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

fusl/src/internal/shgetc.c

+#include "shgetc.h"
+
+void __shlim(FILE *f, off_t lim)
+{
+        f->shlim = lim;
+        f->shcnt = f->rend - f->rpos;
+        if (lim && f->shcnt > lim)
+                f->shend = f->rpos + lim;
+        else
+                f->shend = f->rend;
+}
+
+int __shgetc(FILE *f)
+{
+        int c;
+        if (f->shlim && f->shcnt >= f->shlim || (c=__uflow(f)) < 0) {
+                f->shend = 0;
+                return EOF;
+        }
+        if (f->shlim && f->rend - f->rpos > f->shlim - f->shcnt - 1)
+                f->shend = f->rpos + (f->shlim - f->shcnt - 1);
+        else
+                f->shend = f->rend;
+        if (f->rend) f->shcnt += f->rend - f->rpos + 1;
+        if (f->rpos[-1] != c) f->rpos[-1] = c;
+        return c;
+}