linux内核中的文件描述符(5)-fd的分配-locate_fd
linux内核中的文件描述符(五)--fd的分配--locate_fd
linux内核中的文件描述符(五)--fd的分配--locate_fd
Kernel version:2.6.14
CPU architecture:ARM920T
Author:ce123(http://blog.csdn.net/ce123)
继续上一篇博客的内容,分析另一个文件描述符fd的分配函数locate_fd。dup系统调用用于复制一个文件描述符对应的文件,返回值是个文件描述符。在前面的文章中,我们已经分析过了dup的源码(http://blog.csdn.net/ce123/article/details/8444482),在这里我们深入分析locate_fd函数,其定义如下:
static int locate_fd(struct files_struct *files, struct file *file, unsigned int orig_start)//从orig_start位开始分配fd { unsigned int newfd; unsigned int start; int error; struct fdtable *fdt; error = -EINVAL; if (orig_start >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//检查orig_start是大于进程最大可以打开文件的数量 goto out; repeat: fdt = files_fdtable(files);//文件描述符位图 /* * Someone might have closed fd's in the range * orig_start..fdt->next_fd */ start = orig_start; if (start < fdt->next_fd) start = fdt->next_fd;//如果orig_start小于next_fd,那就从next_fd开始分配 newfd = start; if (start < fdt->max_fdset) {//max_fdset是描述符问题的位数,下面会具体讲解 newfd = find_next_zero_bit(fdt->open_fds->fds_bits, fdt->max_fdset, start);//分配fd } error = -EMFILE; if (newfd >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//进行判断,分配的fd不能大于进程最大可以打开的文件数量 goto out; error = expand_files(files, newfd);//文件描述符表的扩展,这个我们留在下一篇文章中详细讲解 if (error < 0) goto out; /* * If we needed to expand the fs array we * might have blocked - try again. */ if (error) goto repeat; /* * We reacquired files_lock, so we are safe as long as * we reacquire the fdtable pointer and use it while holding * the lock, no one can free it during that time. */ fdt = files_fdtable(files); if (start <= fdt->next_fd) fdt->next_fd = newfd + 1;//更新next_fd值 error = newfd; out: return error; }max_fdset值的分析和rlim_cur差不多,最初的值时从父进程继承过来的。
linux/arch/arm/kernel/init_task.c struct task_struct init_task = INIT_TASK(init_task); #define INIT_TASK(tsk) \ { \ ... .files = &init_files, \ ... }init_files的定义如下:
static struct files_struct init_files = INIT_FILES; linux/init_task.h #define INIT_FDTABLE \ { \ .max_fds = NR_OPEN_DEFAULT, \ .max_fdset = __FD_SETSIZE, \ .next_fd = 0, \ .fd = &init_files.fd_array[0], \ .close_on_exec = &init_files.close_on_exec_init, \ .open_fds = &init_files.open_fds_init, \ .rcu = RCU_HEAD_INIT, \ .free_files = NULL, \ .next = NULL, \ } #define NR_OPEN_DEFAULT BITS_PER_LONG #define __FD_SETSIZE 1024 #define INIT_FILES \ { \ .count = ATOMIC_INIT(1), \ .file_lock = SPIN_LOCK_UNLOCKED, \ .fdt = &init_files.fdtab, \ .fdtab = INIT_FDTABLE, \ .close_on_exec_init = { { 0, } }, \ .open_fds_init = { { 0, } }, \ .fd_array = { NULL, } \ }BITS_PER_LONG是long型数据的字节数,即4*8=3,也就是说max_fds = 32。max_fdset为1024。max_fdset是进程打开文件描述符位图open_fds的大小。open_fds是fd_set的指针。
typedef __kernel_fd_set fd_set; #undef __NFDBITS #define __NFDBITS (8 * sizeof(unsigned long)) #undef __FD_SETSIZE #define __FD_SETSIZE 1024 #undef __FDSET_LONGS #define __FDSET_LONGS (__FD_SETSIZE/__NFDBITS) #undef __FDELT #define __FDELT(d) ((d) / __NFDBITS) #undef __FDMASK #define __FDMASK(d) (1UL << ((d) % __NFDBITS)) typedef struct { unsigned long fds_bits [__FDSET_LONGS]; } __kernel_fd_set;fds_bits是一个long型数组,共有32个元素,共有1024bit。