Re: [PATCH 1/4] stringbuf: A string buffer implementation

[Eric St-Laurent <ericstl34@sympatico.ca>]

On Tue, 2007-10-23 at 17:12 -0400, Matthew Wilcox wrote:
> Consecutive calls to printk are non-atomic, which leads to various
> implementations for accumulating strings which can be printed in one call.
> This is a generic string buffer which can also be used for non-printk
> purposes.  There is no sb_scanf implementation yet as I haven't identified
> a user for it.
> 
> +
> +struct stringbuf {
> +	char *s;
> +	int alloc;
> +	int len;
> +};
> +

I don't know if copy-on-write semantics are really useful for current
in-kernel uses, but I've coded and used a C++ string class like this in
the past:

struct string_data
{
	int nrefs;
	unsigned len;
	unsigned capacity;
	//char data[capacity];	/* allocated along string_data */
};

struct string	/* or typedef in C... */
{
	struct string *data;
};

[ struct string_data is a hidden implementation detail, only struct
string is exposed ]

Multiple string objects can share the same data, by increasing the nrefs
count, a new data is allocated if the string is modified and nrefs > 1.

Not having to iterate over the string to calculate it's length,
allocating a larger buffer to eliminate re-allocation and copy-on-write
semantics make a string like this a vast performance improvement over a
normal C string for a minimal (about 3 ints per data buffer) memory
cost.

By using it correctly it can prevents buffer overflows.

You still always null terminate the string stored in data to directly
use it a normal C string.

You also statically allocate an empty string which is shared by all
"uninitialized" or empty strings.


Even without copy-on-write semantics and reference counting, I think
this approach is better because it uses 1 less "object" and allocation:

struct string - "handle" (pointer really) to string data
struct string_data - string data

versus:

struct stringbuf *sb - pointer to string object
struct stringbuf - string object
char *s (member of stringbuf) - string data



Best regards,

- Eric


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