Re: [PATCH 1/5] qemu/qarray.h: introduce QArray

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Samstag, 21. August 2021 22:18:18 CEST Christian Schoenebeck wrote:
> Implements deep auto free of arrays while retaining common C-style
> squared bracket access.
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---

As I'm going to send a v2 anyway, I'll also just do some minor API comment 
changes in this patch, specifically ...

>  include/qemu/qarray.h | 148 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 148 insertions(+)
>  create mode 100644 include/qemu/qarray.h
> 
> diff --git a/include/qemu/qarray.h b/include/qemu/qarray.h
> new file mode 100644
> index 0000000000..230a556e81
> --- /dev/null
> +++ b/include/qemu/qarray.h
> @@ -0,0 +1,148 @@
> +/*
> + * QArray - deep auto free C-array
> + *
> + * Copyright (c) 2021 Crudebyte
> + *
> + * Authors:
> + *   Christian Schoenebeck <qemu_oss@crudebyte.com>
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> copy + * of this software and associated documentation files (the
> "Software"), to deal + * in the Software without restriction, including
> without limitation the rights + * to use, copy, modify, merge, publish,
> distribute, sublicense, and/or sell + * copies of the Software, and to
> permit persons to whom the Software is + * furnished to do so, subject to
> the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included
> in + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
> OR + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
> MERCHANTABILITY, + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
> IN NO EVENT SHALL + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
> CLAIM, DAMAGES OR OTHER + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
> TORT OR OTHERWISE, ARISING FROM, + * OUT OF OR IN CONNECTION WITH THE
> SOFTWARE OR THE USE OR OTHER DEALINGS IN + * THE SOFTWARE.
> + */
> +#ifndef QEMU_QARRAY_H
> +#define QEMU_QARRAY_H
> +
> +/**
> + * QArray provides a mechanism to access arrays in common C-style (e.g. by
> + * square bracket [] operator) in conjunction with reference variables that
> + * perform deep auto free of the array when leaving the scope of the auto
> + * reference variable. That means not only is the array itself
> automatically + * freed, but also memory dynamically allocated by the
> individual array + * elements.
> + *
> + * Example:
> + *
> + * Consider the following user struct @c Foo which shall be used as scalar
> + * (element) type of an array:
> + * @code
> + * typedef struct Foo {
> + *     int i;
> + *     char *s;
> + * } Foo;
> + * @endcode
> + * and assume it has the following function to free memory allocated by @c
> Foo + * instances:
> + * @code
> + * void free_foo(Foo *foo) {
> + *     free(foo->s);
> + * }
> + * @endcode
> + * Add the following to a shared header file:
> + * @code
> + * DECLARE_QARRAY_TYPE(Foo);
> + * @endcode
> + * and the following to a C unit file:
> + * @code
> + * DEFINE_QARRAY_TYPE(Foo, free_foo);
> + * @endcode
> + * Finally the array may then be used like this:
> + * @code
> + * void doSomething(int n) {
> + *     QArrayRef(Foo) foos = NULL;
> + *     QARRAY_CREATE(Foo, foos, n);
> + *     for (size_t i = 0; i < n; ++i) {
> + *         foos[i].i = i;
> + *         foos[i].s = calloc(4096, 1);
> + *         snprintf(foos[i].s, 4096, "foo %d", i);
> + *     }
> + * }
> + * @endcode
> + */
> +
> +/**
> + * Declares an array for the passed @a scalar_type.

"Declares an array *type* ...".

To not confuse it with declaring an array instance/variable.

> + *
> + * This is typically used from a shared header file.
> + *
> + * @param scalar_type - type of the individual array elements
> + */
> +#define DECLARE_QARRAY_TYPE(scalar_type) \
> +    typedef struct QArray##scalar_type { \
> +        size_t len; \
> +        scalar_type first[]; \
> +    } QArray##scalar_type; \
> +    \
> +    void qarray_create_##scalar_type(scalar_type **auto_var, size_t len); \
> +    void qarray_auto_free_##scalar_type(scalar_type **auto_var); \ +
> +/**
> + * Defines an array for the passed @a scalar_type and appropriate
> + * @a scalar_cleanup_func.

Likewise "Defines an array *type* ...".

Plus missing:

"This is typically used from a C unit file."

> + *
> + * @param scalar_type - type of the individual array elements
> + * @param scalar_cleanup_func - appropriate function to free memory
> dynamically + *                              allocated by individual array
> elements before + */
> +#define DEFINE_QARRAY_TYPE(scalar_type, scalar_cleanup_func) \
> +    void qarray_create_##scalar_type(scalar_type **auto_var, size_t len) \
> +    { \
> +        qarray_auto_free_##scalar_type(auto_var); \
> +        QArray##scalar_type *arr = g_malloc0(sizeof(QArray##scalar_type) +
> \ +            len * sizeof(scalar_type)); \
> +        arr->len = len; \
> +        *auto_var = &arr->first[0]; \
> +    } \
> +    \
> +    void qarray_auto_free_##scalar_type(scalar_type **auto_var) \
> +    { \
> +        scalar_type *first = (*auto_var); \
> +        if (!first) { \
> +            return; \
> +        } \
> +        QArray##scalar_type *arr = (QArray##scalar_type *) ( \
> +            ((char *)first) - offsetof(QArray##scalar_type, first) \
> +        ); \
> +        for (size_t i = 0; i < arr->len; ++i) { \
> +            scalar_cleanup_func(&arr->first[i]); \
> +        } \
> +        g_free(arr); \
> +    } \
> +
> +/**
> + * Used to declare a reference variable (unique pointer) for an array.
> After + * leaving the scope of the reference variable, the associated array
> is + * automatically freed.
> + *
> + * @param scalar_type - type of the individual array elements
> + */
> +#define QArrayRef(scalar_type) \
> +    __attribute((__cleanup__(qarray_auto_free_##scalar_type))) scalar_type*
> +
> +/**
> + * Allocates a new array of passed @a scalar_type with @a len number of
> array + * elements and assigns the created array to the reference variable
> + * @a auto_var.
> + *
> + * @param scalar_type - type of the individual array elements
> + * @param auto_var - destination reference variable
> + * @param len - amount of array elements to be allocated immediately
> + */
> +#define QARRAY_CREATE(scalar_type, auto_var, len) \
> +    qarray_create_##scalar_type((&auto_var), len)
> +
> +#endif /* QEMU_QARRAY_H */

Best regards,
Christian Schoenebeck