Linux下getopt函数的使用

getopt为解析命令行参数函数，它是Linux C库函数。使用此函数需要包含系统头文件unistd.h。

getopt函数声明如下：

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

其中函数的argc和argv参数通常直接从main的参数直接传递而来。optstring是一个包含合法选项字符的字符串。如果字符后跟一个冒号”:”，则该选项要求有参数。在argv中以“-“开头的都视作选项，“-“后的字符是选项字符。getopt会返回在argv中的选项字符，此字符会对应optstring中的字符。如果getopt被重复调用，它返回下一个选项字符。如果不再有可识别的选项字符，将返回-1。

如果getopt找到另一个选项字符，则返回该字符，更新外部变量optind和静态变量nextchar,以便下次调用getopt可以重复使用接下来的选项字符。如果成功找到选项，getopt返回该选项字符。如果所有命令行选项已被解析，getopt返回-1.如果getopt遇到一个选项字符不在optstring中，那么将返回"?".

如果getopt遇到一个缺少参数的选项，则返回值取决于optstring中的第一个字符，如果是":"，则返回":"，否则返回"?".

默认情况下，getopt会调换argv中的内容，将非选项放在最后。这样当getopts读取完所有的选项以后，optind会指向非选项的参数。

在处理选项列表时，getopt可以检测两种错误：(1).一个选项字符在optstring中并没有指定；(2).缺少选项参数。默认情况下，getopt在标准错误上输出错误信息，将错误的选项字符放在optopt中，并返回"?"作为函数结果。如果调用者将全局变量opterr设置为0，那么getopt不会输出错误信息(默认情况下，opterr是一个非零值)。如果optstring中的第一个字符是冒号":",那时getopt同样不会打印错误信息。另外，getopt将返回":"代替返回"?"以表示缺少选项参数。

getopt()所设置的全局变量包括：

(1). char *optarg：当前选项的参数。

(2). int optind: 是在argv中要处理的下一个元素的索引。系统初始化此值为1.调用者可以将其重置为1以重新开始扫描相同的argv，或扫描一个新的参数向量。每次调用getopt时，optind保存下个参数在argv中的索引(index)。如果找到一个选项，getopt会返回找到的选项字符，更新optind。如果选项有参数，将参数存到optarg，否则optarg为0。

(3). int opterr: 这个变量为非零时，getopt为”无效选项”或”缺少参数选项”输出错误信息。

(4). int optopt: 当发现无效选项字符时，getopt或返回'?'字符，或返回':'字符，并且optopt包含了所发现的无效选项字符。

getopt定义分为三种：

(1). 不带参数的选项。

(2). 必须带参数的选项：在选项后加一个冒号。

(3). 可选参数的选项：在选项后加两个冒号。

注意事项：

(1). 不带参数的选项可用连写。

(2). 选项不分先后顺序。

(3). 可选参数的选项与参数之间不能有空格。

下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：

CMakeLists.txt文件内容如下：

PROJECT(samples_cplusplus)
CMAKE_MINIMUM_REQUIRED(VERSION 3.0)# 支持C++11
SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g -Wall -O2 -std=c11")
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}  -g -Wall -O2 -std=c++11")INCLUDE_DIRECTORIES(${PROJECT_SOURCE_DIR})FILE(GLOB samples ${PROJECT_SOURCE_DIR}/*.cpp)FOREACH (sample ${samples})STRING(REGEX MATCH "[^/]+$" sample_file ${sample})STRING(REPLACE ".cpp" "" sample_basename ${sample_file})ADD_EXECUTABLE(test_${sample_basename} ${sample})TARGET_LINK_LIBRARIES(test_${sample_basename} pthread)
ENDFOREACH()

sample_getopt.cpp内容如下：

#include <iostream>
#include <unistd.h>namespace {void test1(int argc, char* argv[])
{// reference: http://man7.org/linux/man-pages/man3/getopt.3.htmlint flags = 0, opt = -1, nsecs = 0, tfnd = 0;while ((opt = getopt(argc, argv, "nt:")) != -1) {switch (opt) {case 'n':flags =1;break;case 't':nsecs = atoi(optarg);tfnd = 1;break;default:fprintf(stderr, "Usage: %s [-t nsecs] [-n] name\n", argv[0]);exit(EXIT_FAILURE);}}fprintf(stdout, "flags = %d; tfnd = %d; nsec = %d; optind = %d\n", flags, tfnd, nsecs, optind);if (optind >= argc) {fprintf(stderr, "Expected argument after options\n");exit(EXIT_FAILURE);}fprintf(stdout, "name argument = %s\n", argv[optind]);exit(EXIT_SUCCESS);
}int test2()
{// reference: https://stackoverflow.com/questions/10502516/how-to-call-correctly-getopt-functionconst char* argv[] = {"ProgramNameHere", "-f", "input.gmn", "-output.jpg"};int argc = sizeof(argv) / sizeof(argv[0]);std::cout<<"argc: "<<argc<<std::endl;for (int i = 0; i < argc; ++i) {//std::cout<<"argv: "<<argv[i]<<std::endl;}int c = -1;while ((c = getopt(argc, (char**)argv, "f:s:o:pw:h:z:t:d:a:b:?")) != -1) {std::cout<<"Option: "<<(char)c;if (optarg) {std::cout<<", argument: "<<optarg;}std::cout<<"\n";}return 0;
}int test3(int argc, char* argv[])
{// reference: https://www.gnu.org/software/libc/manual/html_node/Example-of-Getopt.html// Normally, getopt is called in a loop. When getopt returns -1, indicating no more options are present, the loop terminates.// A switch statement is used to dispatch on the return value from getopt. In typical use, each case just sets a variable that is used later in the program.// A second loop is used to process the remaining non-option arguments.int aflag = 0, bflag = 0, index = -1, c = -1;char* cvalue = nullptr;opterr = 0;while ((c = getopt(argc, argv, "abc:")) != -1) {switch (c) {case 'a':aflag = 1;break;case 'b':bflag = 1;break;case 'c':cvalue = optarg;break;case '?':if (optopt == 'c')fprintf(stderr, "Option -%c requires an argument.\n", optopt);else if (isprint(optopt))fprintf(stderr, "Unknown option '-%c'.\n", optopt);elsefprintf(stderr, "Unknown option character '\\x%x'.\n", optopt);return 1;default:abort();}}		fprintf(stdout, "aflag = %d, bflag = %d, cvalue = %s\n", aflag, bflag, cvalue);for (index = optind; index < argc; ++index) {fprintf(stdout, "index: %d, Non-option argument: %s\n", index, argv[index]);}return 0;
}} // namespaceint main(int argc, char* argv[])
{if (argc < 2) {fprintf(stderr, "the number of params must be greater than or equal to 2\n");return -1;}int flag = atoi(argv[1]);switch(flag) {case 1:fprintf(stdout, "start test 1:\n");test1(argc, argv);break;case 2:fprintf(stdout, "start test 2:\n");test2();break;case 3:fprintf(stdout, "start test 3:\n");test3(argc, argv);break;default:fprintf(stderr, "params error\n");break;}return 0;
}

