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Problem with passing char arrays to functions

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Join Date: Jun 2009

Posts: 2

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spankboy11 spankboy11 is offline

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Problem with passing char arrays to functions

Nov 6th, 2009

Hi there, I'm assuming the answer to this problem is really easy, but I just can't work out where the problem is. It's driving me insane and I'd really appreciate some help. I'm new to C++.

My problem is this: I'm trying to pass a char array to a function, where a line is read from a file and then the array is passed back. However, the function returns gibberish - but ONLY when returning a file-derived array element. Here is my code:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int main ()
{
    char* tags[128];
    char* file="project3.xml";
 
    FILE *fp;
   	fp=fopen(file, "r");
 
   	int tits = NextTag( fp, tags );
   	cout << "tags[0] post-Fn = *" << tags[0] << "*\n";
   	cout << "tags[1] post-Fn = *" << tags[1] << "*\n";
 
	system("PAUSE");
    return 0;
}
 
int NextTag( FILE* fp, char* tags[] )
{
    int count2 = 0;
    char* pch;
    char rida1[512];
    fgets(rida1, sizeof(rida1), fp);
    cout << "rida1 in-Fn = *" << rida1 << "*\n";
    tags[0] = "bottoms";
    tags[1] = rida1;
    cout << "tags[0] in-Fn = *" << tags[0] << "*\n";
    cout << "tags[1] in-Fn = *" << tags[1] << "*\n";
}
int main ()
{
    char* tags[128];
    char* file="project3.xml";
    
    FILE *fp;
   	fp=fopen(file, "r");
   	
   	int tits = NextTag( fp, tags );
   	cout << "tags[0] post-Fn = *" << tags[0] << "*\n";
   	cout << "tags[1] post-Fn = *" << tags[1] << "*\n";
   	
	system("PAUSE");
    return 0;
}

int NextTag( FILE* fp, char* tags[] )
{
    int count2 = 0;
    char* pch;
    char rida1[512];
    fgets(rida1, sizeof(rida1), fp);
    cout << "rida1 in-Fn = *" << rida1 << "*\n";
    tags[0] = "bottoms";
    tags[1] = rida1;
    cout << "tags[0] in-Fn = *" << tags[0] << "*\n";
    cout << "tags[1] in-Fn = *" << tags[1] << "*\n";
}

The .xml file it's reading from just contains the word 'bottoms'. Here is the output:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
rida1 in-Fn = *bottoms*
tags[0] in-Fn = *bottoms*
tags[1] in-Fn = *bottoms*
tags[0] post-Fn = *bottoms*
tags[1] post-Fn = *L1"*
Press any key to continue. . .
 
rida1 in-Fn = *bottoms*
tags[0] in-Fn = *bottoms*
tags[1] in-Fn = *bottoms*
tags[0] post-Fn = *bottoms*
tags[1] post-Fn = *L1"*
Press any key to continue. . .

Why the gibberish on line 5 of the output when it returns the manually entered entry fine? Any ideas? It's driving me insane!!

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Nov 6th, 2009

It returns gibberish because when that NextTag() returns the array radial is destroyed, which invalidates tags[1]. What you need to do is allocate memory for the text. Then main() will have to delete[] that memory when its done with it.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
tags[1] = new char[strlen(radia1)+1];
strcpy(tags[1], radia1);
tags[1] = new char[strlen(radia1)+1];
strcpy(tags[1], radia1);

Another way to do it is for main() to allocate memory for that string

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
char* tags[4] = {0};
char radia1[80];
tags[1] = radia1;
...
...
char* tags[4] = {0};
char radia1[80];
tags[1] = radia1;
...
...

Last edited by Ancient Dragon; Nov 6th, 2009 at 10:34 pm.

Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.

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