The Linux Programming Interface - Chapter 3 System Programming Concepts

Chapter 3 System Programming Concepts

• 3.1 System Calls

  • [Concept] A system call is a controlled entry point into the kernel, allowing a process to request that the kernel perform some action on the process’s behalf.
  • [How] How a system call works?
    • (1) [Kernel Model] it changes the processor state from user model to kernel model. So CPU can access protected kernel memory.
    • (2) [a unique #] The set of system calls is fixed. Each system call is identified by a unique number.
    • (3) [arguments] Each system call may have a set of arguments that specify information to be transferred from user space
  • [Steps] The steps of execution of a system call.
    • (1) Invoking a wrapper function in the C library.
    • (2) The wrapper func should pass all the system call arguments into the sys call trap-handling rountine. via the statck & copy the args to registers.
    • (3) The wrapper func copies the sys call # into a specific CPU register.
    • (4) The wrapper executes a trap machine instruction (int 0x80). switch to kernel model
      • [up-to-date] more recent x86-32 sys introduces the sysenter instruction. it is more faster than 0x80 trap instruction.(2.6 kernel & from glibc2.3.2 onward)
    • (5) The kernel invokes its system_call(). (arch/i386/entry.S)
      • (5.1) save register value onto the kernel stack.
      • (5.2) check the validity of the system call #
      • (5.3) invokes appropriate sys call found by sys call # in sys_call_table. check validity of arguments if any. return value
      • (5.4) restore register values from the kernel stack and places the sys call return value on the stack.
      • (5.5) return to the wrapper func, and return the processor to user model.
      • (5.6) if the returning value is an error. the wrapper sets global variable “errno” using this value. return to caller. return an integer indicating success or failure. (non-negative success) (-1 failure)

• 3.2 Library Functions

  • [Concept] A library function constitutes the standard C library
    • e.g. opening a file, converting a time to a human-readable format, and comparing two character strings.

• 3.3 The Standard C Library & The GNU C Library ( glibc )

  • 1.[Many] There are different implementations of the standard C library on the various UNIX implementations.
  • 2.[Most Common] glibc: The most commonly used implementation on Linux is the GNU C library
  • 3.[Version] running the glibc shared library file as an executable program
    • [note] the GNU C library resides at other pathname in diff Linux destribution.
    • [solution]
      • (1) run “ldd” to inspect the resulting library dependency list to find the location of the glibc shared library
      • (2) run the dependency link
  • 4.[TWO Constants] glibc defines two constants, __GLIBC__ and __GLIBC_MINOR__
    • [Example] glibc 2.12: __GLIBC__ (2) & __GLIBC_MINOR__ (12)

  • 5.[Reliable run time func] gnu_get_libc_version()

                  ========================3.Version======================
                  $ /lib/libc.so.6
                  GNU C Library stable release version 2.10.1, by Roland McGrath et al.
                  Copyright (C) 2009 Free Software Foundation, Inc.
                  This is free software; see the source for copying conditions.
                  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
                  PARTICULAR PURPOSE.
                  Compiled by GNU CC version 4.4.0 20090506 (Red Hat 4.4.0-4).
                  Compiled on a Linux >>2.6.18-128.4.1.el5<< system on 2009-08-19.
                  Available extensions:
                  The C stubs add-on version 2.1.2.
                  crypt add-on version 2.1 by Michael Glad and others
                  GNU Libidn by Simon Josefsson
                  Native POSIX Threads Library by Ulrich Drepper et al
                  BIND-8.2.3-T5B
                  RT using linux kernel aio
                  For bug reporting instructions, please see:
                  <http://www.gnu.org/software/libc/bugs.html>.
                  ---------------------3.solution "ldd"---------------------
                  $ ldd ~/Tools/pycharm-5.0.4/bin/libyjpagent-linux.so | grep libc
                  > libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xf720c000)
                  $ /lib/i386-linux-gnu/libc.so.6 
                  ========================5.Version======================
                  #include <gnu/libc-version.h>
                  const char *gnu_get_libc_version(void);
                  //Returns pointer to null-terminated, statically allocated string containing GNU C library version number
    

• To summarize: