Why does such a struct contain two array fields containing only one element?

Question

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

The following code is excerpted from the Linux kernel source (versi

Answer 1

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.