string compare

[James Antill]

Mark Bluemel wrote:
...
I think Santosh was suggesting a structure approach like this :-

typedef struct {
short length;
char data[]; /* or "char data[1];" if c99 support isn't
available */
} my_string;

so if you had a string of 32 characters to work with, you'd do
something like:-

my_string *string_pointer = malloc(sizeof(short) + 32);

That assumes there are no padding bytes between length and data. The
right way to calculate the allocation is

#include <stddef.h>
...
mystring *string_pointer = malloc(offsetof(my_string, data) +
32);

This is required to be the same as sizeof(mystring),

Citation, please?

6.7.2.1

#16

As a special case, the last element of a structure with more than
one named member may have an incomplete array type; [...] the
size of the structure shall be equal to the offset of the last
element of an otherwise identical structure that replaces the
flexible array member with an array of unspecified length

#17 even gives an example basically identical to what I posted.

 

[Harald van Dijk]

typedef struct {
short length;
char data[]; /* or "char data[1];" if c99 support isn't
available */
} my_string;

#include <stddef.h>
...
mystring *string_pointer = malloc(offsetof(my_string, data) +
32);

This is required to be the same as sizeof(mystring),

Citation, please?

6.7.2.1

#16

As a special case, the last element of a structure with more than one
named member may have an incomplete array type; [...] the size of the
structure shall be equal to the offset of the last element of an
otherwise identical structure that replaces the flexible array member
with an array of unspecified length

This states that sizeof(my_string) must be equal to offsetof(struct {
short length;
char data[n];
}, data) for some fixed n, but that might be different from
offsetof(my_string, data).

#17 even gives an example basically identical to what I posted.

#17 even explicitly mentions that the offset of a flexible array member
might differ from the offset of a non-flexible array member.