bprint.h

/*
 * Copyright (c) 2012 Nicolas George
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#ifndef AVUTIL_BPRINT_H
#define AVUTIL_BPRINT_H

#include "attributes.h"

/**
 * Define a structure with extra padding to a fixed size
 * This helps ensuring binary compatibility with future versions.
 */
#define FF_PAD_STRUCTURE(size, ...) \
    __VA_ARGS__ \
    char reserved_padding[size - sizeof(struct { __VA_ARGS__ })];

/**
 * Buffer to print data progressively
 *
 * The string buffer grows as necessary and is always 0-terminated.
 * The content of the string is never accessed, and thus is
 * encoding-agnostic and can even hold binary data.
 *
 * Small buffers are kept in the structure itself, and thus require no
 * memory allocation at all (unless the contents of the buffer is needed
 * after the structure goes out of scope). This is almost as lightweight as
 * declaring a local "char buf[512]".
 *
 * The length of the string can go beyond the allocated size: the buffer is
 * then truncated, but the functions still keep account of the actual total
 * length.
 *
 * In other words, buf->len can be greater than buf->size and records the
 * total length of what would have been to the buffer if there had been
 * enough memory.
 *
 * Append operations do not need to be tested for failure: if a memory
 * allocation fails, data stop being appended to the buffer, but the length
 * is still updated. This situation can be tested with
 * av_bprint_is_complete().
 *
 * The size_max field determines several possible behaviours:
 *
 * size_max = -1 (= UINT_MAX) or any large value will let the buffer be
 * reallocated as necessary, with an amortized linear cost.
 *
 * size_max = 0 prevents writing anything to the buffer: only the total
 * length is computed. The write operations can then possibly be repeated in
 * a buffer with exactly the necessary size
 * (using size_init = size_max = len + 1).
 *
 * size_max = 1 is automatically replaced by the exact size available in the
 * structure itself, thus ensuring no dynamic memory allocation. The
 * internal buffer is large enough to hold a reasonable paragraph of text,
 * such as the current paragraph.
 */
typedef struct AVBPrint {
    FF_PAD_STRUCTURE(1024,
    char *str;         /** string so far */
    unsigned len;      /** length so far */
    unsigned size;     /** allocated memory */
    unsigned size_max; /** maximum allocated memory */
    char reserved_internal_buffer[1];
    )
} AVBPrint;

/**
 * Init a print buffer.
 *
 * @param buf        buffer to init
 * @param size_init  initial size (including the final 0)
 * @param size_max   maximum size;
 *                   0 means do not write anything, just count the length;
 *                   1 is replaced by the maximum value for automatic storage
 */
void av_bprint_init(AVBPrint *buf, unsigned size_init, unsigned size_max);

/**
 * Convenience macros for special values for size_max.
 */
#define AV_BPRINT_SIZE_UNLIMITED  ((unsigned)-1)
#define AV_BPRINT_SIZE_AUTOMATIC  1
#define AV_BPRINT_SIZE_COUNT_ONLY 0

/**
 * Append a formated string to a print buffer.
 */
void av_bprintf(AVBPrint *buf, const char *fmt, ...) av_printf_format(2, 3);

/**
 * Append char c n times to a print buffer.
 */
void av_bprint_chars(AVBPrint *buf, char c, unsigned n);

/**
 * Test if the print buffer is complete (not truncated).
 *
 * It may have been truncated due to a memory allocation failure
 * or the size_max limit (compare size and size_max if necessary).
 */
static inline int av_bprint_is_complete(AVBPrint *buf)
{
    return buf->len < buf->size;
}

/**
 * Finalize a print buffer.
 *
 * The print buffer can no longer be used afterwards,
 * but the len and size fields are still valid.
 *
 * @arg[out] ret_str  if not NULL, used to return a permanent copy of the
 *                    buffer contents, or NULL if memory allocation fails;
 *                    if NULL, the buffer is discarded and freed
 * @return  0 for success or error code (probably AVERROR(ENOMEM))
 */
int av_bprint_finalize(AVBPrint *buf, char **ret_str);

#endif /* AVUTIL_BPRINT_H */