Linux文件系统(七)---系统调用之open操作(二) 之 get_unused_fd

迈不过友情╰ 2022-08-13 00:52 189阅读 0赞

Open系统调用

**/\* 2 \*/**：这一步是需要找到一个没有使用的文件描述符fd

看一下sys\_open函数中调用的这个函数get\_unused\_fd：

738 /*
    739  * Find an empty file descriptor entry, and mark it busy.
    740  */
    741 int get_unused_fd(void)
    742 {
    743         struct files_struct * files = current->files;  /* (1)获得当前进程的文件打开表，这个前面已经说过了 */
    744         int fd, error;
    745 
    746         error = -EMFILE;
    747         write_lock(&files->file_lock);
    748 
    749 repeat:
    750         fd = find_next_zero_bit(files->open_fds,      /* (2)在这个进程文件打开表中寻找还没有使用的fd */
    751                                 files->max_fdset, 
    752                                 files->next_fd);
    753 
    754         /*
    755          * N.B. For clone tasks sharing a files structure, this test
    756          * will limit the total number of files that can be opened.
    757          */
    758         if (fd >= current->rlim[RLIMIT_NOFILE].rlim_cur)  /* 不能超过限制(考虑到fork进程情况，共享fd情况) */
    759                 goto out;
    760 
    761         /* Do we need to expand the fdset array? */
    762         if (fd >= files->max_fdset) {    /* 如果当前进程的fd集合中最大fd比当前申请的fd小，那么需要扩大fdset，需要扩容：具体见http://blog.csdn.net/shanshanpt/article/details/38943731中files_struct结构体描述 */
    763                 error = expand_fdset(files, fd);  /* 扩容函数1 */
    764                 if (!error) {
    765                         error = -EMFILE;
    766                         goto repeat;
    767                 }
    768                 goto out;
    769         }
    770         
    771         /* 
    772          * Check whether we need to expand the fd array.
    773          */
    774         if (fd >= files->max_fds) {    /* 类似于上面，这里是需要扩大fd-array数组，具体见上面链接 */
    775                 error = expand_fd_array(files, fd);  /* 扩容函数2 */
    776                 if (!error) {
    777                         error = -EMFILE;
    778                         goto repeat;
    779                 }
    780                 goto out;
    781         }
    782 
    783         FD_SET(fd, files->open_fds);     /* 将fd加入到打开文件描述符中 */
    784         FD_CLR(fd, files->close_on_exec);/* 从close-on-exec中清除 */
    785         files->next_fd = fd + 1;         /* 当前描述符是最大的fd，所有next就是fd+1 */
    786 #if 1
    787         /* Sanity check */
    788         if (files->fd[fd] != NULL) {
    789                 printk(KERN_WARNING "get_unused_fd: slot %d not NULL!\n", fd);
    790                 files->fd[fd] = NULL;
    791         }
    792 #endif
    793         error = fd;
    794 
    795 out:
    796         write_unlock(&files->file_lock);
    797         return error;
    798 }
    799

**ATTENTION**：注意上面的max\_fdset和max\_fds的区别，前者是当前可以容纳的最大的文件描述符的数量大小！后者是当前可以容纳的文件对象的数量大小！对于一个文件对象而言，可以存在多个文件描述符指向这一个文件对象！所以对于申请到的fd，要分别和这两个进行判断！

主要看上面的三个函数，第一个是寻找fd函数find\_next\_zero\_bit：  
这个函数的意义就是找到open\_fds打开的文件描述符中第一个bit位=0的那一位，作为新的fd返回。(前面也说过，fd的管理是使用位管理的)，反正是一堆位运算，慢慢看吧：

254 static inline unsigned long find_next_zero_bit(void *addr, unsigned long size, unsigned long offset)
    255 {
    256         unsigned long *p = ((unsigned long *) addr) + (offset >> 5);
    257         unsigned long result = offset & ~31UL;
    258         unsigned long tmp;
    259 
    260         if (offset >= size)
    261                 return size;
    262         size -= result;
    263         offset &= 31UL;
    264         if (offset) {
    265                 tmp = *(p++);
    266                 tmp |= ~0UL >> (32-offset);
    267                 if (size < 32)
    268                         goto found_first;
    269                 if (~tmp)
    270                         goto found_middle;
    271                 size -= 32;
    272                 result += 32;
    273         }
    274         while (size & ~31UL) {
    275                 if (~(tmp = *(p++)))
    276                         goto found_middle;
    277                 result += 32;
    278                 size -= 32;
    279         }
    280         if (!size)
    281                 return result;
    282         tmp = *p;
    283 
    284 found_first:
    285         tmp |= ~0UL << size;
    286         if (tmp == ~0UL)        /* Are any bits zero? */
    287                 return result + size; /* Nope. */
    288 found_middle:
    289         return result + ffz(tmp);
    290 }

