Help me understand this statement:
static int ** volatile ptr
Please let me know how to analyze this pointer statement.
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4Which one `static int static int ** volatile ptr` or `static int ** volatile ptr`? – Kiril Kirov Nov 22 '12 at 12:40
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2Welcome to SO. It is not clear what you actually want to know. There are a lot of resources out there that would explain you `volatile` or pointers in C. – Jens Gustedt Nov 22 '12 at 12:41
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1I think you have the wrong expression in the question's body; it doesn't match the title and is not valid. – unwind Nov 22 '12 at 12:41
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@KirilKirov I edited the question.I want to know about static int ** volatile ptr – Kuldeep Singh Nov 22 '12 at 12:46
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@JensGustedt Thanks.I know about volatile and static separately but what this combination is doing? – Kuldeep Singh Nov 22 '12 at 12:50
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possible duplicate of [How do you read C declarations?](http://stackoverflow.com/questions/89056/how-do-you-read-c-declarations) – AProgrammer Nov 22 '12 at 13:18
2 Answers
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static means that this is static variable - only one copy in source code for local variables in function, not shared between modules for global.
int **ptr means that ptr is pointer to a pointer to int.
volatile on the right side of * means that the pointer itself is volatile and not the pointed value. volatile means that all accesses to this variable must be actual memory operations and most optimizations are not allowed.
zch
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I know this thing but what is special about this combination of static and volatile. – Kuldeep Singh Nov 22 '12 at 13:00
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To address the title text after inserting markers into it: 4(static int 3( * 2( * 1(volatile ptr))). Working inside out:
1: Variable that is free to change at run time for reasons the program may not be aware of. Popular causes are other threads, signals or other events, and pointers to "memory" that is really hardware controllers, etc. Stops compilers/optimizers from emitting code that can fail under such changes.
2: Volatile pointer to this memory. What is pointed to is not volatile.
3: pointer to pointer, or array of pointers. Think char **argv; Again, not volatile.
4: static: this depends on where in the declaration is found:
- outside any function: the ptr value only has compile scope.
- inside a function: the memory will be shared by all calls and/or threads to the function. Allocated just once at compile time.
Interesting: const volatile indicates external influences can change the pointer but your software can not do so.
Gilbert
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