在Linux内核中主要有两个函数涉及到文件描述符的分配:get_unused_fd和locate_fd。本文主要讲解get_unused_fd,将会在下一篇文章中介绍locate_fd。首先给出get_unused_fd的定义(fs/open.c):
int get_unused_fd(void) { struct files_struct * files = current->files;//获得当前进程的打开文件列表files int fd, error; struct fdtable *fdt;
error = -EMFILE; spin_lock(&files->file_lock);
repeat: fdt = files_fdtable(files);//获得文件描述符位图结构 fd = find_next_zero_bit(fdt->open_fds->fds_bits, fdt->max_fdset, fdt->next_fd); //find_next_zero_bit函数在文件描述符位图fds_bits中从next_fd位开始搜索下一个(包括next_fd)为0的位,也是分配一个文教描述符 /* * N.B. For clone tasks sharing a files structure, this test * will limit the total number of files that can be opened. */ if (fd >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//检查是否超过当前进程限定的大可打开文件数 goto out;
/* Do we need to expand the fd array or fd set? */ error = expand_files(files, fd);//根据需要扩展fd,稍后我们会详细介绍该函数。返回值<0,错误;返回值>0,扩展后再次进行fd的分配 if (error < 0) goto out;
if (error) { /* * If we needed to expand the fs array we * might have blocked - try again. */ error = -EMFILE; goto repeat;//之前进行了扩展操作,重新进行一次空闲fd的分配 }
FD_SET(fd, fdt->open_fds);//在open_fds的位图上置位 FD_CLR(fd, fdt->close_on_exec); fdt->next_fd = fd + 1;//next_fd加1 #if 1 /* Sanity check */ if (fdt->fd[fd] != NULL) { printk(KERN_WARNING "get_unused_fd: slot %d not NULL! ", fd); fdt->fd[fd] = NULL; } #endif error = fd;
out: spin_unlock(&files->file_lock); return error; }
current->signal->rlim[RLIMIT_NOFILE].rlim_cur是一个进程可以打开的大文件数量。我们首先来看RLIMIT_NOFILE,该值定义如下:
# define RLIMIT_NOFILE 7 /* max number of open files */
在signal结构中,rlim是struct rlimit类型的数组,
struct signal_struct { ... struct rlimit rlim[RLIM_NLIMITS]; ... };
struct rlimit定义如下
struct rlimit { unsigned long rlim_cur;//当前值 unsigned long rlim_max;//大值
这些值时是在哪设定的呢?我们应该知道,linux内核通过fork创建进程,第一个进程是静态定义的。因此,如果进程创建后没有修改这些值,那么这些和第一个进程中的值应该是一样的。下面是第一个进程的task_struct结构,仅列出部分数据。
linux/arch/arm/kernel/init_task.c
struct task_struct init_task = INIT_TASK(init_task);
#define INIT_TASK(tsk) { ... .signal = &init_signals, ... }
init_signals的定义如下:
#define INIT_SIGNALS(sig) { .count = ATOMIC_INIT(1), .wait_chldexit = __WAIT_QUEUE_HEAD_INITIALIZER(sig.wait_chldexit), .shared_pending = { .list = LIST_HEAD_INIT(sig.shared_pending.list), .signal = {{0}}}, .posix_timers = LIST_HEAD_INIT(sig.posix_timers), .cpu_timers = INIT_CPU_TIMERS(sig.cpu_timers), .rlim = INIT_RLIMITS, }
includeasm-genericesource.h #define INIT_RLIMITS { [RLIMIT_CPU] = { RLIM_INFINITY, RLIM_INFINITY }, [RLIMIT_FSIZE] = { RLIM_INFINITY, RLIM_INFINITY }, [RLIMIT_DATA] = { RLIM_INFINITY, RLIM_INFINITY }, [RLIMIT_STACK] = { _STK_LIM, _STK_LIM_MAX }, [RLIMIT_CORE] = { 0, RLIM_INFINITY }, [RLIMIT_RSS] = { RLIM_INFINITY, RLIM_INFINITY }, [RLIMIT_NPROC] = { 0, 0 }, [RLIMIT_NOFILE] = { INR_OPEN, INR_OPEN }, [RLIMIT_MEMLOCK] = { MLOCK_LIMIT, MLOCK_LIMIT }, [RLIMIT_AS] = { RLIM_INFINITY, RLIM_INFINITY }, [RLIMIT_LOCKS] = { RLIM_INFINITY, RLIM_INFINITY }, [RLIMIT_SIGPENDING] = { 0, 0 }, [RLIMIT_MSGQUEUE] = { MQ_BYTES_MAX, MQ_BYTES_MAX }, [RLIMIT_NICE] = { 0, 0 }, [RLIMIT_RTPRIO] = { 0, 0 }, }
#define NR_OPEN (1024*1024) /* Absolute upper limit on fd num */ #define INR_OPEN 1024 /* Initial setting for nfile rlimits */
从上面的代码我们可以看到rlim_cur = 1024,也是说进程多可以打开1024个文件。