The API contains many synchronization primitives that allow threaded code to be efficiently written. Unfortunately, Microsoft Windows does not support this interface as-is.
Also, notice that the address returned by mmap is not 64KB aligned - it can be arbitrary but of course will always be page- that is, 4KB- aligned. In fact, due to the "twist" added by Linux, it is likely that the address returned by mmap will be directly adjacent to some existing virtual address e.
All of this is explained by how Linux represents the mappings. The kernel calls them VMAs, which stand for a "virtual memory area".
Unlike Windows allocations, VMAs can be split by the kernel whenever a page in the range gets unmapped or its attributes change as a result of a mprotect call among other reasons. All VMAs taken together describe how your program's Win32 api and posix api comparison space looks like and can be thought of as a "shadow" and hardware-independent page table.
In theory, since kernel already maintains the page table for the hardware, it does not need another one. They were however introduced to Linux to solve two main problems: As Linus described in an e-mail from 17 years ago: We used to have these "this is a COW page" and "this is shared writable" bits in the page table etc - there are two sw bits on x86, and I think we used them both.
These days, the vma's just have too much information, and the page tables can't be counted on to have enough bits. So, each time you do a mmapyou're either creating a VMA or enlarging an existing one, because the kernel is smart enough to place the new mapping adjacent to some other one with the same attributes, thus merging them.
However, since VMA only represents a contiguous virtual memory region, it means that not only mmap can create them, but munmap as well.
In the above example where we unmapped the middle of our 16KB allocationwe actually created two new VMAs of 4KB each, because they are no longer adjacent to each other one starts at 0x7ff and ends at 0x7ff01fff, another at 0x7ff and 0x7ff04fff respectively.
So far so good. However, remember that a VMA structure holds "too much information", and it is larger than the page entry in the hardware page table. A programmer's intuition immediately suggests that the very existence of VMAs puts another limit on how fine-grained the mappings can be - and indeed it does.
A degenerate case - putting each virtual page in its own separate VMA - would take too many resources, so Linux limits the number of mappings. The limit is vm. This also introduces another limit on the maximum amount of virtual memory you could allocate which normally, similarly to the commit limit on Windows, depends on the physical RAM and swap space, governed by the vm.
Why is there a limit on the number of VMAs at all? If you have root, you can jack up vm. However, the number of VMAs is limited not just because of their size, but also because they need to be traversed during each page faultwhich is, well, very often.
And most importantly, even if you can increase the limit, you absolutely cannot rely on your users being able or willing to increase the system-wide limit just to run your program. This means that in practice, you need to keep your mappings down to a reasonable number by avoiding unmapping memory with munmapchanging page protection with mprotect or changing page properties with madvise for parts of a mapping which can split it in two or even in three VMAs.
It is rather unfortunate that none of the traditional Linux monitoring tools pay attention to the number of mappings that a process has, which changes unpredictably as your address space fragments itself due to the above calls.
Conclusions It's time to make a side by side comparison of the most important traits: Windows Linux Virtual memory range can be just reserved or actually mapped "committed", i.
Mapping can be "decommitted", converting it back to just a reservation. Creating a mapping both reserves the address space and allows access to its contents, i. Each mapping is a separate allocation that is book-kept by the OS. Number of calls to VirtualFree must match the number of calls to VirtualAlloc and the same pointer must be passed - no "partial" frees.
Mappings can be merged and split as needed by mmap and munmap. Number of calls to munmap can be larger or smaller than the number of calls to mmap as long as all mappings are removed. It is up to the program to book keep the sizes of mappings.
Mappings always start at a 64KB-aligned boundary. Asking for less than 64KB of the virtual adress space on Windows is suboptimal and results in waste fragmentation of the address space. Mappings can start at an arbitrary page-aligned address. You can map single pages one by one with no ill effects on the address space the kernel will try to "attach" new mappings to existing ones.
No limit on the number of mappings the performance of VirtualAlloc will degrade, but it will not refuse to allocate. There is a limit on the number of mappings pretty low, You cannot easily know the number of mappings, as they can be created whenever you partially unmap existing mappings or change properties with mprotect or madvise.
You can reserve up to TB of virtual address space, however the commit limit depends on your physical RAM and swap size.
Since all mappings are always committed, the limit on the virtual address space is much lower and depends on the RAM and swap size by default. Of this, the most important differences are perhaps the two last ones.OSDN > Find Software > Software Development > Code Generators > MinGW - Minimalist GNU for Windows > Download File List > Package r-bridal.com POSIX Threads API (pttheads-win32).
Cygwin consists of a library that implements the POSIX system call API in terms of Win32 system calls, a GNU development toolchain (including GCC and GDB) to allow software development, and running of a large number of application programs equivalent to those on Unix systems.
DESCRIPTION. Perl on Win32 contains several functions to access Win32 APIs.
Some are included in Perl itself (on Win32) and some are only available after explicitly requesting the Win32 module with. The Win32 API is implemented by a "Client-Side DLL" that is specific to the Win32 Subsystem.
The Win32 Subsystem is just one of Windows NT’s Operating System (OS) Emulation Subsystems. All of NT’s OS Environment Subsystems (Win32, POSIX, OS/2, and DOS/WoW) utilize services provided by the Windows NT Executive.
Comparison between threading mechanism in Win32 and POSIX systems PThreads Vs Win32 Threads the basic execution unit is the processSome may claim POSIX threads are a low-level API and Windows threads are a high-level APIIn Windows the thread scheduling code is implemented in the r-bridal.com Linux realm, there are several.
Dec 19, · AF_UNIX comes to Windows Finally, how does the performance of this feature compare with other IPC mechanisms, such as a Windows named pipe, a loopback TCP/IP socket, a shared memory segment, ALPC (perhaps via MSRPC or similar), or so on? I wouldn’t need the exact same Linux/POSIX API.
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