source: azure_iot_hub/trunk/musl-1.1.18/src/stdio/fmemopen.c@ 388

#include "stdio_impl.h"
#include <errno.h>
#include <string.h>
#include <inttypes.h>

struct cookie {
        size_t pos, len, size;
        unsigned char *buf;
        int mode;
};

static off_t mseek(FILE *f, off_t off, int whence)
{
        ssize_t base;
        struct cookie *c = f->cookie;
        if (whence>2U) {
fail:
                errno = EINVAL;
                return -1;
        }
        base = (size_t [3]){0, c->pos, c->len}[whence];
        if (off < -base || off > (ssize_t)c->size-base) goto fail;
        return c->pos = base+off;
}

static size_t mread(FILE *f, unsigned char *buf, size_t len)
{
        struct cookie *c = f->cookie;
        size_t rem = c->len - c->pos;
        if (c->pos > c->len) rem = 0;
        if (len > rem) {
                len = rem;
                f->flags |= F_EOF;
        }
        memcpy(buf, c->buf+c->pos, len);
        c->pos += len;
        rem -= len;
        if (rem > f->buf_size) rem = f->buf_size;
        f->rpos = f->buf;
        f->rend = f->buf + rem;
        memcpy(f->rpos, c->buf+c->pos, rem);
        c->pos += rem;
        return len;
}

static size_t mwrite(FILE *f, const unsigned char *buf, size_t len)
{
        struct cookie *c = f->cookie;
        size_t rem;
        size_t len2 = f->wpos - f->wbase;
        if (len2) {
                f->wpos = f->wbase;
                if (mwrite(f, f->wpos, len2) < len2) return 0;
        }
        if (c->mode == 'a') c->pos = c->len;
        rem = c->size - c->pos;
        if (len > rem) len = rem;
        memcpy(c->buf+c->pos, buf, len);
        c->pos += len;
        if (c->pos > c->len) {
                c->len = c->pos;
                if (c->len < c->size) c->buf[c->len] = 0;
                else if ((f->flags&F_NORD) && c->size) c->buf[c->size-1] = 0;
        }
        return len;
}

static int mclose(FILE *m)
{
        return 0;
}

FILE *fmemopen(void *restrict buf, size_t size, const char *restrict mode)
{
        FILE *f;
        struct cookie *c;
        int plus = !!strchr(mode, '+');
        
        if (!size || !strchr("rwa", *mode)) {
                errno = EINVAL;
                return 0;
        }

        if (!buf && size > SIZE_MAX-sizeof(FILE)-BUFSIZ-UNGET) {
                errno = ENOMEM;
                return 0;
        }

        f = calloc(sizeof *f + sizeof *c + UNGET + BUFSIZ + (buf?0:size), 1);
        if (!f) return 0;
        f->cookie = c = (void *)(f+1);
        f->fd = -1;
        f->lbf = EOF;
        f->buf = (unsigned char *)(c+1) + UNGET;
        f->buf_size = BUFSIZ;
        if (!buf) buf = f->buf + BUFSIZ;

        c->buf = buf;
        c->size = size;
        c->mode = *mode;
        
        if (!plus) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
        if (*mode == 'r') c->len = size;
        else if (*mode == 'a') c->len = c->pos = strnlen(buf, size);

        f->read = mread;
        f->write = mwrite;
        f->seek = mseek;
        f->close = mclose;

        if (!libc.threaded) f->lock = -1;

        return __ofl_add(f);
}