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convert string to pointer to pointer?

 
 
Joe H
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      01-08-2004
Hi i converted a program to a lib. The parameters of main are
maincommand(int argc, char **argv)
I want to use main a s function that takes a string how can i convert
a char * to a char ** or even better how should i substitute the parameters
for a string. thanks
 
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Arthur J. O'Dwyer
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      01-08-2004

On Thu, 8 Jan 2004, Joe H wrote:
>
> Hi i converted a program to a lib. The parameters of main are
> maincommand(int argc, char **argv)
> I want to use main a s function that takes a string how can i convert
> a char * to a char ** or even better how should i substitute the parameters
> for a string. thanks


How many parameters, how organized, etc.? For example, if you're
used to calling the old program at the command line like this:

% myprog 100
% myprog 42
% myprog -8192

then you might prototype the new library function as

int myprog(int arg);

But if you write big complicated programs that you call like this:

% myprog -X 310 -Y 100 -bc -g 5
% myprog -X 80 -Y 80 -b -g 10 -k1 1.5 -k2 .3
% myprog -c -k2 50

then you might be better off with a prototype that requires the client
to explicitly state each possible argument's value:

int myprog(int X, int Y, int g, double k1, double k2, int B, int C);

Finally, if you are into clever little parsing algorithms, you
might write programs that get called like this:

% myprog "echome"
% myprog -o foo bar
% myprog recursive -o foo

and then maybe it would make sense to let your library function do
its own splitting-up-of-arguments, and just take a single string
with all the arguments glommed together, like this:

int myprog(const char *all_the_arguments);

It really depends on what you're trying to do.

If you want to pass a single string to a 'main'-like function
that takes (int, char **), you can do it like this:

int callmain(const char *p)
{
int my_argc;
char *my_argv[100];

my_argc = 2;
my_argv[0] = "program name goes here, usually";
my_argv[1] = (char *)p; /* EVIL CAST! EVIL EVIL EVIL! */
my_argv[2] = NULL;
return fake_main(my_argc, my_argv);
}

Notice the presence of an EVIL EVIL EVIL cast to (char *). That's
only necessary because I made 'callmain' take a (const char *)
parameter. I did that only because it seems nicer to the user. It
works as long as 'fake_main' behaves itself and doesn't go modifying
'p' behind our backs. Maybe it would be better to remove the 'const'
altogether... it depends.

HTH,
-Arthur

 
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joe
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      01-08-2004
thanks for your reponse ill try to make sense out of that
the program takes the pointer to pointer and the int and gets its
parameters like this

while ((ch = getopt(argc, argv,
"b:cde:f:F:hHk:lmnPr:sS:tT:uUv:z:")) != -1)

i would like to give it a string rather than the pointer to pointer. i have
done this in the main program and it compiles but i cant get the function
to run

#include "zib.h"
int main(int argc, char* argv[])
{
char line[100] = "-s -T 1002\n";

int t = zcommand(1, &line);

return 1;

}

 
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Arthur J. O'Dwyer
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      01-09-2004

On Thu, 8 Jan 2004, joe wrote:
>
> thanks for your reponse ill try to make sense out of that
> the program takes the pointer to pointer and the int and gets its
> parameters like this
>
> while ((ch = getopt(argc, argv,
> "b:cde:f:F:hHk:lmnPr:sS:tT:uUv:z:")) != -1)


'getopt' is not a standard C function, and so it's off-topic here.
That's not just zealous topicality guidance, either -- I really have
no idea how 'getopt' works, and I don't care. But I can make some
assumptions, most of them implicit, and let you puzzle out what works
and what doesn't.

> i would like to give it a string rather than the pointer to pointer. i have
> done this in the main program and it compiles but i cant get the function
> to run


> int main(int argc, char* argv[])
> {
> char line[100] = "-s -T 1002\n";
>
> int t = zcommand(1, &line);


Where 'zcommand' is the old 'main' function, no doubt. Okay, I'm
not surprised that it didn't work. I recommend that you take a good
look at the last public C draft standard, N869 (which you can Google
for; it comes in plain text as n869.txt[.gz]). It has a section
somewhere detailing exactly what the implementation guarantees to
be present in 'argc' and 'argv' -- and thus what *you* need to
guarantee will be present in *your* "fake" argc and argv.
I would quote N869 right here, but I'm on a slow connection this
week. Maybe someone else will. Anyway, search the file for the
words 'argc' and 'argv' and I bet you'll find it.

You [probably] need to split up those arguments in that string
literal, like this [but I don't know how lenient 'getopt' might
be on your platform -- maybe it's a non-issue]:

char *line[] = {"dummy", "-s", "-T", "1002", NULL};

int t = zcommand(4, line);

If this *is* the issue [consult your 'getopt' documentation, such
as "man getopt" or your compiler manual], then you might need to
write some code to split up the "command-line arguments" yourself.
That is, a function that can take the input
"-s -T 1002\n"
and produce the output
{"dummy", "-s", "-T", "1002", NULL}

If it turns out you do need such a function, check Google for
"command line argument parsing" or post to a group like
comp.sources.wanted. (comp.lang.c is *not* the place to ask other
people to write whole big functions for you.)

HTH,
-Arthur
 
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joe
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      01-09-2004

>


thanks alot, that was great
 
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