Placing floats from string into void array

Question

I get a segmentation fault when my function reads floats from a string and places them in a void array. The segfault occurs after about 200 iterations of the for loop in the following code:

// Allocate memory
void** data;
data = (void**)malloc(num_vals * sizeof(float));

// Convert text to floats
(*(float**)data)[0] = atof(strtok(text, " "));  
for(int index=1; index<num_vals; index++) {
    (*(float**)data)[index] = atof(strtok(NULL, " "));   
    std::cout << (*(float**)data)[index] << std::endl;
}

The void array is necessary because the size and type of data in the string are determined at run-time. I've tried increasing the malloc size, but it doesn't change anything. Any thoughts?

1. [There is no such thing](http://stackoverflow.com/q/4810664/46642) as a "void array". 2. C and C++ are not the same language; you need to pick one. 3. Ugh, `strtok`. — R. Martinho Fernandes, Sep 08 '11 at 14:01
@R. Martinho Fernandes: What are you talking about? I avoid arrays. — Kevin, Sep 08 '11 at 14:04

score 5 · Answer 1 · answered Sep 08 '11 at 13:51

5

Seriously??

std::vector<float> data;
std::istringstream str(text);
float fv;
while (str >> fv)
{
  data.push_back(fv);
}

Now that's c++

answered Sep 08 '11 at 13:51

Nim

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while this is clearly a better solution it doesn't explain the where the seg fault is or why it occurs in the op's code. – Kevin Sep 08 '11 at 14:02
@Kevin, if the OP finds the answer unacceptable, it's his choice - I prefer to tackle the underlying problem (approach) rather than the specific issue here (which is an all too real side-effect of the approach) – Nim Sep 08 '11 at 14:27
I wish I could do that, but as I said, I don't know the data type of the string content or size until run-time. Still, I need to keep the data in a common structure, so I'm stuck with void pointers. – MatthewScarpino Sep 08 '11 at 15:01

score 2 · Answer 2 · answered Sep 08 '11 at 14:16

2

As much as it pains me to do so, here is a version of your code that probably does what you want.

// Allocate memory
void* data;
data = malloc(num_vals * sizeof(float));

// Convert text to floats
((float*)data)[0] = atof(strtok(text, " "));  
for(int index=1; index<num_vals; index++) {
    ((float*)data)[index] = atof(strtok(NULL, " "));   
    std::cout << ((float*)data)[index] << '\n';
}

Note, however, that if you worked for me and tried to check in that code, we would have a serious discussion about your choice of career.

I'd rather see something like this:

std::vector<float> v;
std::copy(std::istream_iterator<float>(std::istringstream(text)),
          std::istream_iterator<float>(),
          std::back_inserter(v));

P.s. Rob's rule #47: Never say std::endl when you mean '\n'.

answered Sep 08 '11 at 14:16

Robᵩ

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I like your last few lines, but I think it's a little ambitious for the OP given the current implementation... ;) – Nim Sep 08 '11 at 14:19
It has to be a void** because I'm allocating the memory in a function and freeing it elsewhere. If vectors were a possibility, I would certainly use them. – MatthewScarpino Sep 08 '11 at 15:03
@user, from your usage, you really want `void*`, not `void**`. If you have a requirement specifically for pointer-to-pointer-to-void instead of pointer-to-void, please add that to the question. – Robᵩ Sep 08 '11 at 15:51

score 1 · Answer 3 · answered Sep 08 '11 at 13:50

1

Why do you convert to void ** ??? You code contains couple errors on indexing, so let me show some reasonable changes

float* data;
data = (float*)malloc(num_vals * sizeof(float));

// Convert text to floats
data[0] = atof(strtok(text, " "));  
for(int index=1; index<num_vals; index++) {
 data[index] = atof(strtok(NULL, " "));   
 std::cout << data[index] << std::endl;
}

answered Sep 08 '11 at 13:50

Dewfy

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I need to use void pointers because I don't know what the data type is until run-time, and I have to store the data in a common structure no matter what type it is. – MatthewScarpino Sep 08 '11 at 15:02
user934904 than see answer of @Rob – Dewfy Sep 08 '11 at 15:44

score 1 · Answer 4 · answered Sep 08 '11 at 13:51

1

You got your types mixed up in your inexplicable attempt to create this monster under the pretence of writing "C++". Anyway. what you're mallocing is nothing but a float*, so you need to cast data back to float*:

((float*)data)[0] = myfloat;

answered Sep 08 '11 at 13:51

Kerrek SB

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score 0 · Answer 5 · answered Sep 08 '11 at 13:51

0

I think, since you are defining a pointer of void pointer and allocating/casting it to pointer of void pointer, it allocates 4 byte memory for each element because in C/C++, regardles of the type of the pointer, pointers are always 4 bytes which are not big enough floats.

answered Sep 08 '11 at 13:51

reader_1000

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I thought floats are 4 bytes and doubles are 8 bytes. – Kevin Sep 08 '11 at 13:57
You are right. I made a mistake. It has been long time without writing in C. – reader_1000 Sep 08 '11 at 14:15

score 0 · Answer 6 · answered Sep 08 '11 at 13:51

0

There're several issues. One is - data should be void *, you have redundant *. Other might be alignment, i'm not sure you are able to place a float in any location in the memory.

answered Sep 08 '11 at 13:51

Drakosha

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Placing floats from string into void array

6 Answers6