sizeof () returning unexpected value

[tbastian]

I have a structure which I need to know the size of. I wrote this test program.


#include <stdio.h>
#include <stdint.h>

#pragma pack ( 1 )

typedef struct {
uint16_t header_word;
uint16_t size;
uint8_t major_no;
uint8_t minor_no;
uint16_t trailer_word;
} header_t;

#pragma pack ()

int main ( int argc, char ** argv )
{
header_t header;
printf ( "header_t %d\n", sizeof ( header ));
return 0;
}

sizeof (header) returns 8, not 64! why?

[Compyx]

The sizeof operator yields a size in bytes, not bits. To get the number of bits, you need to multiply the value with CHAR_BIT, which the C standard guarantees to be at least 8.

By the way, to print a size_t (which is what sizeof 'returns') you should use either (C99):


printf("%zu\n", sizeof foo);

or (C89):


printf("%lu\n", (unsigned long)sizeof foo);

[tbastian]

The sizeof operator yields a size in bytes, not bits.


ahh, right. I knew that!:oops: