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Nov 6th, 2009

Problem with passing char arrays to functions

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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:

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:

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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spankboy11

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Nov 6th, 2009

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Re: Problem with passing char arrays to functions

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.

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

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.

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Nov 7th, 2009

Re: Problem with passing char arrays to functions

A-ha! I hadn't even considered that it might be because rida1 was going out of scope (I had thought that once the value of rida1 had been passed to the array then rida1's value was irrelevent).

Thanks so much - it really makes sense now!

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