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

fusl/src/stdio/open_memstream.c

+#include "stdio_impl.h"
+#include <errno.h>
+#include <limits.h>
+#include <string.h>
+
+struct cookie {
+        char **bufp;
+        size_t *sizep;
+        size_t pos;
+        char *buf;
+        size_t len;
+        size_t space;
+};
+
+static off_t ms_seek(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_MAX-base) goto fail;
+        return c->pos = base+off;
+}
+
+static size_t ms_write(FILE *f, const unsigned char *buf, size_t len)
+{
+        struct cookie *c = f->cookie;
+        size_t len2 = f->wpos - f->wbase;
+        char *newbuf;
+        if (len2) {
+                f->wpos = f->wbase;
+                if (ms_write(f, f->wbase, len2) < len2) return 0;
+        }
+        if (len + c->pos >= c->space) {
+                len2 = 2*c->space+1 | c->pos+len+1;
+                newbuf = realloc(c->buf, len2);
+                if (!newbuf) return 0;
+                *c->bufp = c->buf = newbuf;
+                memset(c->buf + c->space, 0, len2 - c->space);
+                c->space = len2;
+        }
+        memcpy(c->buf+c->pos, buf, len);
+        c->pos += len;
+        if (c->pos >= c->len) c->len = c->pos;
+        *c->sizep = c->pos;
+        return len;
+}
+
+static int ms_close(FILE *f)
+{
+        return 0;
+}
+
+FILE *open_memstream(char **bufp, size_t *sizep)
+{
+        FILE *f;
+        struct cookie *c;
+        char *buf;
+
+        if (!(f=malloc(sizeof *f + sizeof *c + BUFSIZ))) return 0;
+        if (!(buf=malloc(sizeof *buf))) {
+                free(f);
+                return 0;
+        }
+        memset(f, 0, sizeof *f + sizeof *c);
+        f->cookie = c = (void *)(f+1);
+
+        c->bufp = bufp;
+        c->sizep = sizep;
+        c->pos = c->len = c->space = *sizep = 0;
+        c->buf = *bufp = buf;
+        *buf = 0;
+
+        f->flags = F_NORD;
+        f->fd = -1;
+        f->buf = (void *)(c+1);
+        f->buf_size = BUFSIZ;
+        f->lbf = EOF;
+        f->write = ms_write;
+        f->seek = ms_seek;
+        f->close = ms_close;
+
+        if (!libc.threaded) f->lock = -1;
+
+        return __ofl_add(f);
+}