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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 test756          * 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 1787         /* 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 #endif793         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！

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 }

162 /*163  * Expand the fdset in the files_struct.  Called with the files spinlock164  * 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 is205                  * 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 }

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         else140                 new_fdset = (fd_set *) vmalloc(size);141         return new_fdset;142 }

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 new106                    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 }