[习题22]C语言关键词|[习题22]C语言关键词 extern [习题22]C语言关键词extern

使用教材

【[习题22]C语言关键词|[习题22]C语言关键词 extern】《“笨办法” 学C语言（Learn C The Hard Way）》
https://www.jianshu.com/p/b0631208a794

完整源码 : learn-c-the-hard-way-lectures/ex22/

https://github.com/zedshaw/learn-c-the-hard-way-lectures/tree/master/ex22

习题22

主要讨论作用域问题；

代码文件结构

文章图片
关键词 extern.png

extern关键词告诉编译器，变量是存在的，但是它位于另外一个文件；

运行代码在命令行中操作

$ cc -Wall -g -DNDEBUG -c -o ex22.o ex22.c $ cc -Wall -g -DNDEBUG ex22_main.c ex22.o -o ex22_main$ ls dbg.hex22.cex22.hex22_mainex22_main.cex22.o

输出结果

anno@anno-m:~/Documents/mycode/ex22$ ./ex22_main [INFO] (ex22_main.c:26) My name: Zed A.Shaw, age: 37 [INFO] (ex22_main.c:30) My age is now: 100 [INFO] (ex22_main.c:33) THE_SIZE is: 1000 [INFO] (ex22.c:31) I think size is : 1000 [INFO] (ex22_main.c:38) THE SIZE if now : 9 [INFO] (ex22.c:31) I think size is : 9 [INFO] (ex22_main.c:42) Ratio at first: 1.000000 [INFO] (ex22_main.c:43) Ratio again: 2.000000 [INFO] (ex22_main.c:44) Ratio once more: 10.000000 [INFO] (ex22_main.c:8) Count is: 4 [INFO] (ex22_main.c:16) count is at exit: 4 [INFO] (ex22_main.c:20) count after assign: 3000 [INFO] (ex22_main.c:8) Count is: 80 [INFO] (ex22_main.c:13) count in this scope is 100 [INFO] (ex22_main.c:16) count is at exit: 80 [INFO] (ex22_main.c:20) count after assign: 3000 [INFO] (ex22_main.c:51) count after calling scope_demo: 4

完整源码 ex22.h

#ifndef _ex22_h #define _ex22_h// make THE_SIZE in ex22.c avaiable to other .c files extern int THE_SIZE; // gets and sets an internal static variable in ex22.c int get_age(); void set_age(int age); // updates a static variable that's inside update_ratio double update_ratio(double ratio); void print_size(); #endif

ex22.c

#include #include "ex22.h" #include "dbg.h"int THE_SIZE = 1000; static int THE_AGE = 37; int get_age() { return THE_AGE; }void set_age(int age) { THE_AGE = age; }double update_ratio(double new_ratio) { static double ratio = 1.0; double old_ratio = ratio; ratio = new_ratio; return old_ratio; }void print_size() { log_info("I think size is : %d", THE_SIZE); }

ex22_main.c

#include "ex22.h" #include "dbg.h"const char *MY_NAME ="Zed A.Shaw"; void scope_demo(int count) { log_info("Count is: %d", count); if(count > 10) { int count = 100; // BAD! BUGS!log_info("count in this scope is %d", count); }log_info("count is at exit: %d", count); count = 3000; log_info("count after assign: %d", count); }int main(int agrc, char *argv[]) { // test out THE_AGE accessors log_info("My name: %s, age: %d", MY_NAME, get_age()); set_age(100); log_info("My age is now: %d", get_age()); // test out THE_SIZE extern log_info("THE_SIZE is: %d", THE_SIZE); print_size(); THE_SIZE = 9; log_info("THE SIZE if now : %d", THE_SIZE); print_size(); // test the ration function staitc log_info("Ratio at first: %f", update_ratio(2.0)); log_info("Ratio again: %f", update_ratio(10.0)); log_info("Ratio once more: %f", update_ratio(300.0)); // test the scope demo int count = 4; scope_demo(count); scope_demo(count *20); log_info("count after calling scope_demo: %d", count); return 0; }

[附加任务]编写 Makefile Makefile

CC=cc CFLAGS=-Wall -g -DNDEBUGex22_main : ex22.oex22_main.c $(CC) $(CFLAGS) ex22_main.c ex22.o -o ex22_mainex22.o : ex22.c $(CC) $(CFLAGS) -c -o ex22.o ex22.cclean: rm ex22_main ex22.o

最新的文件应该是，可执行文件ex22_main
ex22_main : ex22.o ex22_main.c 意味着：可执行文件ex22_main要生成，需要（即依赖）文件ex22.o以及文件ex22_main.c

命令行运行

$ make ex22_main gcc -Wall -g -DNDEBUG -c -o ex22.o ex22.c gcc -Wall -g -DNDEBUG ex22_main.c ex22.o -o ex22_main$ ls dbg.hex22.cex22.hex22_mainex22_main.cex22.oMakefileMakefile~$ ./ex22_main