Overview and Tools

• Prepare your UNIX environment
• A Brief Introduction to the UNIX environment
• Fundamental UNIX programming practices
• Tools

使用 Ubuntu Linux (Debian-based) 為例，建議最少需要以下套件

• gcc, g++, gdb, make
• manpages-dev, manpages-posix, manpages-posix-dev

$ sudo apt-get install gcc g++ gdb make manpages-dev manpages-posix manpages-posix-dev

• The interface to the kernel is a layer of software called the system calls.
• Libraries of common functions are built on top of the system call interface, but applications are free to use both.
• The shell is a special application that provides an interface for running other applications.

Booting Process

開機後依序會經過下面流程

• BIOS
• OS Loader(e.g. grub): Loads kernel and an (optional) initial RAM disk into the memory
• Kernel initializes system hardware components
• Launch the first process: /sbin/init, /etc/init, /bin/init, and finally /bin/sh
• init process
  • Mount file systems
  • Setup networks
  • Launch services
  • Provide the login interface

File System Architecture

Everything starts from the “root directory” (/)

UNIX Commands

• Builtin commands: 由 login shell 提供
• Other commands: 由 system administrators 安裝的套件
• The Linux standard base (LSB): Linux operating system 的基本工具或 libraries

Common Notations

UNIX 文件中的提示符號

• %, $ : 一般使用者輸入
• # : root 權限輸入
• [] : 可選用
• … : 允許複數參數
• -, – : 參數

Man page

$ man [section] page

1. User commands
2. System calls
3. C library functions
4. Devices and special files
5. File formats and conventions
6. Games et al.
7. Miscellaneous
8. System administration tools and daemons

#include <stdio.h>
 
int main() {
    printf("Hello, World.\n");
    return 0;
}

Return value of the main() function

• a one byte value
• zero: True
• non-zero: False

Read return values from your program

• Run echo $? immediately right after your program execution

Shell’s short cut branch

Boolean OR (||) – Evaluate until a condition is true

$ ./return 0 || echo 'A'
$ ./return 1 || echo 'B'

• 第一個前面為 0 在 shell 中是 True，整個 OR 結果可知，後面 echo 'A' 不執行
• 第二個前面為 1 在 shell 中是 False，整個 OR 結果未知，後面 echo 'B' 需執行

Boolean AND (&&) – Evaluate until a condition is not false

$ ./return 0 && echo 'C'
$ ./return 1 && echo 'D'

• 第一個前面為 0 在 shell 中是 True，整個 AND 結果未知，後面 echo 'C' 需執行
• 第二個前面為 1 在 shell 中是 False，整個 AND 結果已知，後面 echo 'D' 不執行

Handle program options

參數包含程式名稱本身

#include <unistd.h>
 
int getopt(int argc, char * const argv[], const char *optstring);
 
extern char *optarg;
extern int optind, opterr, optopt;
 
/*Return: option now if OK, -1 if no more options, Colon (:) or question mark (?) if Invalid option encountered*/
 
#include <getopt.h>
 
int getopt_long(int argc, char * const argv[],
           const char *optstring,
           const struct option *longopts, int *longindex);
 
int getopt_long_only(int argc, char * const argv[],
           const char *optstring,
           const struct option *longopts, int *longindex);
 
struct option {
    const char *name;
    int         has_arg;
    int        *flag;
    int         val;
};

• argc: 參數個數
• *argv[]: 參數
• optstring: 由字母和冒號組成，字母表示可用參數，冒號表示該字母參數後面還需要額外補充參數
• optind: 目前 getopt() 消耗的參數個數
• optarg: 目前取到的補充參數
• getopt_long(): 可取得一個 (-) 或兩個 (–) 減號的參數
• getopt_long_only(): 只取得兩個 (–) 減號的參數

UNIX time representations

• Wall clock time: time_t
  • 以秒為單位
  • 一般是 32-bit signed integer
  • 表示從 00:00:00, January 1st, 1970 UTC 至今經過的時間
  • 另外有 struct timeval，基本上是 time_t 加上微秒，較精準

struct timeval {
    long tv_sec; // seconds
    long tv_usec; // microseconds
};

• CPU time: clock_t, in CPU-ticks unit
  • 單位是 CLOCKS_PER_SEC constant
  • 回傳程式開始執行後所使用的 cpu 時間

相關函式

• time(3) function: Get time in time_t format
• gettimeofday(2) function: Get time in struct timeval format
• clock(3) function: Get time in clock_t format

#include<time.h>
 
time_t time(time_t *t);
int gettimeofday(struct timeval *tv, struct timezone *tz);
clock_t clock(void);

• gcc
• make/Makefile
• GDB

• C 語言標準函數庫分類導覽 - 時間處理 time.h