What is a segmentation fault

Question

What is a segmentation fault  Is it different in C and C    How are segmentation faults and dangling pointers related

User · Answer

Segmentation fault  means that you tried to access memory that you do not have access to   The first problem is with your arguments of main  The main function should be int main int argc  char  argv     and you should check that argc is at least 2 before accessing argv 1    Also  since you re passing in a float to printf  which  by the way  gets converted to a double when passing to printf   you should use the  f format specifier  The  s format specifier is for strings    0 -terminated character arrays

User · Answer

Wikipedia s Segmentation fault page has a very nice description about it  just pointing out the causes and reasons  Have a look into the wiki for a detailed description   In computing  a segmentation fault  often shortened to segfault  or access violation is a fault raised by hardware with memory protection  notifying an operating system  OS  about a memory access violation   The following are some typical causes of a segmentation fault    Dereferencing NULL pointers     this is special-cased by memory management hardware Attempting to access a nonexistent memory address  outside process s address space  Attempting to access memory the program does not have rights to  such as kernel structures in process context  Attempting to write read-only memory  such as code segment    These in turn are often caused by programming errors that result in invalid memory access    Dereferencing or assigning to an uninitialized pointer  wild pointer  which points to a random memory address  Dereferencing or assigning to a freed pointer  dangling pointer  which points to memory that has been freed deallocated deleted  A buffer overflow  A stack overflow  Attempting to execute a program that does not compile correctly   Some compilers will output an executable file despite the presence of compile-time errors

User · Answer

To be honest  as other posters have mentioned  Wikipedia has a very good article on this so have a look there  This type of error is very common and often called other things such as Access Violation or General Protection Fault   They are no different in C  C   or any other language that allows pointers  These kinds of errors are usually caused by pointers that are   Used before being properly initialised Used after the memory they point to has been realloced or deleted  Used in an indexed array where the index is outside of the array bounds  This is generally only when you re doing pointer math on traditional arrays or c-strings  not STL   Boost based collections  in C

User · Answer

Segmentation fault is also caused by hardware failures  in this case the RAM memories  This is the less common cause  but if you don t find an error in your code  maybe a memtest could help you   The solution in this case  change the RAM   edit   Here there is a reference  Segmentation fault by hardware

User · Answer

Segmentation fault is a specific kind of error caused by accessing memory that    does not belong to you     It   s a helper mechanism that keeps you from corrupting the memory and introducing hard-to-debug memory bugs  Whenever you get a segfault you know you are doing something wrong with memory     accessing a variable that has already been freed  writing to a read-only portion of the memory  etc  Segmentation fault is essentially the same in most languages that let you mess with memory management  there is no principal difference between segfaults in C and C    There are many ways to get a segfault  at least in the lower-level languages such as C      A  common way to get a segfault is to dereference a null pointer  int  p   NULL   p   1   Another segfault happens when you try to write to a portion of memory that was marked as read-only  char  str    quot Foo quot      Compiler marks the constant string as read-only  str    b      Which means this is illegal and results in a segfault  Dangling pointer points to a thing that does not exist anymore  like here  char  p   NULL        char c      p    amp c       Now p is dangling  The pointer p dangles because it points to the character variable c that ceased to exist after the block ended  And when you try to dereference dangling pointer  like  p  A    you would probably get a segfault

User · Answer

There are enough definitions of segmentation fault  i would like to quote few examples which i came across while programming  which might seem silly mistakes  but will waste a lot of time    you can get segmentation fault in below case while argumet type mismatch in printf   include lt stdio h gt  int main      int a   5    printf   s  a     return 0      output   Segmentation Fault  SIGSEGV    when you forgot to allocate memory to a pointer  but trying to use it     include lt stdio h gt    typedef struct     int a    myStruct     int main      myStruct  s       few lines of code      s- gt a   5    return 0       output   Segmentation Fault  SIGSEGV

User · Answer

In simple words  segmentation fault is the operating system sending a signal to the program saying that it has detected an illegal memory access and is prematurely terminating the program to prevent memory from being corrupted

