# Study - 你所不知道的C語言

###### tags: `C語言`

### 1. [指標篇](https://hackmd.io/@sysprog/c-pointer)
- &a => address of a
- *ptr => dereference ptr等於value of ptr
- [ ] array[ ] => it's an array subscripting
- In C11 chap 6.5.2.1, E1[E2] is identical to (*((E1)+(E2))).

- sizeof()是一個operator
- Incomplete type
- Ex. char x[];
- 沒辦法建立instance(object)；但可以建立a pointer to incomplete type
- [ ] void 算是incomplete type嗎?
- Ex. void *ptr => ptr is a pointer to a void

Types are partitioned into 
object types (types that describe objects) and 
function types (types that describe functions). 
At various points within a translation unit an object type may be 
incomplete (lacking sufficient information to determine the size of objects of that type) or 
complete (having sufficient information)

- [ ] Lvalue (Locator value)?

      *(int32_t * const) (0x67a9) = 0xaa6;
      /* Lvalue */

- function call傳入的參數是個複本，input argument只存在function內
- Ex. void func(int *p) { p = &B; }
v.s. void func(int **p) { *p = &B; }
- 所以若會對input argument做assign動作，就會需要 pointer to pointer
- Linked List Remove head
- 若不用使用pointer to pointer，解法:
- Input p_head; Return Updated p_head
- 若使用pointer to pointer，解法
- Input pp_head; Return don't care
- 可以達到ADT(Abstract Data Type)，不用aware
- [ ] 是否有***這種用法呢?
- [ ] Forward declaration?
- Array v.s. Pointer
- char x[10] => 不能變更為 pointer 的形式，因為pointer是描述一個start address，但沒有指定size。
- [ ] 為什麼GDB可以修改記憶體?

    (gdb) p (&b[0])->v = {1, 2, 3}
    $12 = {1, 2, 3}
    (gdb) p b[0]
    $13 = {
      v = {1, 2, 3},
      length = 0
    }

- [x] LLP64 data model
- ==L==ong==L==ong
- ==P==ointer
- [ ] 為什麼不用帶入argc就可以執行?
- 
- [ ] C 語言規格中定義 string literals 會被分配於 “static storage” 當中 (C99 [6.4.5])，並說明如果程式嘗試修改 string literals 的內容，將會造成未定義行為
- 以 gcc 的 ELF target 來說，將 string literals 分配在 read-only data section 中 (當然包含 \0 結尾)
- [ ] branch predictor?

### 2. [linked list 和非連續記憶體](https://hackmd.io/@sysprog/c-linked-list)

- linked list適用在分散式系統(資料龐大, RTOS排成器...)
- 改linked list的p_head -> 這是個常數時間
- [ ] Locality of reference (cache 用語)?
- [ ] Merge Sort
- [ ] [cache line](https://hackmd.io/@sysprog/HkW3Dr1Rb?type=view) 介紹

### 3. [函式呼叫](https://hackmd.io/@sysprog/c-function)
- 在 C 語言中，“function” 其實是特化的形式，並非數學意義上的函數，而隱含一個狀態到另一個狀態的關聯，因此，我們將一般的 C function 翻譯為「函式」，以區別數學意義上的函數 (如 abs, cos, exp)
- ABI(application binary interface)
- [ ] Calling convention
- [ ] Forward declaration
- forward declaration is a declaration of an identifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a complete definition.
- C preprocessor 以獨立程式的形式存在
> As the C preprocessor can be invoked separately from the compiler with which it is supplied, it can be used separately, on different languages.
- [const pointer](https://stackoverflow.com/questions/8808167/c-const-correctness-and-pointer-arguments)

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### Reference link:
- [C99 standard](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf)
- [C11 standard](https://www.open-std.org/jtc1/sc22/WG14/www/docs/n1570.pdf)
- [GNU debugger GDB](http://sourceware.org/gdb/current/onlinedocs/gdb/)
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