[libc-commits] [libc] add mremap (PR #112804)

[Michael Jones via libc-commits]

================
@@ -0,0 +1,53 @@
+//===---------- Linux implementation of the POSIX mremap function
+//-----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/sys/mman/mremap.h"
+
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+
+#include "src/__support/macros/config.h"
+#include "src/errno/libc_errno.h"
+#include <linux/param.h> // For EXEC_PAGESIZE.
+#include <stdarg.h>
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(void *, mremap,
+                   (void *old_address, size_t old_size, size_t new_size,
+                    int flags, ... /* void *new_address */)) {
+
+#ifdef SYS_mremap
+  long syscall_number = SYS_mremap;
+#else
+#error "mremap syscalls not available."
+#endif
+  long ret = 0;
+  if (flags & MREMAP_FIXED) {
+    void *arg;
+    va_list varargs;
+    va_start(varargs, flags);
+    arg = va_arg(varargs, void *);
+    va_end(varargs);
+    ret = LIBC_NAMESPACE::syscall_impl(syscall_number, old_address, old_size,
+                                       new_size, flags, arg);
+  } else {
+    ret = LIBC_NAMESPACE::syscall_impl(syscall_number, old_address, old_size,
+                                       new_size, flags);
+  }
----------------
michaelrj-google wrote:

if you make `arg` default to `nullptr`, you can pass it to the syscall regardless of if the user specified it.
```suggestion
  void *new_address = nullptr;
  if (flags & MREMAP_FIXED) {
    va_list varargs;
    va_start(varargs, flags);
    new_address = va_arg(varargs, void *);
    va_end(varargs);
  }
  ret = LIBC_NAMESPACE::syscall_impl(syscall_number, old_address, old_size,
                                       new_size, flags, new_address);
```

https://github.com/llvm/llvm-project/pull/112804