how to write a program that takes arguments from commandline?

[Martin Jørgensen]

Hi,

I'm learning C-programming. I have a program which I would like to
modify so it takes arguments from the commandline. Let call the program:
program.exe.

Could somebody shortly explain how I get this behaviour:

C:>program -help or C:>program -h
printf("\nBla. bla. Here is some help and arguments\n").... etc.

C:>program -dt=0.1 -tend=10 (etc. additional switches might be added).
In the program:

float (or double) dt should become 0.1. The integer variable named
"tend" should be assigned respectively to 10.

In case of any problems such as for instance C:>program dt=adg, the
program should respond with something like "dt: Invalid syntax. Exiting.".

I suspect that one should change the program such that "int main(??
something goes in here, right?)" instead of int main(void), but I'm not
really sure of how to address this problem. If somebody has any sample
code in C to post, I would be very happy.

Thanks in advance for any hints...
Med venlig hilsen / Best regards
Martin Jørgensen

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[Walter Roberson]

In article <j4************@news.tdc.dk>,
=?ISO-8859-1?Q?Martin_J=F8rgensen?= <un*********@spam.jay.net> wrote:

I'm learning C-programming. I have a program which I would like to
modify so it takes arguments from the commandline. I suspect that one should change the program such that "int main(??
something goes in here, right?)" instead of int main(void), but I'm not
really sure of how to address this problem.

Ummm, I cannot think of any introductory C text which does not
cover the mechanics of passing in arguments.

The harder part is in cleanly and efficiently parsing what gets passed in,
especially if some options modify the meaning of others or some
options cannot co-exist with others. For the mechanics, see most
any program and look for argc and argv .
--
"law -- it's a commodity"
-- Andrew Ryan (The Globe and Mail, 2005/11/26)

[Alvin]

You're right, when you define main, you do:
int main(int argc, char *argv[])
argc is the number of arguments specified, program.exe would count as
an argument, too.
argv is the arguments given. It's a constant array, so it starts at [0]
(that would be program.exe). [1] would be -h or -help, and so on. I've
seen argv written as char **argv rather than the conventional char
*argv[], but I guess it's down to you.

One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work. If you try:
---
if(argv[1] == "-help")
printf("\n<help>");
else
printf("\nyou typed %s", argv[1]);
---
then no matter what, it drops to the else statment.

[stathis gotsis]

"Martin Jørgensen" <un*********@spam.jay.net> wrote in message
news:j4************@news.tdc.dk...

Hi,

I'm learning C-programming. I have a program which I would like to
modify so it takes arguments from the commandline. Let call the program:
program.exe.

Could somebody shortly explain how I get this behaviour:

C:>program -help or C:>program -h
printf("\nBla. bla. Here is some help and arguments\n").... etc.

C:>program -dt=0.1 -tend=10 (etc. additional switches might be added).
In the program:

float (or double) dt should become 0.1. The integer variable named
"tend" should be assigned respectively to 10.

In case of any problems such as for instance C:>program dt=adg, the
program should respond with something like "dt: Invalid syntax. Exiting.".

I suspect that one should change the program such that "int main(??
something goes in here, right?)" instead of int main(void), but I'm not
really sure of how to address this problem. If somebody has any sample
code in C to post, I would be very happy.

Thanks in advance for any hints...

Maybe this is relevant:
http://c-faq.com/misc/argv.html

[Joe Wright]

Alvin wrote:
[ much snippage ]

One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work. If you try:
---
if(argv[1] == "-help")
printf("\n<help>");
else
printf("\nyou typed %s", argv[1]);
---
then no matter what, it drops to the else statment.

Wrong completely. 'if (argv[1] == "-help")' can never be expected to
'work' in your wildest dreams. argv[1] is a char* supplied by the
command processor, "-help" is a char* to an anonymous array of char
somewhere in memory. They will not ever be equal. Never.

strcmp() is your friend.

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[boa]

Joe Wright wrote:

Alvin wrote:
[ much snippage ]

One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work. If you try:
---
if(argv[1] == "-help")
printf("\n<help>");
else
printf("\nyou typed %s", argv[1]);
---
then no matter what, it drops to the else statment. -------^^^^^^^^^^^^^^^^
Did you miss this part?

Wrong completely. 'if (argv[1] == "-help")' can never be expected to
'work' in your wildest dreams. argv[1] is a char* supplied by the
command processor, "-help" is a char* to an anonymous array of char
somewhere in memory. They will not ever be equal. Never.

strcmp() is your friend.

boa

[Joe Wright]

boa wrote:

Joe Wright wrote:

Alvin wrote:
[ much snippage ]

One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work. If you try:
---
if(argv[1] == "-help")
printf("\n<help>");
else
printf("\nyou typed %s", argv[1]);
---
then no matter what, it drops to the else statment.
-------^^^^^^^^^^^^^^^^
Did you miss this part?

