Incorrectly initialized bit fields in structure

[negge]

Creating a struct that uses packed bit fields does not match the C specification with differently sized types. For example:

#include <stdio.h>

typedef struct key key;

struct key {
  struct {
    unsigned char type : 2;
    unsigned short row : 12;
  };
};

#define MAX_ROW (4*60*60/8)

typedef enum {
  STEP = 0,
  SMOOTH = 1,
  LINEAR = 2,
  RAMP = 3,
} simp_type;

static char *TYPE_STR[] = {
  "STEP", "SMOOTH", "LINEAR", "RAMP",
};

key keys[] = {
   { .row = MAX_ROW, .type = RAMP },
};

int main() {
  printf("sizeof(key) = %i\r\n", sizeof(key));
  printf("keys[0] = { .row = %3i, .type = %s }\r\n",
   keys[0].row, TYPE_STR[keys[0].type]);
  printf("%04x", *(int *)&keys[0]);
}

when built with Version 2.0 beta Mar 1 2024 01:51:55 (64-bit) prints the output:

sizeof(key) = 4
keys[0] = { .row = 1800, .type = RAMP }
7080003

However building with gcc -m32 keys.c -o keys gives the following output:

$ ./keys
sizeof(key) = 2
keys[0] = { .row = 1800, .type = RAMP }
1c23

[jmalak]

For which standard?
Do you thing that gcc in default mode follow standard?
Anyway your key structure is invalid by C90 standard.
Both compilers are using some extension and default pack size which is not same.
What command line you use for OW compiler and what target.

Anyway OW compiler process your structure following way

unsigned char type : 2;  // allocate byte member and use 2 bits

unsigned short row : 12; // different type therefore cannot continue previous member, 
                            // need allocate 2-bytes member and need align it on 2-byte boundary

Therefore size is 4-bytes.

What is done by gcc is completely different then C standard.
It looks like gcc pack all bits into one member and ignore defined type alignment.

[jmalak]

If you want bits be packed by OW as by gcc then use same type as

struct key {
struct {
unsigned short type : 2;
unsigned short row : 12;
};
};

[negge]

If you want bits be packed by OW as by gcc then use same type as

struct key { struct { unsigned short type : 2; unsigned short row : 12; }; };

Thank you, I modified the example for both fields as short and the bit layout is now correct. However, I am still seeing strange things in the initializer. Code is now:

#include <stdio.h>

typedef struct key key;

struct key {
  struct {
    unsigned short type : 2;
    unsigned short row : 12;
  };
};

#define MAX_ROW (4*60*60/8)

typedef enum {
  STEP = 0,
  SMOOTH = 1,
  LINEAR = 2,
  RAMP = 3,
} simp_type;

static char *TYPE_STR[] = {
  "STEP", "SMOOTH", "LINEAR", "RAMP",
};

key keys[] = {
   { .row = MAX_ROW, .type = RAMP },
};

int main() {
  printf("sizeof(key) = %i\r\n", sizeof(key));
  printf("keys[0] = { .row = %3i, .type = %s }\r\n",
   keys[0].row, TYPE_STR[keys[0].type]);
  printf("%04x\r\n", *(int *)&keys[0]);
  /* set values again */
  keys[0].row = MAX_ROW;
  keys[0].type = RAMP;
  printf("keys[0] = { .row = %3i, .type = %s }\r\n",
   keys[0].row, TYPE_STR[keys[0].type]);
  printf("%04x", *(int *)&keys[0]);
}

Now prints out:

sizeof(key) = 2
keys[0] = { .row =   0, .type = RAMP }
0003
keys[0] = { .row = 1800, .type = RAMP }
1c23

But gcc -m32 keys.c -o keys gives the output:

$ ./keys
sizeof(key) = 2
keys[0] = { .row = 1800, .type = RAMP }
1c23
keys[0] = { .row = 1800, .type = RAMP }
1c23

[jmalak]

It looks like bug in static initialization of bit fields.

[jmalak]

#1283
#1274