linux内核中的文件描述符(6)-fd的分配-expand_files

linux内核中的文件描述符(六)--fd的分配--expand_files

linux内核中的文件描述符(六)--fd的分配--expand_files

Kernel version：2.6.14

CPU architecture：ARM920T

Author：ce123(http://blog.csdn.net/ce123)

我们先贴出expand_files函数的源码：

static struct fdtable *alloc_fdtable(int nr){struct fdtable *fdt = NULL;int nfds = 0;  fd_set *new_openset = NULL, *new_execset = NULL;struct file **new_fds;fdt = kmalloc(sizeof(*fdt), GFP_KERNEL);if (!fdt)  goto out;memset(fdt, 0, sizeof(*fdt));nfds = __FD_SETSIZE;  //#define __FD_SETSIZE1024　　　　      //  #define PAGE_SHIFT12　　　　      //  #define PAGE_SIZE(1UL << PAGE_SHIFT)  /* Expand to the max in easy steps */  do {if (nfds < (PAGE_SIZE * 8))//dfds = 1024nfds = PAGE_SIZE * 8;else {nfds = nfds * 2;if (nfds > NR_OPEN)nfds = NR_OPEN;}} while (nfds <= nr);//第一次expand时，nr应该等于32  new_openset = alloc_fdset(nfds);//分配打开文件位图  new_execset = alloc_fdset(nfds);  if (!new_openset || !new_execset)  goto out;fdt->open_fds = new_openset;fdt->close_on_exec = new_execset;fdt->max_fdset = nfds;//更新max_fdset值，此时这个值为32knfds = NR_OPEN_DEFAULT;//nfds = 32/* * Expand to the max in easy steps, and keep expanding it until * we have enough for the requested fd array size. */do {#if NR_OPEN_DEFAULT < 256if (nfds < 256)nfds = 256;//nfds = 256（32->256->1024）//无法超过1024，因为在最开始的就进行了检查，一定要小于current->signal->rlim[RLIMIT_NOFILE].rlim_cur）else#endifif (nfds < (PAGE_SIZE / sizeof(struct file *)))nfds = PAGE_SIZE / sizeof(struct file *);else {nfds = nfds * 2;if (nfds > NR_OPEN)nfds = NR_OPEN;  }} while (nfds <= nr);new_fds = alloc_fd_array(nfds);//分配文件描述符数组if (!new_fds)goto out;fdt->fd = new_fds;fdt->max_fds = nfds;//更新max_fdsfdt->free_files = NULL;return fdt;out:  if (new_openset)  free_fdset(new_openset, nfds);  if (new_execset)  free_fdset(new_execset, nfds);kfree(fdt);return NULL;}