I don't think so. What might I have missed? Sarcasm? Maybe.

Wrong completely. 'if (argv[1] == "-help")' can never be expected to
'work' in your wildest dreams. argv[1] is a char* supplied by the
command processor, "-help" is a char* to an anonymous array of char
somewhere in memory. They will not ever be equal. Never.

strcmp() is your friend.

boa

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[Alvin]

*hits forhead*
Should have thought of using that before!
Thanks very much, Joe! I was worried I was going to get a stingy reply
with no help at all.

[Keith Thompson]

"Alvin" <al***********@gmail.com> writes:

*hits forhead*
Should have thought of using that before!
Thanks very much, Joe! I was worried I was going to get a stingy reply
with no help at all.

Should have thought of using what before?

Don't assume we can easily see the article to which you're replying.
Please read <http://cfaj.freeshell.org/google/>.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[boa]

Joe Wright wrote:

boa wrote:

Joe Wright wrote:

Alvin wrote:
[ much snippage ]
One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work. If you try:
---
if(argv[1] == "-help")
printf("\n<help>");
else
printf("\nyou typed %s", argv[1]);
---
then no matter what, it drops to the else statment.

-------^^^^^^^^^^^^^^^^
Did you miss this part?

I don't think so. What might I have missed? Sarcasm? Maybe.

My mistake. I messed up and for some reason thought that you responded
to someone other than the OP, and thought that you didn't "get" the
(rather poor) example of why one cannot compare string values like that
in C.

Sorry about that.
boa
[snip]

[Martin Jørgensen]

stathis gotsis wrote:
-snip-

Maybe this is relevant:
http://c-faq.com/misc/argv.html

Thanks. That looks exactly like the place to start. Also thanks to the
other answers. I assume argc contains the number of (space-separated)
arguments.
Med venlig hilsen / Best regards
Martin Jørgensen

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[Keith Thompson]

Martin Jørgensen <un*********@spam.jay.net> writes:

stathis gotsis wrote:
-snip-

Maybe this is relevant:
http://c-faq.com/misc/argv.html

Thanks. That looks exactly like the place to start. Also thanks to the
other answers. I assume argc contains the number of (space-separated)
arguments.

The arguments aren't necessarily space-separated; that depends on the
mechanism (for example, a shell) used to invoke your program.

argc is the number of valid elements of the array pointed to by argv.
If you invoke your program with 2 command-line arguments, argc will be 3.
argv[0] probably points to (some form of) the name of your program;
argv[1] and argv[2] point to your command-line arguments.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Mark McIntyre]

On 11 Feb 2006 11:21:59 -0800, in comp.lang.c , "Alvin"
<al***********@gmail.com> wrote:

One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work.

you can't compare strings like that. This is why strcmp and its
friends exist.,
Mark McIntyre
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

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[Mark McIntyre]

On Sat, 11 Feb 2006 21:08:53 +0100, in comp.lang.c , boa
<bo*****@gmail.com> wrote:

Joe Wright wrote:

Alvin wrote:
[ much snippage ]

One problem I've run into is this: when testing for an argument, say:
if(argv[1] == "-help")
in your case, it doesn't work. If you try:
---
if(argv[1] == "-help")
printf("\n<help>");
else
printf("\nyou typed %s", argv[1]);
---
then no matter what, it drops to the else statment.

-------^^^^^^^^^^^^^^^^
Did you miss this part?

No, he didn't. Its meaningless to compare strings using ==, unless
both are the same object it will almost inevitably fail (all you're
doing is comparing the pointers). So naturally it drops down to the
else.
Mark McIntyre
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

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[pete]

Martin Jørgensen wrote:

I suspect that one should change the program such that "int main(??
something goes in here, right?)" instead of int main(void),
but I'm not
really sure of how to address this problem. If somebody has any sample
code in C to post, I would be very happy.

