Parsing command line arguments in C

Question

I have a program that needs to use variables that should be set when the program is called from the command line. I want the user to be able to specify options -r, -o, -s, and -p along with some integer. I've never dealt with command line arguments before but after doing some research I came up with the following:

int main(int argc, char *argv[]){
    for(k = 1; k < argc; k = k+2){
        if(std::string(argv[k]) == "-r"){
           n = atoi(std::string(argv[k+1]));
        }else if(std::string(argv[k]) == "-o"){
           o = atoi(std::string(argv[k+1]));
        }else if(std::string(argv[k]) == "-s"){
           p = atoi(std::string(argv[k+1]));
        }else{
          n = 1; o = 2; p = 3;
        }
    }
}

My idea is that the user would call the program like so:

$ ./my_program -r 1000 -o 10000000 -s 1

in order to set the variables in my program accordingly. However, when I try to compile this with my makefile, I get an error:

    error: std undeclared (first use in this function)
        if(std::string(argv[k]) == "-r"){
           ^

There are many more but I can't type them all. I'm sure if I figure out this error I can figure out the others. Why doesn't C like me?

EDIT:

As a quick note, I did include , but this does not fix any of the errors and the output when calling make is still the same.

Then you should have said so, and included the "fatal error," in your question. We can't help you solve problems without all the information. — TypeIA, Mar 09 '14 at 06:58
I didn't include it before you mentioned it, so I didn't mention it in the question. I will edit my question though to say that I did include it though. — Jeremy Fisher, Mar 09 '14 at 06:59
Are you by chance trying to feed C++ code into a pure-C compiler? That's what it sounds like now. — TypeIA, Mar 09 '14 at 07:00
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/49339/discussion-between-dvnrrs-and-jeremy-fisher) — TypeIA, Mar 09 '14 at 07:02
`std::string` does not exist in C; to compare strings you would use the `strcmp()` function. You may want to consider using a library that already has commandline arguments worked out, such as GNU getopt. — M.M, Mar 09 '14 at 07:04
Apart from the `std::string` in C issue, your code's logic is wrong: Remove the last `else` clause and set the default values for `n`, `o` and `p` before parsing the arguments. (Or rather then removing the `else`, turn it into an "argument not recognised" error message.) — M Oehm, Mar 09 '14 at 07:08
You should use a library function for parsing arguments. In C, on POSIX-ish systems you'd use [`getopt()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getopt.html), or GNU [`getopt_long()`](http://www.gnu.org/software/libc/manual/html_node/Getopt.html). In C++, you'd look at the [Boost options parser](http://www.boost.org/doc/libs/1_55_0/doc/html/program_options.html). There are a myriad alternatives. You would not, however, write ad hoc code to parse the options. — Jonathan Leffler, Mar 09 '14 at 07:08
Possible duplicate of [Parsing command-line arguments?](http://stackoverflow.com/questions/9642732/parsing-command-line-arguments) There are also many other questions that this could be a duplicate of. — Jonathan Leffler, Mar 09 '14 at 07:18
Why use `std::string` at all? Why not use `strcmp` ? It's much more efficient than creating objects when you only need to compare two strings. — Trinopoty, Mar 09 '14 at 07:05

score 2 · Answer 1 · answered Mar 09 '14 at 07:03

2

You should use getopt() here. The getopt() and getopt_long functions automate some of the chore involved in parsing typical unix command line options. It will save you all the effort.

http://www.ibm.com/developerworks/aix/library/au-unix-getopt.html

answered Mar 09 '14 at 07:03

brokenfoot

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Parsing command line arguments in C

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