manipulating arg in the: function(char* arg); without returning it[C,freebsd7.1,gcc]

mali2 · 06-07-2009, 05:28 AM

As the title of the topic is not so clear, now here i can clear my problem...

I have a program source code, main.c . In which i m intending to do following thing with out changing the source code...
----------------------------------------------------------------------------------------------------

Code:

#include <stdio.h>
int main(int argc,char **argv)
{
char* backup;
char* arg = argv && argc > 0 ? argv[1] : NULL;
backup=arg;

foo(arg);

if (strcmp(arg, backup) != 0) 
  {
   printf("Yeah! u success; as u have manipulated arg");
   }   
else
   printf("You Failed!; no change in arg; arg and backup both are same");
exit(0);
}

void foo(char *arg)
{
arg="what can i do in it :( ??????? " ;
}

-----------------------------------------------------------------------------------------------------
I am beginner in C, I want to do some thing in this funtion so i would be able to change the arg, without returning arg from this method, as i m not supposed to change in the main method, i can only do things in foo function.
waiting for ur suggestions..
thanks for your concentration.
I m using freebsd7.1 and compiler is gcc.

matsp · 06-07-2009, 05:32 AM

I doubt you can do that. arg and backup point to the same bit of memory. You have no way to modify arg inside foo(), so you can't make it point to something else.

Maybe you have misunderstood the task?

--
Mats

mali2 · 06-07-2009, 05:44 AM

Quote:

Originally Posted by matsp

I doubt you can do that. arg and backup point to the same bit of memory. You have no way to modify arg inside foo(), so you can't make it point to something else.

Maybe you have misunderstood the task?

--
Mats

Thanks for your reply - my actual code is this... but it is assumes to be do in this way....

Code:

static void* function(char* arg){
    fprintf(stderr, "int 3h\n");
    printf("You've made it passed the first obstacle!\n");
    printf("-----\nStep 2. Case of the missing dynlib\n");
 
    char* backup = strdup(arg);
    void* handle = dlopen("mylib.so", RTLD_NOW);
    voidfun hsym = dlsym(handle, "NULL");
    hsym(arg);
    if (strcmp(arg, backup) != 0) cake(arg);    
}

Now i want to make this condition true , so cake function will parsed by compiler...
that method which i have described above is supposed to be done in the shared library which i have created....

matsp · 06-07-2009, 05:55 AM

If you pass a char * to a function in C, you can not change it to point somewhere else.

I'm not sure what you are supposed to do, but it's either breaking some rules, or you have misunderstood what you are supposed to do.

--
Mats

MacGyver · 06-07-2009, 06:01 AM

If you know about the details of the implementation, you could theoretically alter the value of arg in main() by determining its location in relation to the copy of arg being passed into it (ie. calculations of the stack, but assuming a heck of a lot of things, most of which can't really be determined easily). This isn't really in the domain of standard C, though, as this requires one to deviate into, at the very least, a murky area of implementation defined behavior, if not outright undefined behavior.

To make sure there is no mistake, I will state again that this task, as I understand it, is impossible in standard C. I don't even understand its purpose.

**Salem** · 06-07-2009, 06:13 AM

> char* arg = argv && argc > 0 ? argv[1] : NULL;
Your strcmp() will blow up anyway if arg ends up being NULL.

As would your modified strdup() variant.

Why does this look like an exercise in stack smashing to get the code to do something non-obvious based on it's intention.

mali2 · 06-07-2009, 06:28 AM

Quote:

Originally Posted by matsp

I doubt you can do that. arg and backup point to the same bit of memory. You have no way to modify arg inside foo(), so you can't make it point to something else.

Maybe you have misunderstood the task?

--
Mats

Quote:

Originally Posted by MacGyver

If you know about the details of the implementation, you could theoretically alter the value of arg in main() by determining its location in relation to the copy of arg being passed into it (ie. calculations of the stack, but assuming a heck of a lot of things, most of which can't really be determined easily). This isn't really in the domain of standard C, though, as this requires one to deviate into, at the very least, a murky area of implementation defined behavior, if not outright undefined behavior.

To make sure there is no mistake, I will state again that this task, as I understand it, is impossible in standard C. I don't even understand its purpose.

hmm... the purpose: "It is my assignment given by the prof in my university", I am sure there is no mistake...the above hsym(arg) got the reference from dynamic shared lib, and my purpose to doing it is to make the if condition true, so cake function will parsed.