/* BEGIN type_.c */
/*
** This is a demonstration of a way to use fscanf
** to read lines from text files.
*/
#include <stdio.h>

#define ARGV_0 type_
#define LINE_LEN 250
#define str(s) # s
#define xstr(s) str(s)

int main(int argc, char *argv[])
{
int rc;
FILE *fd;
char line[LINE_LEN + 1];

if (argc > 1) {
while (*++argv != NULL) {
fd = fopen(*argv, "r");
if (fd != NULL) {
do {
rc = fscanf(fd,
"%" xstr(LINE_LEN) "[^\n]%*[^\n]", line);
if (!feof(fd)) {
getc(fd);
}
if (rc == 0) {
*line = '\0';
}
if (rc != EOF) {
puts(line);
}
} while (rc == 1 || rc == 0);
fclose(fd);
} else {
fprintf(stderr,
"\nfopen() problem with \"%s\"\n", *argv);
break;
}
}
} else {
puts(
"Usage:\n>" xstr(ARGV_0)
" <FILE_0.txt> <FILE_1.txt> <FILE_2.txt> ...\n"
);
}
return 0;
}

/* END type_.c */
--
pete

[Martin Jørgensen]

Keith Thompson wrote:

Martin Jørgensen <un*********@spam.jay.net> writes:

stathis gotsis wrote:
-snip-

Maybe this is relevant:
http://c-faq.com/misc/argv.html
Thanks. That looks exactly like the place to start. Also thanks to the
other answers. I assume argc contains the number of (space-separated)
arguments.

The arguments aren't necessarily space-separated; that depends on the
mechanism (for example, a shell) used to invoke your program.

If the arguments isn't space-separated, how else would they be separated?
argc is the number of valid elements of the array pointed to by argv.
If you invoke your program with 2 command-line arguments, argc will be 3.
argv[0] probably points to (some form of) the name of your program;
argv[1] and argv[2] point to your command-line arguments.

Just as I thought.
Med venlig hilsen / Best regards
Martin Jørgensen

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[Martin Jørgensen]

pete wrote:
-snip-

/* END type_.c */

Thanks for the example. Looks a little advanced but I'll try it out...
Med venlig hilsen / Best regards
Martin Jørgensen

--
---------------------------------------------------------------------------
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[William J. Leary Jr.]

"Martin Jørgensen" <un*********@spam.jay.net> wrote in message
news:e0************@news.tdc.dk...

Keith Thompson wrote:

The arguments aren't necessarily space-separated; that depends on the
mechanism (for example, a shell) used to invoke your program.

If the arguments isn't space-separated, how else would they be separated?

I've encountered two variations:

program argument1,argument2,argument with space,,argument 5

program/argument1/argument2/argument with space//argument 5

These both got argc == 6 with

argv[0] = "" (neither of them gave the program name in any form)
argv[1] = "argument1"
argv[2] = "argument2"
argv[3] = "argument with space"
argv[4] = ""
argv[5] = "argument 5"

The touted advantage of both was that you could have spaces, or even empties,
in the list. One of them discarded leading and trailing blanks. The other
didn't. I don't recall which now, but for the one that didn't, committing the
typo of

program argument1, argument 2

got you

argv[1] = "argument1"
argv[2] = " argument 2"

I was targeting both of these at the same time, so I just coded to remove
leading and trailing blanks no matter what.

- Bill

[pete]

Martin Jørgensen wrote:

pete wrote:
-snip-

/* END type_.c */

Thanks for the example. Looks a little advanced but I'll try it out...

The command line agurment part
isn't especially advanced, I don't think.

The fprintf part *is* a little bit advanced.

--
pete

[pete]

pete wrote:

Martin Jørgensen wrote:

pete wrote:
-snip-

/* END type_.c */

Thanks for the example.
Looks a little advanced but I'll try it out...

The command line agurment part
isn't especially advanced, I don't think.

But I should explain it anyway.

For this form of main: int main(int argc, char *argv[]);

argc is the number of command line arguments.
argv is the NULL terminated array of pointers to strings,
where the strings are command line arguments.
The name of the program is the first argument.

A system need not support command line arguments at all,
in which case arc may be equal to zero,
this something that should be checked for.
If command line arguments are supported,
then the first one will be the name of the program.

--
pete

[pete]

pete wrote:

A system need not support command line arguments at all,
in which case arc may be equal to zero,

argc

--
pete

[tmp123]

Martin Jørgensen wrote:

Hi,

I'm learning C-programming. I have a program which I would like to
modify so it takes arguments from the commandline. Let call the program:
program.exe.

Could somebody shortly explain how I get this behaviour:

C:>program -help or C:>program -h
printf("\nBla. bla. Here is some help and arguments\n").... etc.

C:>program -dt=0.1 -tend (etc. additional switches might be added).
In the program:

float (or double) dt should become 0.1. The integer variable named
"tend" should be assigned respectively to 10.

In case of any problems such as for instance C:>program dt*g, the
program should respond with something like "dt: Invalid syntax. Exiting.".

I suspect that one should change the program such that "int main(??
something goes in here, right?)" instead of int main(void), but I'm not
really sure of how to address this problem. If somebody has any sample
code in C to post, I would be very happy.

