What causes a SIGSEGV

Question

I need to know the root cause of the segmentation fault  SIGSEGV   and how to handle it

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The initial source cause can also be an out of memory

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SigSegV means a signal for memory access violation  trying to read or write from to a memory area that your process does not have access to  These are not C or C   exceptions and you can   t catch signals  It   s possible indeed to write a signal handler that ignores the problem and allows continued execution of your unstable program in undefined state  but it should be obvious that this is a very bad idea   Most of the time this is because of a bug in the program  The memory address given can help debug what   s the problem  if it   s close to zero then it   s likely a null pointer dereference  if the address is something like 0xadcedfe then it   s intentional safeguard or a debug check  etc    One way of    catching    the signal is to run your stuff in a separate child process that can then abruptly terminate without taking your main process down with it  Finding the root cause and fixing it is obviously preferred over workarounds like this

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There are various causes of segmentation faults  but fundamentally  you are accessing memory incorrectly   This could be caused by dereferencing a null pointer  or by trying to modify readonly memory  or by using a pointer to somewhere that is not mapped into the memory space of your process  that probably means you are trying to use a number as a pointer  or you incremented a pointer too far    On some machines  it is possible for a misaligned access via a pointer to cause the problem too - if you have an odd address and try to read an even number of bytes from it  for example  that can generate SIGBUS  instead

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using an invalid null pointer  Overrunning the bounds of an array  Kindof hard to be specific without any sample code   Essentially  you are attempting to access memory that doesn t belong to your program  so the OS kills it

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segmentation fault arrives when you access memory which is not declared by the program  You can do this through pointers i e through memory addresses  Or this may also be due to stackoverflow for eg    void rec func    int q   5  rec func      int main    rec func       This call will keep on consuming stack memory until it s completely filled and thus finally stackoverflow happens  Note  it might not be visible in some competitive questions as it leads to timeouterror first but for those in which timeout doesn t happens its a hard time figuring out sigsemv

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Here is an example of SIGSEGV   root pierr-desktop  opt playGround  cat test c int main          int   p          p   0x1234       return 0     root pierr-desktop  opt playGround  g   -o test test c   root pierr-desktop  opt playGround    test  Segmentation fault   And here is the detail      How to handle it     Avoid it as much as possible in the first place   Program defensively  use assert    check for NULL pointer   check for buffer overflow   Use static analysis tools to examine your code   compile your code with -Werror -Wall   Has somebody review your code  When that actually happened   Examine you code carefully    Check what you have changed since the last time you code run successfully without crash   Hopefully  gdb will give you a call stack so that you know where the crash happened      EDIT   sorry for a rush  It should be  p   0x1234  instead of  p   0x1234

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Wikipedia has the answer  along with a number of other sources   A segfault basically means you did something bad with pointers  This is probably a segfault   char  c   NULL       c     dereferencing a NULL pointer   Or this   char  c    Hello       c 10     z      out of bounds  or in this case  writing into read-only memory   Or maybe this   char  c   new char 10       delete    c      c 2     z      accessing freed memory   Same basic principle in each case - you re doing something with memory that isn t yours

[segmentation-fault] What causes a SIGSEGV

Examples related to segmentation-fault