Creating array of structs

I'm having some trouble using an array of structs. I need to allocate sufficient memory then pass a pointer to this to a function that will create the array of structs, so that I can use the pointer later to search this array. It looks something like this (I've cut out much that is irrelevant):

Code:

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typedef struct
{
  char filename[128];
  char group[32];
  int type;
} Experiment;

void fill(Experiment * exptarr)
{
  int count = 0;
  // parse a file to extract the data for each of maxexpts Experiments,
  // and attempt to copy the data to the array of structs
  {
    Experiment experiment;
    strcpy(experiment.filename,filename);
    strcpy(experiment.group,group);
    experiment.type = type;
    exptarr[count] = experiment;
    count++;
  }
}

int main(int argc, char * argv[])
{
  Experiment * exptarr;
  int maxexpts = 0;
  // code to count the maximum number of experiments goes here,
  // and updates maxexpts
  exptarr = malloc(sizeof(Experiment) * maxexpts);
  fill(&exptarr);
  // expect to find that exptarr is an pointer to an array of Experiment structs...
}

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This doesn't work at all, probably because I don't understand the syntax properly (I'm not a C or C++ programmer) or have perpetrated some other basic error(s). Would someone mind pointing the error(s) out?

Code:

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fill(&exptarr);

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has one too many levels of indirection - remove the &. exptarr is already a pointer, and your function expects a pointer. If you enable warnings (or even compile as C++ which is the forum you are writing in), it should tell you that you have the wrong pointer type.

--
Mats

Thanks.
I have been compiling it as C++ and it tells me that at the "malloc" line that I'm trying an invalid conversion from 'void*' to 'experiment*'. What could be causing that?

Quote:

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Originally Posted by knirirr

Thanks.
I have been compiling it as C++ and it tells me that at the "malloc" line that I'm trying an invalid conversion from 'void*' to 'experiment*'. What could be causing that?

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Exactly what it says: You are trying to convert between a void * to a experiment * - in C that is valid (any pointer can be converted to void * in both C and C++, but C allows you to convert a void * to any type of pointer, C++ doesn't let you do that). You can overcome this by either adopting the C++ style of using new, or by casting the pointer type to experiment *.

--
Mats

Quote:

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Originally Posted by matsp

You can overcome this by either adopting the C++ style of using new, or by casting the pointer type to experiment *.

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Would you mind telling me the syntax for either of these methods, or where I might find information on it? I am not having any success with what I think it should be (e.g. (Experiment)* malloc...).

(Experiment *) malloc. The type is "pointer to Experiment", a/k/a Experiment *.

Thanks - that now works. Perhaps if I had tried learning C before scripting languages this might be rather less confusing...

Quote:

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Originally Posted by knirirr

Thanks - that now works. Perhaps if I had tried learning C before scripting languages this might be rather less confusing...

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Possibly, but it's probable that you are just going through the learning curve of pointers, memory allocation and generally understanding C as a language.

But yes, many of the higher level languages do not bother with letting you allocate memory, or have pointers to data. It just "automagically works anyways".

--
Mats

The C++ version using new/delete:

Code:

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Experiment * exptarr;

...

exptarr = new Experiment[maxexpts];

...

delete [] exptarr;

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You should be using free after you are done using you're pointer if using malloc to allocate memory.

Code:

---

  // parse a file to extract the data for each of maxexpts Experiments,
  // and attempt to copy the data to the array of structs
  {
    Experiment experiment;
    strcpy(experiment.filename,filename);
    strcpy(experiment.group,group);
    experiment.type = type;
    exptarr[count] = experiment;
    count++;
  }

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Well don't do it that way. The experiment array already has N structs to be filled. What you're doing here is assigning pointers some dummy info from local variables which don't persist after the function returns. You're better off just using exptarr[count] as your memory.

Even if you do it your way and it appears to work, you've leaked memory.

nice program

>> The C++ version using new/delete:
I would rather show the C++ version using vector. There's no reason to use new/delete for this.

Quote:

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Originally Posted by citizen

Even if you do it your way and it appears to work, you've leaked memory.

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A good point - now corrected, thanks.