{"id":1315,"date":"2015-04-18T00:01:54","date_gmt":"2015-04-18T07:01:54","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=1315"},"modified":"2015-04-25T07:27:38","modified_gmt":"2015-04-25T14:27:38","slug":"parsing-the-command-line-i","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=1315","title":{"rendered":"Parsing the Command Line I"},"content":{"rendered":"

About a year ago, I wrote a post<\/a> on reading the command line arguments. It’s a process that takes place all the time, not only when running programs in a terminal window but also for graphical operating systems. Knowing how to manipulate command line arguments is important.
\n
\nOne thing I didn’t mention in that earlier post is that the two arguments for the main()<\/em> function don’t always have to be named argc<\/code> and **argv<\/code>. Those are merely the traditional names, probably first used by Brian Kernighan and Dennis Ritchie when they wrote their seminal book, The C Programming Language<\/em>.<\/p>\n

If you like, you can name main()<\/em> function’s arguments anything, as long as the first is an integer value and the second is a pointer array. To wit:<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main(int c, char **s)\r\n{\r\n    printf(\"This program is named %s\\n\",*s);\r\n\r\n    return(0);\r\n}<\/pre>\n
The code compiles and it runs and it spews out the program name, argument zero on the command line.<\/p>\n
A more devilish trick is being able to read command line arguments and determine whether or not they’re valid. This type of processing, officially known as parsing the command line<\/em>, can occupy a great amount of the code. It gets really hairy, especially when dozens of options are available. Worse: The user can type them in any order, lump them together, forget some, or do all kinds of mischief. Truly, parsing a command line can be an art form.<\/p>\n
In the following code, one argument must be specified, hello<\/em>. It must match that case exactly. The code checks for the argument and further confirms that it’s the proper one.<\/p>\n
\r\n#include <stdio.h>\r\n#include <string.h>\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    if(argc != 2)\r\n    {\r\n        fprintf(stderr,\"Please specify a valid argument\\n\");\r\n        return(1);\r\n    }\r\n\r\n    if( strcmp(argv[1],\"hello\")==0 )\r\n    {\r\n        puts(\"Valid argument specified\");\r\n        return(0);\r\n    }\r\n    else\r\n    {\r\n        fprintf(stderr,\"Invalid argument specified\\n\");\r\n        return(1);\r\n    }\r\n}<\/pre>\n
The first argument on any command line is always the program name. So when another argument is specified, the argument count (argc<\/code> above) is greater than one. Specific to this code, the second argument is required, so argc<\/code> must equal 2 or the program is unable to run.<\/p>\n
The single command line argument dwells as a string referenced by variable argv[1]<\/code>. Remember that argv[0]<\/code> is the program’s name. One thing that throws even experienced programmers is keeping in mind that argument 2 is accessed via element one in the argv[]<\/code> array.<\/p>\n
Above, the strcmp()<\/em> function checks for the proper argument. If specified, the program is pleased, otherwise an error message is output. And the error output goes to the stderr<\/em> device, which is typically the display. You don’t want to use a puts()<\/em>  or printf()<\/em> function for an error message, which can be redirected and potentially overlooked.<\/p>\n
Of course, there’s no logic for a program to require a single, specific argument; arguments are options that affect the program’s processing or output. In next week’s Lesson<\/a> I’ll cover how to parse multiple options.<\/p>\n","protected":false},"excerpt":{"rendered":"
Knowing your argc<\/code> and argv<\/code>. 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-1315","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\/1315","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=1315"}],"version-history":[{"count":7,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1315\/revisions"}],"predecessor-version":[{"id":1345,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1315\/revisions\/1345"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1315"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1315"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1315"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}