为什么这样的结构包含两个仅包含一个元素的数组字段? [英] Why does such a struct contain two array fields containing only one element?

问题描述

请注意:此问题不是重复的内容(一个struct中的元素数组)

以下代码摘自Linux内核源代码(版本:3.14)

struct files_struct
{
    atomic_t count;
    struct fdtable __rcu *fdt;
    struct fdtable fdtab;

    spinlock_t file_lock ____cacheline_aligned_in_smp;
    int next_fd;
    unsigned long close_on_exec_init[1];
    unsigned long open_fds_init[1];
    struct file __rcu * fd_array[NR_OPEN_DEFAULT];
};

我只是想知道为什么将close_on_exec_init和open_fds_init定义为包含一个元素的数组，而不是仅仅将其定义为unsigned long close_on_exec_init;和unsigned long open_fds_init;.

解决方案

这些字段是一种优化，因此Linux不必为不超过BITS_PER_LONG打开文件描述符的典型进程执行尽可能多的分配. /p>

close_on_exec_init字段为分配了files_struct的fdt->close_on_exec提供初始存储. (请参阅fs/file.c中的dup_fd.)

如果相应的文件描述符已设置"close-on-exec"标志，则设置fdt->close_on_exec的每个位.因此，如果进程具有比unsigned long中的位数更多的打开文件描述符，则Linux仅需要为fdt->close_on_exec分配额外的空间.

open_fds_init字段与fdt->open_fds字段具有相同的功能. fd_array字段与fdt->fd字段具有相同的功能. (请注意，fd_array的大小为BITS_PER_LONG.)

close_on_exec_init和open_fds_init字段以前的类型为struct embedded_fd_set，但在此提交.提交消息没有解释为什么作者选择使用单元素数组而不是纯标量.也许作者(David Howells)只是想避免使用&运算符.

Please Note: This question is not a duplicate of ( One element array in struct )

The following code is excerpted from the Linux kernel source (version: 3.14)

struct files_struct
{
    atomic_t count;
    struct fdtable __rcu *fdt;
    struct fdtable fdtab;

    spinlock_t file_lock ____cacheline_aligned_in_smp;
    int next_fd;
    unsigned long close_on_exec_init[1];
    unsigned long open_fds_init[1];
    struct file __rcu * fd_array[NR_OPEN_DEFAULT];
};

I just wonder why close_on_exec_init and open_fds_init are defined as arrays containing one element, rather than just defined as unsigned long close_on_exec_init; and unsigned long open_fds_init;.

解决方案

These fields are an optimization so Linux doesn't have to perform as many allocations for a typical process that has no more than BITS_PER_LONG open file descriptors.

The close_on_exec_init field provides the initial storage for fdt->close_on_exec when a files_struct is allocated. (See dup_fd in fs/file.c.)

Each bit of fdt->close_on_exec is set if the corresponding file descriptor has the "close-on-exec" flag set. Thus Linux only needs to allocate additional space for fdt->close_on_exec if the process has more open file descriptors than the number of bits in an unsigned long.

The open_fds_init field serves the same function for the fdt->open_fds field. The fd_array field serves the same function for the fdt->fd field. (Note that fd_array has a size of BITS_PER_LONG.)

The close_on_exec_init and open_fds_init fields formerly had type struct embedded_fd_set, but were changed to bare arrays in this commit. The commit message doesn't explain why the author chose to use one-element arrays instead of bare scalars. Perhaps the author (David Howells) simply wanted to avoid using the & operator.

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