Kernel version：2.6.14
CPU architecture：ARM920T
我們先貼出expand_files函數的源碼：
int expand_files(struct files_struct *files, int nr)
{
int err, expand = 0;
struct fdtable *fdt;
fdt = files_fdtable(files);
if (nr >= fdt->max_fdset || nr >= fdt->max_fds) { //我們在前面的文章中已經分析過，初始時max_fdset = 1024，max_fds = 32
if (fdt->max_fdset >= NR_OPEN || //#define NR_OPEN (1024*1024) /* Absolute upper limit on fd num */
fdt->max_fds >= NR_OPEN || nr >= NR_OPEN) {
err = -EMFILE; //max_fdset和max_fds都不能大於 NR_OPEN，否則返回 -EMFILE，即打開太多的文件
goto out;
}
expand = 1;
if ((err = expand_fdtable(files, nr)))//真正進行擴展
goto out;
}
err = expand;
out:
return err;
}
expand_files函數進行一些檢查後調用expand_fdtable進行文件描述符表的擴展，下面分析expand_fdtable函數。
static int expand_fdtable(struct files_struct *files, int nr)
__releases(files->file_lock)
__acquires(files->file_lock)
{
int error = 0;
struct fdtable *fdt;
struct fdtable *nfdt = NULL;
spin_unlock(&files->file_lock);
nfdt = alloc_fdtable(nr);//根據nr重新創建一個新的fdtable
if (!nfdt) {
error = -ENOMEM;
spin_lock(&files->file_lock);
goto out;
}
spin_lock(&files->file_lock);
fdt = files_fdtable(files);
/*
* Check again since another task may have expanded the
* fd table while we dropped the lock
*/
if (nr >= fdt->max_fds || nr >= fdt->max_fdset) { //nr值必須大於max_fds和max_fdset值，這裡再次進行檢查是防止另一個進程進行了expand
copy_fdtable(nfdt, fdt); //將舊的fdtable中的內容拷貝至新的fdtable
} else {
/* Somebody expanded while we dropped file_lock */
spin_unlock(&files->file_lock);
__free_fdtable(nfdt);
spin_lock(&files->file_lock);
goto out;
}
rcu_assign_pointer(files->fdt, nfdt);//用新的fdtable替換舊的fdtable
free_fdtable(fdt);//釋放舊的fdtable
out:
return error;
}
