extracting invididual arguments from command line argument array

[blacklantern]

First and foremost, I'm working in C here.

I'm using command line arguments to pass in a port number that I'd like a socket to bind to in my program. My assignment requires that the TA be able to put in some arbitrary port number. Problem is, the argv is a pointer to a character array. My program will be called 'sender' and 'receiver,' and for example, the sender will be called like so:

sender 10001 11.234.124.23:10002

Where 10001 is the port number of the sender and 10002 is the port number of the receiver. The argv array will hold ascii, I'm guessing. How can I extract the first argument and change it to binary?

[jerome bettis]

#include <stdio.h>

int main(int argc, char* argv[]) {
if (argc > 2) {
int port = 0;
sscanf(argv[1], "%d", &port);
printf("port = %d\n", port);
} else {
printf("no arg given\n");
}
return 0;
}

[LordHunter317]

sscanf() is the devil and should be avoided at all possible costs. Use atoi() if possible, though it's not exceptionally better.

[blacklantern]

Thanx.

And for followup:


sender 10001 11.234.124.23:10002

How would I extract the ip address from the second argument? That is, how would I stop once the colon is reached? I'm assuming sscanf doesn't stop until it sees the null-terminated end of the string.

[LordHunter317]

You can make sscanf() stop, with a proper format string. A better option would be to saerch for the ':', copy the first part of the string, and then copy the second half; or use strspn() or strtok(). Unfortunately, C has no awesome string handling functions when it comes to even rudimentary parsing.

If you get stuck, I'll post hints.

[blacklantern]

Thanx Lordhunter - I knew there had to be a function out there. If I get stuck, I'll let you know.

[jerome bettis]

sscanf() is the devil and should be avoided at all possible costs. Use atoi() if possible, though it's not exceptionally better.

are any native C functions NOT the devil??? hah i was going to mention atoi() but i thought sscanf() was not as bad ... i looked for snscanf() but it doesn't exist .. this is my main beef with C as opposed to C++ .. there's acceptable easy ways to do this type of stuff in C++. whereas C has been around forever and all the important functions are still not recommended to use. i can't wait till C dissappears [-o<

strtok should never be used ever but if you're stuck with C what are you supposed to do?

i'm wondering what you're doing for sockets, maybe the library can take the ip address like a string? if not it probably has some wacky defined type for an ip address you might have to look at before you can create the socket.

[LordHunter317]

are any native C functions NOT the devil???A very select few, yes.


sscanf is worse, and even a length-checking version would be worse. The whole scanf() family is to be ignored.

[quote]i'm wondering what you're doing for sockets, maybe the library can take the ip address like a string?Not in that form, though.

[LordHunter317]

I should point out I mentioned strtok() and strspn() but they're to be avoided too, I wasn't clear on that (strtok() should never be used in new programs, strspn() shouldn't be used except as a replacement for strtok()). The "correct" way to do is it search for the ':', do the pointer artihmetic, make the copies, and deal with those.

At anyrate, use of strtok() or strspn() is going to require pointer math and copying anyway and the resulting code will be less clear.

[blacklantern]

Yeah, I jus now realized that something is wrong with strtok(). For some reason, the when I do a gethostbyname() with the hostname string extracted from the argument with strtok(), I get an herror of 'Unknown Host.' However, when I created a new string initialized for testing purposes with the name of the host I'm trying to connect to, the gethostbyname() completes. Strange thing is, when I do a strcmp() between the tested string and the one gotten with strtok, the function returns a zero - no difference between the strings.