**laserlight** · 06-07-2009, 09:04 AM

Quote:

Originally Posted by mali2

the purpose: "It is my assignment given by the prof in my university", I am sure there is no mistake.

Maybe you should provide some context: what module is this prof teaching?

itCbitC · 06-07-2009, 02:09 PM

Only way a variable can be altered in the calling function is by passing its address as an argument to the callee.

dwks · 06-08-2009, 01:53 PM

. . . for example:

Code:

char *rating = "Good";
modify_rating(&rating);

void modify_rating(char **rating) {
    *rating = "Bad";
}

King Mir · 06-08-2009, 02:42 PM

Perhalps the code is ment to be

Code:

backup= malloc(strlen(arg)+1);
strcpy(backup, arg);

Instead of

Code:

backup=arg;

Then you can change the string passed to foo without effecting the backup variable. (Note: there is still the restriction that the new string must be shorter than the old one)

06-07-2009, 05:28 AM	#1
mali2 Registered User Join Date: Jun 2009 Posts: 3	*manipulating arg in the: function(char arg); without returning it[C,freebsd7.1,gcc] As the title of the topic is not so clear, now here i can clear my problem... I have a program source code, main.c . In which i m intending to do following thing with out changing the source code... ---------------------------------------------------------------------------------------------------- Code: #include <stdio.h> int main(int argc,char argv) { char* backup; char* arg = argv && argc > 0 ? argv[1] : NULL; backup=arg; foo(arg); if (strcmp(arg, backup) != 0) { printf("Yeah! u success; as u have manipulated arg"); } else printf("You Failed!; no change in arg; arg and backup both are same"); exit(0); } void foo(char *arg) { arg="what can i do in it :( ??????? " ; } ----------------------------------------------------------------------------------------------------- I am beginner in C, I want to do some thing in this funtion so i would be able to change the arg, without returning arg from this method, as i m not supposed to change in the main method, i can only do things in foo function. waiting for ur suggestions.. thanks for your concentration. I m using freebsd7.1 and compiler is gcc.
mali2 is offline

06-07-2009, 05:32 AM	#2
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	I doubt you can do that. arg and backup point to the same bit of memory. You have no way to modify arg inside foo(), so you can't make it point to something else. Maybe you have misunderstood the task? -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
matsp is offline

06-07-2009, 05:55 AM	#4
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	If you pass a char * to a function in C, you can not change it to point somewhere else. I'm not sure what you are supposed to do, but it's either breaking some rules, or you have misunderstood what you are supposed to do. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
matsp is offline

06-07-2009, 06:01 AM	#5
MacGyver Deathray Engineer Join Date: Mar 2007 Posts: 3,211	If you know about the details of the implementation, you could theoretically alter the value of arg in main() by determining its location in relation to the copy of arg being passed into it (ie. calculations of the stack, but assuming a heck of a lot of things, most of which can't really be determined easily). This isn't really in the domain of standard C, though, as this requires one to deviate into, at the very least, a murky area of implementation defined behavior, if not outright undefined behavior. To make sure there is no mistake, I will state again that this task, as I understand it, is impossible in standard C. I don't even understand its purpose. __________________
MacGyver is offline

06-07-2009, 06:13 AM	#6
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	> char* arg = argv && argc > 0 ? argv[1] : NULL; Your strcmp() will blow up anyway if arg ends up being NULL. As would your modified strdup() variant. Why does this look like an exercise in stack smashing to get the code to do something non-obvious based on it's intention. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

06-07-2009, 02:09 PM	#9
itCbitC Registered User Join Date: Oct 2008 Location: TX Posts: 1,262	Only way a variable can be altered in the calling function is by passing its address as an argument to the callee.
itCbitC is offline

06-08-2009, 01:53 PM	#10
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,629	. . . for example: Code: char rating = "Good"; modify_rating(&rating); void modify_rating(char rating) { rating = "Bad"; } __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, etc. New project: nort
dwks is offline

06-08-2009, 02:42 PM	#11
King Mir Registered User Join Date: Apr 2006 Posts: 1,193	Perhalps the code is ment to be Code: backup= malloc(strlen(arg)+1); strcpy(backup, arg); Instead of Code: backup=arg; Then you can change the string passed to foo without effecting the backup variable. (Note: there is still the restriction that the new string must be shorter than the old one) __________________ It is too clear and so it is hard to see. A dunce once searched for fire with a lighted lantern. Had he known what fire was, He could have cooked his rice much sooner.
King Mir is online now

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Ranged numbers	Desolation	Game Programming	8	07-25-2006 10:02 PM