C Programming #84: flexible array member

Following article explores the Flexible array member or also called array of length zero. It is very useful when you don't know the size during compile time.

Consider following code

#include <stdio.h>
int main()
{
   char a[0];
   printf("sizeof a is %d\n", sizeof(a));
   return 0;
}

sizeof a is 0

Here a is array of size 0, there are no elements in a. a takes no memory at all. You might think there is no use for such a C construct. It is used in very useful one of useful structure called variable length object.

struct buf_tag {
  int len;
  char data[0];
};

Here data can be of any length, memory allocated using malloc (dynamic behaviour). Only last member of struct can be flexible array member.

Usage of is explored in below program.

#include <stdio.h>
typedef struct buf_tag {
  int len;
  char data[0];
}buf_t;
int main()
{
   buf_t *a;

   a = malloc(sizeof(buf_t) + 3);
   a->len = 3;
   a->data[0] = 10;
   a->data[1] = 11;
   a->data[2] = 12;

   printf("a len is %d\n", a->len);
   for(int i = 0; i < a->len; i++) {
      printf("data[%d] is %d\n", i, a->data[i]);
   }

   return 0;
}

a len is 3
data[0] is 10
data[1] is 11
data[2] is 12

Above syntax that was used was GNU C standard. C99 dictates that it should be as follows

typedef struct buf_tag {
  int len;
  char data[];
}buf_t;

Hence for writing more portable code it is recommended the above way. Make sure allocation matches the len correctly otherwise we could run into issue easily.

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