Dynamically create an array of TYPE in C

Question

I've seen many posts for c++/java, but nothing for C. Is it possible to allocate memory for an array of type X dynamically during run time? For example, in pseudo,

switch(data_type) 
     case1:float, create a new array of floats to use in the rest of the program
     case2:int, create new array of ints to use in the rest of the program
     case3:unsigned, ....
     // etc.

In my program I determine the data type from a text header file during run time, and then I need to create an appropriate array to store/manipulate data. Is there some kind of generic type in C?

EDIT: I need to dynamically create and DECIDE which array should be created.

Thanks, csand

I apologize for the formatting of the question; something went wrong there. — csand
– csand, Commented Jan 31, 2011 at 19:41
There is no need to cast malloc, nor the implicit conversions from/to void*. — user562374
– user562374, Commented Jan 31, 2011 at 19:48

Gavin H · Accepted Answer · 2011-01-31 22:20:58Z

5

Assuming you calculate the total size, in bytes, required from the array, you can just allocate that much memory and assign it to the correct pointer type.

Ex:

void * data_ptr = malloc( data_sz );

then you can assign it to a pointer for whatever type you want:

int *array1 = (int *)data_ptr;

or

float *array2 = (float *)data_ptr;

NOTE: malloc allocates memory on the heap, so it will not be automatically freed. Make sure you free the memory you allocate at some point.

UPDATE

enum {
    DATA_TYPE_INT,
    DATA_TYPE_FLOAT,
    ...
};

typedef struct {
    int data_type;
    union {
        float * float_ptr;
        int * int_ptr;
        ...
    } data_ptr;
} data;

While this might allow you to store the pointer and tell what type of pointer you should be using, it still leaves the problem of not having to branch the behavior depending on the data type. That will be difficult because the compiler has to know the data type for assignments etc.

edited Jan 31, 2011 at 22:20

answered Jan 31, 2011 at 19:47

Gavin H

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12 Comments

csand Over a year ago

Void *, totally forgot about. I think this will solve my issue. Cheers to everyone who replied.

csand Over a year ago

Unfortunately this didn't help me. My problem still resides with the decision/scope of the variable. It is true that I could use void* and then cast a new array. However, how would my program know which array to use? After declaring "int *array1" or "float *array2" I need my program to use that array. Declaring array1 and array2 inside a switch or if block leaves my scope encapsulated there. What I need is: Declare generic array space -> decide data type - use array of appropriate data type.

csand Over a year ago

I'm trying to avoid textually having 8 different blocks of the exact same code, except they have a different array type for each one.

Gavin H Over a year ago

@csand: Well that will be an issue because whatever the right hand value is will also depend on the type used. In terms of the pointer itself you could use a struct with a type flag and a union of the different pointer types.

csand Over a year ago

I'm not very familiar with unions (and am currently reading some man pages), but can you explain what you mean by type flags?

|

Craig Putnam · Accepted Answer · 2011-01-31 22:55:45Z

You're going to have a hard time doing this in C because C is statically typed and has no run-time type information. Every line of C code has to know exactly what type it is dealing with.

However, C comes with a nifty and much-abused macro preprocessor that lets you (among other things) define new functions that differ only in the static type. For example:

#define FOO_FUNCTION(t) t foo_function_##t(t a, t b) { return a + b; }

FOO_FUNCTION(int)
FOO_FUNCTION(float)

This gets you 2 functions, foo_function_int and foo_function_float, which are identical other than the name and type signature. If you're not familiar with the C preprocessor, be warned it has all sorts of fun gotchas, so read up on it before embarking on rewriting chunks of your program as macros.

Without knowing what your program looks like, I don't know how feasible this approach will be for you, but often the macro preprocessor can help you pretend that you're using a language that supports generic programming.

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