Kernel version：2.6.14
CPU architecture：ARM920T
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，也就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最大可打开文件数
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!\n", 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最大文件数量。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;//最大值
};
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, \
}
include\asm-generic\resource.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 */