Need help with Void*

The malfunctioning code snippet:

Code:

---

        ifstream uFile;
        struct stat results;
        char filePath[fileName.size()];
        chstr(fileName,*filePath);
       
        uFile.open(filePath, ios::in | ios::binary);
       
    if ( !uFile.is_open() )
    {
            return false;
    }
   
    stat(filePath, &results);
   
    if (results.st_size <= 0)
    {
            return false;
    }
   
    archFiles[fileAmt] = malloc(results.st_size+1);
    uFile.read(((char*)archFiles[fileAmt]),results.st_size);

---

I don't have a clue whats wrong with it!

Anyway, more in depth the last two lines are the problem.
Everytime it gets to uFile.read the program crashes.

A note:
archfiles is defined as such:

Code:

---

void * archFiles[];

---

I'm certain the problem lies in archFiles[fileAmt] but I haven't a clue whats wrong with it. Even an attempt to print out the value of the pointer (after the malloc ofc) results in a crash at the print statement!

Edit: More info, I'm using windows and my compiler is g++

Code:

---

        ifstream uFile;
        struct stat results;
        char filePath[fileName.size()];
//...
void * archFiles[];

---

The code in red is illegal C++. You cannot define array sizes using variables, and you cannot define arrays as empty.

Use std::string to define string variables, and std::vector to define adjustable arrays.

Quote:

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my compiler is g++

---

That explains it. Turn off the extensions, and you will see that g++ will give you compiler errors (-Wall -pedantic). Otherwise, consult your g++ manual as to how to what those extensions do, as they are not legal C++.

Regards,

Paul McKenzie

Ok then, thanks for that. Do you have any suggestions on how to facilitate dynamic arrays?

I don't have a g++ manual. I acquired it through minGW. Also I'm using a programming environment called Wind River Work Bench. As far as I can tell there's no way to turn off any specific parameters from within the environment so I'll take your word for it.

Ok I changed

Code:

---

char filePath[fileName.size()];

---

to

Code:

---

char *filePath = (char*)malloc(fileName.size());
//...
free((void*)filePath);

---

Oh you said c++ doesn't allow for a variable to define an array size. What about in the following case?

Code:

---

int *a;
int n;
n = 25;
a = new int[n];

---

If that code works, my only problem would be expanding the array. The only solution to that, that I can think of is making a new array of the expanded size, copying all the data from the previous array to the new array then deleting the previous array.
Or perhaps I could make a linked list, though this is somewhat more difficult to deal with and has 4-8 bytes of necessary overhead per array entry.

Hmm after having thought about I'll probably go with a linked list of structs. The 8 bytes of overhead (per array element) is minor when being compared with the size of each array element.

Quote:

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Originally Posted by wd40bomber7

Ok then, thanks for that. Do you have any suggestions on how to facilitate dynamic arrays?

---

use std::vector.

To add, your code looks more like 'C' than C++. Usage of void* is not needed at all in the code you did show. Also, malloc() in a supposedly trivial C++ code is dubious, at best.

Another thing as that you never mentioned where "fileAmt" comes from. It just looks like you want to read something into a buffer, and the size of the buffer is determined by some runtime value.

Code:

---

#include <string>
#include <vector>
#include <fstream>
//... Other includes
typedef std::vector<std::vector<char> > ArrayofCharArray;

using namespace std;
ArrayofCharArray archFiles;

void someFunction()
{
  ifstream uFile;
  stat results;
  string filePath;
  //...
  uFile.open(filePath, ios::in | ios::binary);
       
  if ( !uFile.is_open() )
  {
            return false;
    }
   
    stat(filePath.c_str(), &results);
   
    if (results.st_size <= 0)
    {
            return false;
    }
   
    // Add an array of char to the vector
    archFiles.push_back(vector<char>());

    // Get the new array as a pointer for convenience
    std::vector<char>* buffer = &archFiles.back();

    // Resize it
    buffer->resize(results.st_size+1));

    // Use it in function
    uFile.read(&buffer->front(), results.st_size);
}

---

You now have an "array" of char arrays in archFiles. There is no call to malloc(), and the code is standard C++.

To get the data archFiles[0] is the buffer that contains the first line. archFiles[0].size() is the number of characters in the first line. archFiles[1] is the second line, etc.

Regards,

Paul McKenzie

Quote:

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Originally Posted by wd40bomber7

Ok then, thanks for that. Do you have any suggestions on how to facilitate dynamic arrays?

---

I stated to use std::vector. That is the dynamic array in C++ (see my example).

Quote:

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As far as I can tell there's no way to turn off any specific parameters from within the environment so I'll take your word for it.

---

You must be able to do that somehow. The g++ compiler relies on proper setting of command-line parameters, both by the IDE and by users. If not, then the IDE is worthless.

Quote:

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Ok I changed

Code:

---

char filePath[fileName.size()];

---

to

Code:

---

char *filePath = (char*)malloc(fileName.size());
//...
free((void*)filePath);

---

---

That is still 'C'. I stated to use std::string, which is the C++ way of defining strings.

Quote:

---

Oh you said c++ doesn't allow for a variable to define an array size. What about in the following case?

Code:

---

int *a;
int n;
n = 25;
a = new int[n];

---

---

'a' is not an array. It is a pointer to an integer. Two different things altogether.

Quote:

---

If that code works, my only problem would be expanding the array. The only solution to that, that I can think of is making a new array of the expanded size, copying all the data from the previous array to the new array then deleting the previous array.

---

Get a good C++ (not 'C') book. The std::vector class should (must) be explained there, as all of this is done automatically for you.

Regards,

Paul McKenzie

Quote:

---

Also I'm using a programming environment called Wind River Work Bench. As far as I can tell there's no way to turn off any specific parameters from within the environment so I'll take your word for it.

---

If it doesn't even allow you to change compiler settings you just need to trash it and go with something else (Code::Blocks and Visual Studio are the popular choices).

I find it very important to note
uFile.open(filePath, ios::in | ios::binary);
can not accept a string as its first parameter.

As for the rest of the code I am looking it over now. I'm glad your taking the time to enlighten me.

Ah.. I didn't know about string.c_str() /cry that could have saved me a few years.

Quote:

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Originally Posted by wd40bomber7

I don't have a g++ manual.

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That can be rectified by consulting the appropriate documentation at the GCC website: GCC online documentation.