扩大fdset集合函数：expand\_fdset

162 /*
    163  * Expand the fdset in the files_struct.  Called with the files spinlock
    164  * held for write.
    165  */
    166 int expand_fdset(struct files_struct *files, int nr)
    167 {
    168         fd_set *new_openset = 0, *new_execset = 0;
    169         int error, nfds = 0;
    170 
    171         error = -EMFILE;
    172         if (files->max_fdset >= NR_OPEN || nr >= NR_OPEN)   /* 不能大于系统最大限制 */
    173                 goto out;
    174 
    175         nfds = files->max_fdset;   /* 当前最大文件描述符 */
    176         write_unlock(&files->file_lock);
    177 
    178         /* Expand to the max in easy steps，下面是一个 简单的扩展过程 */
    179         do {
    180                 if (nfds < (PAGE_SIZE * 8))
    181                         nfds = PAGE_SIZE * 8;
    182                 else {
    183                         nfds = nfds * 2;
    184                         if (nfds > NR_OPEN)
    185                                 nfds = NR_OPEN;
    186                 }
    187         } while (nfds <= nr);
    188 
    189         error = -ENOMEM;
    190         new_openset = alloc_fdset(nfds);  /* 根据新的大小分配新的打开set集合 */
    191         new_execset = alloc_fdset(nfds);  /* 根据新的大小分配新的执行时候需要close的set集合 */
    192         write_lock(&files->file_lock);
    193         if (!new_openset || !new_execset)
    194                 goto out;
    195 
    196         error = 0;
    197         
    198         /* Copy the existing tables and install the new pointers：将老的数据拷贝到新的内存中来 */
    199         if (nfds > files->max_fdset) {
    200                 int i = files->max_fdset / (sizeof(unsigned long) * 8);
    201                 int count = (nfds - files->max_fdset) / 8;
    202                 
    203                 /* 
    204                  * Don't copy the entire array if the current fdset is
    205                  * not yet initialised.  
    206                  */
    207                 if (i) {
    208                         memcpy (new_openset, files->open_fds, files->max_fdset/8);
    209                         memcpy (new_execset, files->close_on_exec, files->max_fdset/8);
    210                         memset (&new_openset->fds_bits[i], 0, count);
    211                         memset (&new_execset->fds_bits[i], 0, count);
    212                 }
    213                 /* 下面几步骤很重要，将新分配的挂载到files结构体中去 */
    214                 nfds = xchg(&files->max_fdset, nfds);
    215                 new_openset = xchg(&files->open_fds, new_openset);
    216                 new_execset = xchg(&files->close_on_exec, new_execset);
    217                 write_unlock(&files->file_lock);
    218                 free_fdset (new_openset, nfds);
    219                 free_fdset (new_execset, nfds);
    220                 write_lock(&files->file_lock);
    221                 return 0;
    222         } 
    223         /* Somebody expanded the array while we slept ... */
    224 
    225 out:
    226         write_unlock(&files->file_lock);
    227         if (new_openset)
    228                 free_fdset(new_openset, nfds);
    229         if (new_execset)
    230                 free_fdset(new_execset, nfds);
    231         write_lock(&files->file_lock);
    232         return error;
    233 }

再看一下具体的alloc\_fdset函数：

128 /*
    129  * Allocate an fdset array, using kmalloc or vmalloc.
    130  * Note: the array isn't cleared at allocation time.
    131  */
    132 fd_set * alloc_fdset(int num)
    133 {
    134         fd_set *new_fdset;
    135         int size = num / 8;
    136 
    137         if (size <= PAGE_SIZE)
    138                 new_fdset = (fd_set *) kmalloc(size, GFP_KERNEL);
    139         else
    140                 new_fdset = (fd_set *) vmalloc(size);
    141         return new_fdset;
    142 }

回到上面，看一下扩大fd数组的函数expand\_fd\_array：

52 /*
     53  * Expand the fd array in the files_struct.  Called with the files
     54  * spinlock held for write.
     55  */
     56 
     57 int expand_fd_array(struct files_struct *files, int nr)
     58 {
     59         struct file **new_fds;
     60         int error, nfds;
     61 
     62         
     63         error = -EMFILE;
     64         if (files->max_fds >= NR_OPEN || nr >= NR_OPEN)   /* 不能超过最大系统限制 */
     65                 goto out;
     66 
     67         nfds = files->max_fds;     /* 当前进程中最大的fd */
     68         write_unlock(&files->file_lock);
     69 
     70         /* 
     71          * Expand to the max in easy steps, and keep expanding it until
     72          * we have enough for the requested fd array size. 
     73          */
     74         /* 简单的扩展策略 */
     75         do {
     76 #if NR_OPEN_DEFAULT < 256
     77                 if (nfds < 256)
     78                         nfds = 256;
     79                 else 
     80 #endif
     81                 if (nfds < (PAGE_SIZE / sizeof(struct file *)))
     82                         nfds = PAGE_SIZE / sizeof(struct file *);
     83                 else {
     84                         nfds = nfds * 2;
     85                         if (nfds > NR_OPEN)
     86                                 nfds = NR_OPEN;
     87                 }
     88         } while (nfds <= nr);
     89 
     90         error = -ENOMEM;
     91         new_fds = alloc_fd_array(nfds);    /* 分配新的fd_array数组 */
     92         write_lock(&files->file_lock);
     93         if (!new_fds)
     94                 goto out;
     95 
     96         /* Copy the existing array and install the new pointer */
     97 
     98         if (nfds > files->max_fds) {
     99                 struct file **old_fds;
    100                 int i;
    101                 /* 将当前进行的文件数组指针指向新申请的fd数组！ */
    102                 old_fds = xchg(&files->fd, new_fds);
    103                 i = xchg(&files->max_fds, nfds);
    104 
    105                 /* Don't copy/clear the array if we are creating a new
    106                    fd array for fork() */
    107                 if (i) {   /* 下面将老的数据拷贝过去 */
    108                         memcpy(new_fds, old_fds, i * sizeof(struct file *));
    109                         /* clear the remainder of the array */
    110                         memset(&new_fds[i], 0,
    111                                (nfds-i) * sizeof(struct file *)); 
    112 
    113                         write_unlock(&files->file_lock);
    114                         free_fd_array(old_fds, i);
    115                         write_lock(&files->file_lock);
    116                 }
    117         } else {
    118                 /* Somebody expanded the array while we slept ... */
    119                 write_unlock(&files->file_lock);
    120                 free_fd_array(new_fds, nfds);
    121                 write_lock(&files->file_lock);
    122         }
    123         error = 0;
    124 out:
    125         return error;
    126 }