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# Pointer # Pointer
In a very simple sense, a pointer represents the memory address of a certain block of data (also can be a single byte) in memory. In a very simple sense, a pointer represents the memory address of a certain datum of a certain size in memory.
``` ```
Here, the pointer `str` has the value of 0xdecade, therefore pointing to the memory at Here, the pointer `str` has the value of 0xdecade, therefore pointing to the memory at
@ -27,7 +27,7 @@ char c4 = str[4]; /* array indexing is syntax sugar for *(arr + index) in C
str[0] = 'h'; /* equivalent in C */ str[0] = 'h'; /* equivalent in C */
``` ```
In C (and almost all languages that have pointers), a pointer is an integer type, big enough to hold the entire valid In [C](c.md) (and almost all languages that have pointers), a pointer is an integer type, big enough to hold the entire valid
address space of the host system. address space of the host system.
```c ```c
@ -50,10 +50,12 @@ while (s < send) {
``` ```
## Null pointer ## Null pointer
A null pointer is a special case, which points to an invalid memory address (nowadays almost 0). A null pointer is a special case, which points to an invalid memory address (nowadays almost always 0).
Usually a null pointer is meaningful only in hosted environments, in that case reading or writing from Usually a null pointer is meaningful only in hosted environments, in that case reading or writing from
a null pointer will case a segmentation fault error, in which the OS will immediately kill the offending a null pointer will case a segmentation fault error, in which the OS will immediately kill the offending
process. On embedded systems (without a MMU), a null pointer may represent a valid memory address. process. On embedded systems (without a MMU), a null pointer may represent a valid memory address. On C
the expression `(void *)0` is guaranteed to always produce a null pointer, even if the actual null pointer
address is not zero.
## Function pointers ## Function pointers
Function pointers are more special. On most systems, just like a regular pointer, a function pointer is simply Function pointers are more special. On most systems, just like a regular pointer, a function pointer is simply