Add a "fork" of musl as //fusl.

fusl/src/stdio/fmemopen.c

+#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);
+}