build.sh内容如下：

#! /bin/bashreal_path=$(realpath $0)
dir_name=`dirname "${real_path}"`
echo "real_path: ${real_path}, dir_name: ${dir_name}"new_dir_name=${dir_name}/build
mkdir -p ${new_dir_name}
cd ${new_dir_name}
cmake ..
makecd -

run_getopt.sh内容如下：

#! /bin/bashreal_path=$(realpath $0)
dir_name=`dirname "${real_path}"`
echo "real_path: ${real_path}, dir_name: ${dir_name}"echo "test1:"
${dir_name}/build/test_sample_getopt 9 # params error
${dir_name}/build/test_sample_getopt 1 # flags = 0; tfnd = 0; nsec = 0; optind = 1
${dir_name}/build/test_sample_getopt 1 -b # invalid option -- 'b'
${dir_name}/build/test_sample_getopt 1 -x YYY # invalid option -- 'x'
${dir_name}/build/test_sample_getopt 1 -vZZZ # invalid option -- 'v'
${dir_name}/build/test_sample_getopt 1 -t 999 -n Jim # flags = 1; tfnd = 1; nsec = 999; optind = 4
${dir_name}/build/test_sample_getopt 1 -t888 -nSom # invalid option -- 'S'
${dir_name}/build/test_sample_getopt 1 -t6666 # flags = 0; tfnd = 1; nsec = 6666; optind = 2
${dir_name}/build/test_sample_getopt 1 -nant -t555 # invalid option -- 'a'
${dir_name}/build/test_sample_getopt 1 -n222 -t111 # invalid option -- '2'echo -e "\n\ntest2:"
${dir_name}/build/test_sample_getopt 2
# argc: 4
# Option: f, argument: input.gmn
# Option: o, argument: utput.jpgecho -e "\n\ntest3:"
${dir_name}/build/test_sample_getopt 3
# aflag = 0, bflag = 0, cvalue = (null)
# index: 1, Non-option argument: 3
${dir_name}/build/test_sample_getopt 3 -a -b
# aflag = 1, bflag = 1, cvalue = (null)
# index: 3, Non-option argument: 3 
${dir_name}/build/test_sample_getopt 3 -ab
# aflag = 1, bflag = 1, cvalue = (null)
# iindex: 2, Non-option argument: 3
${dir_name}/build/test_sample_getopt 3 -c foo
# aflag = 0, bflag = 0, cvalue = foo
# index: 3, Non-option argument: 3
${dir_name}/build/test_sample_getopt 3 -cfoo
# aflag = 0, bflag = 0, cvalue = foo
# index: 2, Non-option argument: 3
${dir_name}/build/test_sample_getopt 3 arg1
# aflag = 0, bflag = 0, cvalue = (null)
# index: 1, Non-option argument: 3
# index: 2, Non-option argumnet: arg1
${dir_name}/build/test_sample_getopt 3 -a arg1
# aflag = 1, bflag = 0, cvalue = (null)
# index: 2, Non-option argument: 3
# index: 3, Non-option argument: arg1
${dir_name}/build/test_sample_getopt 3 -c foo arg1
# aflag = 0, bflag = 0, cvalue = foo
# index: 3, Non-option argument: 3
# index: 4, Non-option argument: arg1
${dir_name}/build/test_sample_getopt 3 -a -- -b
# aflag = 1, bflag = 0, cvalue = (null)
# index: 3, Non-option argument: 3
# index: 4, Non-option argument: -b
${dir_name}/build/test_sample_getopt 3 -a -
# aflag = 1, bflag = 0, cvalue = (null)
# index: 2, Non-option argument: 3
# index: 3, Non-option argument: -

执行过程：首先执行build.sh，然后再执行run_getopt.sh即可。

GitHub： https://github.com/fengbingchun/Linux_Code_Test