Thanks in advance for any hints...
Med venlig hilsen / Best regards
Martin Jørgensen

Hi,

The answers from other persons have been very complete, and fully
according to the objective of this group. However, I thing that the
subject, from the point of view of learning, can not be complete
without expend a few lines to reference the existence of the function
"getopt" (and, sometimes, "getopts"). Their usage and validity for one
specific operating system could be considered off-topic.

[Keith Thompson]

Martin Jørgensen <un*********@spam.jay.net> writes:

Keith Thompson wrote:

Martin Jørgensen <un*********@spam.jay.net> writes:

stathis gotsis wrote:
-snip-
Maybe this is relevant:
http://c-faq.com/misc/argv.html

Thanks. That looks exactly like the place to start. Also thanks to the
other answers. I assume argc contains the number of (space-separated)
arguments.

The arguments aren't necessarily space-separated; that depends on the
mechanism (for example, a shell) used to invoke your program.

If the arguments isn't space-separated, how else would they be separated?

Who knows? A C program gets zero or more arguments when it's
executed; it can retrieve them via argc and argv. The way those
arguments were generated is outside the scope of the C language.

Wandering a little off-topic ...

In a Unix-like system, programs are often invoked from a shell
command line. Shells *can* use spaces to delimit arguments, but the
syntax is more complex; for example:
my_program 'foo bar' baz
will invoke my_program with two arguments: "foo bar" and "baz".

Programs can also be invoked directly using one of the exec*() family
of functions. For example:

execlp("my_program", "foo bar", "baz", (char*)NULL);

Here, the arguments are provided directly as separate strings; they're
not delimited at all.

As I said, the details are off-topic; I discuss them here merely to
provide an example of the kind of thing that can happen before your
program is executed.

If you're writing a shell yourself, you can pick any mechanism you
like for constructing the arguments for the programs you invoke.
Delimiting the arguments with spaces (or, more generally, with
whitespace) is a reasonable approach, but it's not the only one.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Mark McIntyre]

On Sun, 12 Feb 2006 15:23:41 +0100, in comp.lang.c , Martin Jørgensen
<un*********@spam.jay.net> wrote:

Keith Thompson wrote:

The arguments aren't necessarily space-separated; that depends on the
mechanism (for example, a shell) used to invoke your program.

If the arguments isn't space-separated, how else would they be separated?

colons, tabs, semicolons, slashes, any number of possibities.
Different OSes do different things.
Mark McIntyre
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

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[Arndt Jonasson]

"tmp123" <tm****@menta.net> writes:

Martin Jørgensen wrote:

I'm learning C-programming. I have a program which I would like to
modify so it takes arguments from the commandline. Let call the program:
program.exe.

The answers from other persons have been very complete, and fully
according to the objective of this group. However, I thing that the
subject, from the point of view of learning, can not be complete
without expend a few lines to reference the existence of the function
"getopt" (and, sometimes, "getopts"). Their usage and validity for one
specific operating system could be considered off-topic.

The manual page for 'getopt' on HP-UX lists the following:
STANDARDS CONFORMANCE
getopt(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2
optarg: AES, SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2
opterr: AES, SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2
optind: AES, SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2
optopt: AES, SVID3, XPG4, POSIX.2

There are most likely public domain implementations of it as well.

[Richard Bos]

Mark McIntyre <ma**********@spamcop.net> wrote:

On Sun, 12 Feb 2006 15:23:41 +0100, in comp.lang.c , Martin Jørgensen
<un*********@spam.jay.net> wrote:

Keith Thompson wrote:

The arguments aren't necessarily space-separated; that depends on the
mechanism (for example, a shell) used to invoke your program.

If the arguments isn't space-separated, how else would they be separated?

colons, tabs, semicolons, slashes, any number of possibities.
Different OSes do different things.

In fact, there's nothing in the Standard that requires the contents of
argv to come from a command line. One could have an implementation on an
all-WIMP environment that pops up a little dialog with an extensible
list of text entry fields each time a program is run.

Richard

[Martin Jørgensen]

tmp123 wrote:
-snip-

The answers from other persons have been very complete, and fully
according to the objective of this group. However, I thing that the
I fully agree.
subject, from the point of view of learning, can not be complete
without expend a few lines to reference the existence of the function
"getopt" (and, sometimes, "getopts"). Their usage and validity for one
specific operating system could be considered off-topic.

I will try google for that. Thanks for all the answers from everyone...
I'll get back if I run into trouble.
Med venlig hilsen / Best regards
Martin Jørgensen

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