User · Answer

Simple meaning of Segmentation fault is that you are trying to access some memory which doesn t belong to you  Segmentation fault occurs when we attempt to read and or write tasks in a read only memory location or try to freed memory  In other words  we can explain this as some sort of memory corruption    Below I mention common mistakes done by programmers that lead to Segmentation fault    Use scanf   in wrong way forgot to put  amp      int num  scanf   d   num     must use  amp num instead of num    Use pointers in wrong way    int  num   printf   d   num      num should be correct as num only   Unless You can use  num but you have to point this pointer to valid memory address before accessing it     Modifying a string literal pointer try to write or modify a read only memory     char  str       Stored in read only part of data segment str    GfG            Problem   trying to modify read only memory   str 1     n      Try to reach through an address which is already freed       allocating memory to num  int  num   malloc 8     num   100       de-allocated the space allocated to num  free num        num is already freed there for it cause segmentation fault  num   110      Stack Overflow -  Running out of memory on the stack Accessing an array out of bounds  Use wrong format specifiers when using printf   and scanf

User · Answer

A segmentation fault is caused by a request for a page that the process does not have listed in its descriptor table  or an invalid request for a page that it does have listed  e g  a write request on a read-only page    A dangling pointer is a pointer that may or may not point to a valid page  but does point to an  unexpected  segment of memory

User · Answer

Segmentation fault occurs when a process  running instance of a program  is trying to access read-only memory address or memory range which is being used by other process or access the non-existent  invalid  memory address  Dangling Reference  pointer  problem means that trying to access an object or variable whose contents have already been deleted from memory  e g   int  arr   new int 20   delete arr  cout lt  lt arr 1      dangling problem occurs here

User · Answer

Consider the following snippets of Code  SNIPPET 1 int  number   NULL   number   1   SNIPPET 2 int  number   malloc sizeof int     number   1   I d assume you know the meaning of the functions  malloc   and sizeof   if you are asking this question  Now that that is settled  SNIPPET 1 would throw a Segmentation Fault Error  while SNIPPET 2 would not  Here s why  The first line of snippet one is creating a variable  number  to store the address of some other variable but in this case it is initialized to NULL  on the other hand  The second line of snippet two is creating the same variable  number  to store the address of some other and in this case it is given a memory address because malloc   is a function in C C   that returns a memory address of the computer  The point is you cannot put water inside a bowl that has not been bought OR a bowl that has been bought but has not been authorized for use by you  When you try to do that  the computer is alerted and it throws a SegFault error  You should only face this errors with languages that are close to low-level like C C    There is an abstraction in other High Level Languages that ensure you do not make this error  It is also paramount to understand that Segmentation Fault is not language-specific

User · Answer

There are several good explanations of  Segmentation fault  in the answers  but since with segmentation fault often there s a dump of the memory content  I wanted to share where the relationship between the  core dumped  part in Segmentation fault  core dumped  and memory comes from      From about 1955 to 1975 - before semiconductor memory - the dominant technology in computer memory used tiny magnetic doughnuts strung on copper wires  The doughnuts were known as  ferrite cores  and main memory thus known as  core memory  or  core     Taken from here

User · Answer

It would be worth noting that segmentation fault isn t caused by directly accessing another process memory  this is what I m hearing sometimes   as it is simply not possible  With virtual memory every process has its own virtual address space and there is no way to access another one using any value of pointer  Exception to this can be shared libraries which are same physical address space mapped to  possibly  different virtual addresses and kernel memory which is even mapped in the same way in every process  to avoid TLB flushing on syscall  I think   And things like shmat    - these are what I count as  indirect  access  One can  however  check that they are usually located long way from process code and we are usually able to access them  this is why they are there  nevertheless accessing them in a improper way will produce segmentation fault    Still  segmentation fault can occur in case of accessing our own  process  memory in improper way  for instance trying to write to non-writable space   But the most common reason for it is the access to the part of the virtual address space that is not mapped to physical one at all   And all of this with respect to virtual memory systems

User · Answer

According to Wikipedia   A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access  or attempts to access a memory location in a way that is not allowed  for example  attempting to write to a read-only location  or to overwrite part of the operating system

User · Answer

A segmentation fault or access violation occurs when a program attempts to access a memory location that is not exist  or attempts to access a memory location in a way that is not allowed        Array out of bounds  error     valid indices for array foo    are 0  1      999       int foo 1000      for  int i   0  i  lt   1000   i        foo i    i    Here i 1000  not exist  so segfault occurs   Causes of segmentation fault   it arise primarily due to errors in use of pointers for virtual memory addressing  particularly illegal access   De-referencing NULL pointers     this is special-cased by memory management hardware   Attempting to access a nonexistent memory address  outside process   s address space    Attempting to access memory the program does not have rights to  such as kernel structures in process context    Attempting to write read-only memory  such as code segment

[c++] What is a segmentation fault?

Examples related to c++

Examples related to c

Examples related to segmentation-fault