C/C++ function definitions without assembly

by

First, you have to understand the concept of rings.
A kernel runs in ring 0, meaning it has a full access to memory and opcodes.
A program runs usually in ring 3. It has a limited access to memory, and cannot use all the opcodes.

So when a software need more privileges (for opening a file, writing to a file, allocating memory, etc), it needs to asks the kernel.
This can be done in many ways. Software interrupts, SYSENTER, etc.

Let’s take the example of software interrupts, with the printf() function:
1 – Your software calls printf().
2 – printf() processes your string, and args, and then needs to execute a kernel function, as writing to a file can’t be done in ring 3.
3 – printf() generates a software interrupt, placing in a register the number of a kernel function (in that case, the write() function).
4 – The software execution is interrupted, and the instruction pointer moves to the kernel code. So we are now in ring 0, in a kernel function.
5 – The kernel process the request, writing to the file (stdout is a file descriptor).
6 – When done, the kernel returns to the software’s code, using the iret instruction.
7 – The software’s code continues.

So functions of the C standard library can be implemented in C. All it has to do is to know how to call the kernel when it need more privileges.

More Related Contents:

  1. Using base pointer register in C++ inline asm
  2. Labels in GCC inline assembly
  3. Referencing memory operands in .intel_syntax GNU C inline assembly
  4. Convert inline assembly code to C++
  5. Address of labels (MSVC)
  6. Direct C function call using GCC’s inline assembly
  7. computer science
  8. Windows LoadCursor – Does it return the same HCURSOR value each time?
  9. How to enable C++17 compiling in Visual Studio?
  10. How do I sort a vector of pairs based on the second element of the pair?
  11. Is main() really start of a C++ program?
  12. Lock-free Progress Guarantees in a circular buffer queue
  13. What is the ‘this’ pointer?
  14. multiple definition in header file
  15. How do you find what version of libstdc++ library is installed on your linux machine?
  16. What are the pitfalls of ADL?
  17. Get the status of a std::future
  18. How to link using GCC without -l nor hardcoding path for a library that does not follow the libNAME.so naming convention?
  19. C++ console keyboard events
  20. Why does C++ allow variable length arrays that aren’t dynamically allocated?
  21. Why does a push_back on an std::list change a reverse iterator initialized with rbegin?
  22. How can std::unique_ptr have no size overhead?
  23. constexpr initialization of array to sort contents
  24. Comparison of arrays in google test?
  25. Templates: template function not playing well with class’s template member function [duplicate]
  26. Is opencv matrix data guaranteed to be continuous?
  27. How does the friend keyword (Class/Function) break encapsulation in C++?
  28. C++ cout and cin buffers, and buffers in general
  29. C++ Passing std::string by reference to function in dll
  30. Changing C++ output without changing the main() function [closed]

Leave a Comment