How can I implement an array whose size is not known at compile time in C89?

Question

Sorry, I'm a bit of a newbie to C and was wondering how you could create an array whose size is not known at compile time before the C99 standard was introduced.

Welcome on SO. Questions asking for code must demonstrate a minimal understanding of the problem being solved. Include attempted solutions, why they didn't work, and the expected results. — epsilon, Dec 27 '13 at 14:42

score 1 · Answer 1 · answered Dec 27 '13 at 14:23

1

Use malloc function from stdlib.h to create a dynamic array object.

answered Dec 27 '13 at 14:23

ouah

138,975
15
262
325

score 1 · Answer 2 · answered Dec 27 '13 at 14:27

the ordinary way would be to allocate the data on the heap

  #include <stdlib.h>
  void myfun(unsigned int n) {
    mytype_t*array = (mytype_t*)malloc(sizeof(mytype_t) * n);
    // ... do something with the array
    free(array);
  }

you could also allocate on the stack (so you don't need to free manually):

  #include <alloca.h>
  void myfun(unsigned int n) {
    mytype_t*array = (mytype_t*)alloca(sizeof(mytype_t) * n);
    // ... do something with the array
  }

Fiddling Bits · Accepted Answer · 2013-12-27T14:35:58.917

1

It's very easy. For example, if you want to create a variable length 1D int array, do the following. First, declare a pointer to type int:

int *pInt;

Next, allocate memory for it. You should know how many elements you will need (NUM_INTS):

pInt = malloc(NUM_INTS * sizeof(*pInt));

Don't forget to free your dynamically allocated array to prevent memory leaks:

free(pInt);

edited Dec 27 '13 at 14:35

answered Dec 27 '13 at 14:29

Fiddling Bits

8,422
3
27
43

***Big, bold, italic "thanks"*** for suggesting `sizeof(*pointer)` instead of `sizeof(type)`. – Dec 27 '13 at 14:30
@H2CO3 Learned that from you when you yelled at me one day. ;-) – Fiddling Bits Dec 27 '13 at 14:30
So that was me... Oh... ;-) – Dec 27 '13 at 14:31
@FiddlingBits; He almost yelled at every one here :( – haccks Dec 27 '13 at 14:31
@haccks (If we accept that, it means that almost everyone did something fundamentally wrong then...) – Dec 27 '13 at 14:31
@haccks Keeping my mouth shut… :-) – Fiddling Bits Dec 27 '13 at 14:32
@H2CO3; (No one is perfect except God). – haccks Dec 27 '13 at 14:32
1

@haccks Agreed. Everybody proves that fact… repeatedly! lol – Fiddling Bits Dec 27 '13 at 14:33
Do malloc takes two arguments? – Digital_Reality Dec 27 '13 at 14:35
1

@haccks That's right. :) – Dec 27 '13 at 14:35
@Digital_Reality No just one. I fixed the typo. – Fiddling Bits Dec 27 '13 at 14:36
@FiddlingBits What is the reason to use `sizeof(*ptr)` instead of `sizeof(type)`? Could you provide a link please? – Utku Apr 23 '16 at 07:51
1

@FiddlingBits Oh, is that because if you change the type of a variable, you don't need to change your `sizeof` code if you had written it as `sizeof(*ptr)` right? Is this the reason? – Utku Apr 23 '16 at 08:00
1

@Utku Correct! You may introduce a subtle bug if you change the type of the variable, but not what you're using `sizeof` on. – Fiddling Bits Apr 23 '16 at 22:54

score 0 · Answer 4 · answered Dec 27 '13 at 14:23

0

You can do this by dynamic memory allocation. Use malloc function.

answered Dec 27 '13 at 14:23

haccks

100,941
24
163
252

score 0 · Answer 5 · answered Dec 27 '13 at 14:25

0

malloc or calloc

YourType* ptr = malloc(sizeof(YourType)*NumberOfItemsYouNeed)

answered Dec 27 '13 at 14:25

Digital_Reality

4,220
1
24
31

2

[Do ***NOT*** cast the return value of `malloc()`](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858)! – Dec 27 '13 at 14:25
@H2CO3 Is that a misdemeanor or a felony? ;-) – Fiddling Bits Dec 27 '13 at 14:27
2

@FiddlingBits It's a deadly sin. :P – Dec 27 '13 at 14:27

How can I implement an array whose size is not known at compile time in C89?

5 Answers5