
          tempstring[valuelen - 1] = '\0';
        *szValue = tempstring;
        ret = 1;


    return ret;

  • Lifetime Qt Champion

    @sudharsan10 said in Segmentation fault SIGSEGV:

    I got segmentation fault

    Please post the stack trace

  • Hi @jsulm Do you mean this?


  • Lifetime Qt Champion

    @sudharsan10 What is in TCP_Connector.c at line 729?
    Also check your code in nextTransport_demo.c line 200
    And I don't see anything related to Qt...

  • You shall find line 729 in the image and also in the code I have posted above :)
    I was wondering if it could be something related to my project settings, Since there was no issue while running the same project in another PC.

  • @sudharsan10

    • Your question seems to have nothing at all to do with Qt.

    • If the reason the changed link flags make things work is to do with pthread, you have said nothing about whether your program uses any threads.

    • If any line is longer than PARSER_BUFFER_SIZE your code will not work correctly.

    • int allocsize = strlen(key + strlen(szKey)) + 1;
      This line looks potentially dodgy. Print out allocsize, and consider how you are using that malloc()ed area to copy things into.

    • /* strip all trailing whitespaces */
      What happens in your code if the value is all whitespace? I do not see why your should not incorrectly keep decrementing and go too far to the left, potentially overwriting unallocated memory?

    • Since there was no issue while running the same project in another PC.

    More likely than a project/compilation issue is that your code, somewhere, uses an uninitialized value or overwrites/underwrites a random area of memory. This can and will behave/show up differently from one PC to another, for no discernible reason....

  • @JonB Thank you for pointing out at the potential problems. For now, I found it to be a project/compilation issue. I changed the compiler version to Clang just like the other PC. Also I changed the debugger settings as suggested here and it seems to work.

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