{"id":4856,"date":"2021-07-10T00:01:18","date_gmt":"2021-07-10T07:01:18","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4856"},"modified":"2021-07-17T07:14:55","modified_gmt":"2021-07-17T14:14:55","slug":"discovering-command-line-options-part-iii","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4856","title":{"rendered":"Discovering Command Line Options, Part III"},"content":{"rendered":"

Continuing from last week’s Lesson<\/a>, to read multiple command line arguments, you must put the getopt()<\/em> function in a loop. Specifically, you set the function as the looping condition. The loop’s guts evaluate the switches found.
\n
\nHere’s how the looping condition can look:<\/p>\n

while( (r=getopt(argc,argv,\"ABC\")) != -1 )<\/code><\/p>\n

The getopt()<\/em> function is nestled at the heart of the while<\/em> loop’s condition. Its return value is saved in int<\/em> variable r<\/code>. This expression is enclosed in parentheses, which evaluates to the value of r<\/code>. This value is compared with -1. As long as the getopt()<\/em> function returns values other than -1, the loop continues. Here is the full code:<\/p>\n

2021_07_10-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <unistd.h>\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    int r,count;\r\n\r\n    \/* suppress getopt()'s error message *\/<\/span>\r\n    opterr = 0;\r\n\r\n    \/* process multiple command line arguments *\/<\/span>\r\n    count = 0;\r\n    while( (r=getopt(argc,argv,\"ABC\")) != -1 )\r\n    {\r\n        if( r=='?' )\r\n            printf(\"Unknown option specified\\n\");\r\n        else\r\n            printf(\"Option '%c' found\\n\",r);\r\n        count++;\r\n    }\r\n    printf(\"%d arguments processed\\n\",count);\r\n\r\n    return(0);\r\n}<\/pre>\n
The program digests command line arguments -A<\/code>, -B<\/code>, and -C<\/code> in any combination. Invalid arguments are output as such. Here is a sample run with all the arguments specified:<\/p>\n
$ .\/a.out -A -B -C
\nOption 'A' found
\nOption 'B' found
\nOption 'C' found
\n3 arguments processed<\/code><\/p>\n
Here is a sample run specifying legitimate argument -B<\/code> and a bogus argument -Z<\/code>:<\/p>\n
$ .\/a.out -B -Z
\nOption 'B' found
\nUnknown option specified
\n2 arguments processed<\/code><\/p>\n
To further examine the unknown option, use the global optind<\/code> variable. This variable is an index used by the getopt()<\/em> function as it churns through the command line arguments. The optind<\/code> “options index” value is one greater than the element number in the argv[]<\/code> array.<\/p>\n
The following code presents a modification that spits out invalid arguments by name:<\/p>\n
2021_07_10-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <unistd.h>\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    int r,count;\r\n\r\n    \/* suppress getopt()'s error message *\/<\/span>\r\n    opterr = 0;\r\n\r\n    \/* process multiple command line arguments *\/<\/span>\r\n    count = 0;\r\n    while( (r=getopt(argc,argv,\"ABC\")) != -1 )\r\n    {\r\n        if( r=='?' )\r\n            printf(\"Unknown option '%s' specified\\n\",\r\n                    argv[optind-1]\r\n                  );\r\n        else\r\n            printf(\"Option '%c' found\\n\",r);\r\n        count++;\r\n    }\r\n    printf(\"%d arguments processed\\n\",count);\r\n\r\n    return(0);\r\n}<\/pre>\n
Here’s a sample run:<\/p>\n
$ .\/a.out -A -O -B
\nOption 'A' found
\nUnknown option '-O' specified
\nOption 'B' found
\n3 arguments processed<\/code><\/p>\n
Easier than using the optind<\/code> value, global variable optopt<\/code> holds the character value (decimal) of the invalid switch. To update the code, replace argv[optind-1]<\/code> with optopt<\/code>, and change the printf()<\/em> format string placeholder from %s<\/code> to %c<\/code>, as is done in this code:<\/p>\n
2021_07_10-Lesson-c.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <unistd.h>\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    int r,count;\r\n\r\n    \/* suppress getopt()'s error message *\/<\/span>\r\n    opterr = 0;\r\n\r\n    \/* process multiple command line arguments *\/<\/span>\r\n    count = 0;\r\n    while( (r=getopt(argc,argv,\"ABC\")) != -1 )\r\n    {\r\n        if( r=='?' )\r\n            printf(\"Unknown option '%c' specified\\n\",\r\n                    optopt\r\n                  );\r\n        else\r\n            printf(\"Option '%c' found\\n\",r);\r\n        count++;\r\n    }\r\n    printf(\"%d arguments processed\\n\",count);\r\n\r\n    return(0);\r\n}<\/pre>\n
The output from the above code’s program and the preceding program are identical.<\/p>\n
Remember that both optind<\/code> and optopt<\/code> are external variables defined in the unistd.h<\/code> header file.<\/p>\n
In next week’s Lesson<\/a>, I cover how the getopt()<\/em> function read’s option values. For example: -a 177<\/code>, where -a<\/code> is the switch and 177<\/code> is specified as the switch’s value (or string).<\/p>\n","protected":false},"excerpt":{"rendered":"
Once you know how it works, it’s possible to use the getopt()<\/em> function to process multiple command line arguments. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-4856","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4856","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4856"}],"version-history":[{"count":5,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4856\/revisions"}],"predecessor-version":[{"id":4884,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4856\/revisions\/4884"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4856"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4856"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4856"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}