C Keywords

Abstract

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• Keywords that may change the behaviours of Compiler or program

volatile

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• Used to indicate to the Compiler that a variable’s value may change unexpectedly
• Often the case when a variable is being accessed by multiple Thread or when it represents hardware that is external to the computer
• Compiler will not optimize the code that accesses the variable. This means that the compiler will not reorder instructions or eliminate redundant reads or writes to the variable
• This can be important for ensuring that the variable’s value is always read and written correctly
• Example: In the below example, we have 2 exact same Instruction, for performance sake, Compiler will remove the second one since it doesn’t cause side effect inside the uartinit() function. However, this will cause us unable to set the Baud Rate!
• Used a lot in memory mapped io

// xv6-riscv kernel source codes, uart.c
uartinit(void)
{
  // disable interrupts.
  WriteReg(IER, 0x00); // Where IER is 1
 
  // ...
 
  // MSB for baud rate of 38.4K.
  WriteReg(1, 0x00);
 
  // ...
}

asm volatile

---

• Used to insert Assembly language directly into C code
• Below example is Turn off interrupt in XV6-RISCV

// XV6-RISCV kernel codes, riscv.h
 
// ...
static inline void 
w_sstatus(uint64 x)
{
  asm volatile("csrw sstatus, %0" : : "r" (x));
}
// ..

extern

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• Declare a variable or function that is defined in another file, allowing it to be accessed across multiple files. It informs the compiler about the existence of the variable or function without allocating storage for it

Shines in large projects

Useful for managing global scope in large projects and linking functions or variables across different source files.

static

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int func() {
  static int i;
}

• The variable i will persist between calls of func()

static int i;
 
int func() {
}

• The variable i can’t be named outside of the current file