What are the common mistakes that lead to segmentation faults? What should errno be in mkdtemp function? The mkdtemp function returns a pointer to the modified template string on success, and NULL on failure, in which case errno is set to indicate the error. Now template is unchanged.
Also see mkdir 2 for other possible values for errno. What are the last six characters of template in mkdtemp? The last six characters of template must be XXXXXX and these are replaced with a string that makes the directory name unique. The directory is then created with permissions Since it will be modified, template must not be a string constant, but should be declared as a character array. A segmentation fault occurs mainly when our code tries to access some memory location which it is not suppose to access.
For example : Working on a dangling pointer. Writing past the allocated area on heap. Operating on an array without boundary checks. Freeing a memory twice. Working on Returned address of a local variable Running out of memory stack or heap Examples of Segmentation Fault in C 1 Working on a dangling pointer. Well, before discussing this scenario of segmentation fault, lets understand what is dangling pointers. A pointer which holds memory address of a memory which is already freed is known as a dangling pointer.
You cannot figure out whether a given pointer is dangling or not until you use it. When a dangling pointer is used, usually a segmentation fault is observed.
Now, lets look at a code to understand it : Code:. SHARE 1. Nice article! A fifth way of causing a segfault is a recursive function that uses all of the stack space.
On some systems, this will cause a "stack overflow" report, and on others, it will merely appear as another type of segmentation fault. The strategy for debugging all of these problems is the same: load the core file into GDB, do a backtrace, move into the scope of your code, and list the lines of code that caused the segmentation fault. The core file contains all the information needed by GDB to reconstruct the state of execution when the invalid operation caused a segmentation fault.
Program terminated with signal 11, Segmentation fault. Some information about loading symbols 0 0xc in foo at t. This is a goldmine of information: we already know exactly where the problem happened and which pointer was involved.
The pointer x is initialized to 0, equivalent to NULL in fact, NULL is a stand-in for 0 , and we know that it's a no-no to then try to access that pointer. But what if it weren't so obvious? Simply printing the value of the pointer can often lead to the solution. The address 0x0 is invalid -- in fact, it's NULL.
If you dereference a pointer that stores the location 0x0 then you'll definitely get a segmentation fault, just as we did. If we'd gotten something more complicated, such as execution crashing inside a system call or library function perhaps because we passed an uninitialized pointer to fgets , we'd need to figure out where we called the library function and what might have happened to cause a segfault within it.
Does this mean the library function did something wrong? It means that we probably passed a bad value to the function. To debug this, we need to see what we passed into strcat. So let's see what function call we made that led to the segfault. Each function is directly above the function that called it. So foo was called by main in this case. The numbers on the side 0, 1, 2 also indicate the order of calls, from most recent to longest ago.
To move from viewing the state within each function encapsulated in the idea of a stack frame , we can use the up and down commands.
Right now, we know we're in the strcat stack frame, which contains all of the local variables of strcat, because it's the top function on the stack. We want to move "up" toward the higher numbers ; this is the opposite of how the stack is printed. We should probably lookup the strcat function at this point to make sure that we got the order of arguments correct. Since we did, the problem must be with x.
The strcat function must be derefencing a NULL pointer that we gave it, and even though it's a library function, it doesn't do anything magical.
NULL pointers are generally pretty easy to work with -- once we've found one, we know that somewhere along the line, we didn't allocate some memory that we should have. It's just a question of where. A common mistake is to not check the return from malloc to make sure that the system isn't out of memory.
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