How do I put an array in a struct in C?

Redis uses a string encoding which it calls “Simple Dynamic Strings”. Simplified, it looks like:

struct sds {
  size_t len;
  char buf[];
};

The interesting thing here is the buf field: it has no defined length. Surely, then, this is a “decayed array”, i.e. a pointer to an array of chars? But no!:

#include <stdio.h>

struct sds {
  size_t len;
  char buf[];
};

int main() {
  struct sds str;

  printf("sizeof(str) = %zu\n", sizeof(str));

  printf("sizeof(str.len) = %zu\n", sizeof(str.len));

  // printf("sizeof(str.buf) = %zu\n", sizeof(str.buf));
  // "invalid application of 'sizeof' to an incomplete type 'char []'"

  return 0;
}

% ./a.out
sizeof(str) = 8
sizeof(str.len) = 8

The buf field has the incomplete type char [], and yet struct sds is a complete type with size 8 bytes.

It seems C allows a single such array type at the end of the struct definition. It is invalid to have two such arrays:

struct sds {
  size_t len;
  char buf[];
  char buf2[];  // error: field has incomplete type 'char []'
};

This makes sense, because C must be able to determine the offset of every field, but the unknown length of buf makes it impossible to determine the offset of buf2. For the same reason, it is invalid to put the incomplete type in the middle of the list:

struct sds {
  char buf[];  // error: field has incomplete type 'char []'
  size_t len;
};

The size of the buf field is 0 bytes, and you become responsible for maintaining the size of the allocation. So to create such an allocation, we do:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct sds {
  size_t len;
  char buf[];
};

int main() {
  struct sds * str = malloc(sizeof(struct sds) + sizeof("foo"));
  str->len = sizeof("foo");
  strcpy(str->buf, "foo");
  return 0;
}

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