undeclared identifier posix_fallocate() and fallocate() in macOS, clang 12.00 [duplicate] - c++

Question

Is there a fallocate() equivalent in OS X?
I would like to aggregate all of those equivalent in OS X questions into some doc/table or whatever for everyone. Anybody knows something familiar?

Answer 1

What about using:
mkfile -n 1m test.tmp
It's not the same command but serves the same purpose.
Note that fallocate uses decimal multipliers, whereas mkfile uses binary multipliers.
mkfile man

Answer 2

fallocate() doesn't exist on OSX. You can "fake" it though; Mozilla fakes it in their FileUtils class. See this file:
http://hg.mozilla.org/mozilla-central/file/3d846420a907/xpcom/glue/FileUtils.cpp#l61
Here's the code, in case that link goes stale:
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* ***** BEGIN LICENSE BLOCK *****
* Version: MPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
* http://www.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is Mozilla code.
*
* The Initial Developer of the Original Code is
* Mozilla Foundation.
* Portions created by the Initial Developer are Copyright (C) 2010
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
* Taras Glek <tglek#mozilla.com>
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
* in which case the provisions of the GPL or the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of either the GPL or the LGPL, and not to allow others to
* use your version of this file under the terms of the MPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the GPL or the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of any one of the MPL, the GPL or the LGPL.
*
* ***** END LICENSE BLOCK ***** */
#if defined(XP_UNIX)
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#elif defined(XP_WIN)
#include <windows.h>
#endif
#include "nscore.h"
#include "private/pprio.h"
#include "mozilla/FileUtils.h"
bool
mozilla::fallocate(PRFileDesc *aFD, PRInt64 aLength)
{
#if defined(HAVE_POSIX_FALLOCATE)
return posix_fallocate(PR_FileDesc2NativeHandle(aFD), 0, aLength) == 0;
#elif defined(XP_WIN)
return PR_Seek64(aFD, aLength, PR_SEEK_SET) == aLength
&& 0 != SetEndOfFile((HANDLE)PR_FileDesc2NativeHandle(aFD));
#elif defined(XP_MACOSX)
int fd = PR_FileDesc2NativeHandle(aFD);
fstore_t store = {F_ALLOCATECONTIG, F_PEOFPOSMODE, 0, aLength};
// Try to get a continous chunk of disk space
int ret = fcntl(fd, F_PREALLOCATE, &store);
if(-1 == ret){
// OK, perhaps we are too fragmented, allocate non-continuous
store.fst_flags = F_ALLOCATEALL;
ret = fcntl(fd, F_PREALLOCATE, &store);
if (-1 == ret)
return false;
}
return 0 == ftruncate(fd, aLength);
#elif defined(XP_UNIX)
// The following is copied from fcntlSizeHint in sqlite
/* If the OS does not have posix_fallocate(), fake it. First use
** ftruncate() to set the file size, then write a single byte to
** the last byte in each block within the extended region. This
** is the same technique used by glibc to implement posix_fallocate()
** on systems that do not have a real fallocate() system call.
*/
struct stat buf;
int fd = PR_FileDesc2NativeHandle(aFD);
if (fstat(fd, &buf))
return false;
if (buf.st_size >= aLength)
return false;
const int nBlk = buf.st_blksize;
if (!nBlk)
return false;
if (ftruncate(fd, aLength))
return false;
int nWrite; // Return value from write()
PRInt64 iWrite = ((buf.st_size + 2 * nBlk - 1) / nBlk) * nBlk - 1; // Next offset to write to
do {
nWrite = 0;
if (PR_Seek64(aFD, iWrite, PR_SEEK_SET) == iWrite)
nWrite = PR_Write(aFD, "", 1);
iWrite += nBlk;
} while (nWrite == 1 && iWrite < aLength);
return nWrite == 1;
#endif
return false;
}

Answer 3

For those wanting to create fake data files for testing, mkfile is pretty elegant. An alternative is to use dd:
dd if=/dev/zero of=zfile count=1024 bs=1024
As you can see with od -b zfile, it's full of zeros. If you want random data (which you may want for testing workflows with data compression, for example), then use "/dev/random" in place of "/dev/zero":
dd if=/dev/random of=randfile count=1024 bs=1024