Question

Pass string (char array) by reference

Asked by: adamport

Hi,

I'd like to know how to pass a string to a function by reference.

e.g.

main()
{
char * string;
myfunction(string);
}

myfunction(string)
{
//modify string and send back to main
}

I would like myfunction to be able to malloc 'string' and modify the contents of it so as it can be used in the main() function.

Any ideas?

Thanks

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Asked On
2009-10-07 at 08:33:55ID24792717
Tags

C

,

Programming

Topics

C Programming Language

,

Programming Languages

Participating Experts
3
Points
250
Comments
7

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Answers

 

by: evilrixPosted on 2009-10-07 at 08:45:37ID: 25516743

C doesn't support references only pointers and this is what a char * already is. If you want to be able to modify the pointer itself then you need to pass a pointer to a pointer (char **).

myfunction(ppstring **)
{
//modify string and send back to main

   * ppstring  = <new value>;
}

main()
{
char * string;
myfunction(&string);
}

 

by: Infinity08Posted on 2009-10-07 at 09:41:20ID: 25517325

char **ppstring will probably work better than ppstring ** ;)

 

by: evilrixPosted on 2009-10-07 at 09:49:03ID: 25517400

Ooops, copy and paste typo, well spotted and thanks :)

Again, with the correction....

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

C  doesn't support references only pointers and this is what a char * already is. If you want to be able to modify the pointer itself then you need to pass a pointer to a pointer (char **).

myfunction(char ** ppstring)
{
//modify string and send back to main

   * ppstring  = <new value>;

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
}

main()
{
char * string;
myfunction(&string);
}

 

by: mnhPosted on 2009-10-07 at 10:08:04ID: 25517588

Hi there,

I wrote an example in the following snippet.
Please always remember to free the allocated memory... :)

Hope this helps

main()
{
   char *string = NULL;
   myfunction( &string );
 
   // Do stuff with newly allocated string
   strcat( string, " I'm fine, thanks!" );
   printf( string );
 
   // Remember to free the allocated memory
   free(string);
}
 
void myfunction(char **string)
{
   //modify string and send back to main
   *string = malloc(512);
   if (!(*string))
   {
      // Print error, not enough memory
      return;
   }
 
   strcpy( *string, "How do you do?" );
}

                                              
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by: evilrixPosted on 2009-10-07 at 10:13:41ID: 25517643

@mnh,

Generally speaking on EE if you are going to post an answer that duplicates someone elses you don't... or you at least mention the original post and explain why you feel your post adds additional value to the question. Unlike other forums, experts are rewarded for their efforts here on EE and duplicate posts both dilute the value of the thread and undermine the answers already provided.

By all means, if you have something additional to add value to a thread then do so; however, just posting code that duplicates a answer benefits no one.

evilrix

 

by: mnhPosted on 2009-10-07 at 10:23:00ID: 25517719

Hi evilrix,

My post's added value is an example about how to allocate and free memory (and how to check if memory allocation was successful). Part of this was explicitly asked by the question's author.

Regards,
-- mnh

 

by: evilrixPosted on 2009-10-07 at 10:29:42ID: 25517785

>> My post's added value is an example about how to allocate and free memory
That was never part of the question so it adds no value. The asker wanted to know how to pass the string by reference in C so it could be modified.

>> Part of this was explicitly asked by the question's author.
No, it was stated as a requirement that needed to be fulfilled but the question was how to pass the string by reference to allow it to be assign a value or modified.

mnh, I can see you are new here so please accept these words as guidance to help you avoid getting involved in confrontation early on in your EE expert career.

At this point there is no need to reply to me and pollute this thread with a stream of off-topic posts. If you'd like to discuss this further I'd be happy to open a private discussion (e-mail me, my address is in my profile).

Thanks.

-Rx.

20120131-EE-VQP-002

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