linux getopt函數詳細分析

標準庫函數：getopt  

標頭檔 #include <unistd.h>

int getopt(int argc, char * const argv[],const char *optstring);

解釋：take the argc and argv as passed to main function (argc和argv參數與int main(int argc,char * argv[])相同)

and an options specifier string that tells

getopt函數 what options are defined for the program and whether they have associated values.

the optstring is simply a list of characters,each representing a single character option.

if a character is followed by a colon(冒號),it indicatess that the option has an associated value that

will be taken as the next argument.

舉例：getopt(argc,argv,"if:lr");

it allows for simple options -i,-l,-r    and -f（後緊跟一個filename參數）

假設執行程式名為test

./test -lrf:i

則argc=2  argv[0]="./test"  argv[1]="-lrf:i"  (已驗證)

 

#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <fcntl.h>
int main(int argc,char*argv[])

{
int opt;
while((opt=getopt(argc,argv,"if:lr")) != -1)
{

switch(opt)
{
case 'i':
              printf("i: %d\n",optind);
              printf("option: %c\n",opt);
              break;
case 'l':
               printf("l: %d\n",optind);
               printf("option: %c\n",opt);
               break;
case 'r':
               printf("r: %d\n",optind);
               printf("option: %c\n",opt);
               break;
case 'f':
                printf("f: %d\n",optind);
                printf("filename: %s\n",optarg);
                break;

}

}

printf("final optind： %d",optind);

for(;optind<argc;++optind)
 printf("argument: %s\n",argv[optind]);

exit(0);

}

假設執行程式名為aaa

./aaa -i -lr 'hello world' -f fred.c
輸出如下：

i: 2
option: i
l: 2
option: l
r: 3
option: r
f: 6
filename: fred.c

5

argument: hello world
分析：

man 3 getopt 擷取協助

The variable optind is the index of the next element to be processed in
       argv.  The system initializes this value to 1.  The caller can reset it
       to 1 to restart scanning of the same argv, or when scanning a new argu‐
       ment vector.

If  getopt() finds another option character, it returns that character,
       updating the external variable optind and a static variable nextchar so
       that  the  next call to getopt() can resume the scan with the following
       option character or argv-element.

 If there are no more option  characters,  getopt()  returns  -1.   Then
       optind  is  the  index in argv of the first argv-element that is not an
       option.

意思是：變數optind（初始值為1）是argv的索引，代表下個元素（待處理）。

如果getopt找到可選項字元，則getopt返回這個字元並更新（註：不一定add）

optind。

如果不再有可選項字元，則getopt返回-1。

optind就是第一個argv元素（不是可選項）的索引（這句話很重要 說明最後optind會

被修改）。

重要：By default, getopt() permutes（排列） the contents of argv as it scans, so that
       eventually all the nonoptions are at the end.

分析上面的例子：

argv指向{"./aaa","-i","-lr","hello world","-f","fred.c"}

i=2 意思是下一個待處理的元素在argv中的索引是2

l=2意思同i，因為l的下一個待處理元素是r，而r的索引是2

r=3 即r之後的元素在argv的索引是3

f=6 因為f後跟的fred.c的索引是5，fred.c是f的參數，所以f=6

由於f之後不再有可選項，所以getopt返回-1，而optind則是hello world在argv中的索引（註：

非可選項已被調整到可選項後面 所以其索引為5 即 ./aaa -i -lr -f fred.c 'hello world'）