Kernel version:2.6.14
CPU architecture:ARM920T
Author:ce123(http://blog.csdn.net/ce123)
继续上一篇博客的内容,分析另一个文件描述符fd的分配函数locate_fd。dup系统调用用于复制一个文件描述符对应的文件,返回值是个文件描述符。在前面的文章中,我们已经分析过了dup的源码(http://blog.csdn.net/ce123/article/details/8444482),在这里我们深入分析locate_fd函数,其定义如下:
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- static int locate_fd(struct files_struct *files,
- struct file *file, unsigned int orig_start)//从orig_start位开始分配fd
- {
- unsigned int newfd;
- unsigned int start;
- int error;
- struct fdtable *fdt;
-
- error = -EINVAL;
- if (orig_start >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//检查orig_start是大于进程最大可以打开文件的数量
- goto out;
-
- repeat:
- fdt = files_fdtable(files);//文件描述符位图
- /*
- * Someone might have closed fd's in the range
- * orig_start..fdt->next_fd
- */
- start = orig_start;
- if (start next_fd)
- start = fdt->next_fd;//如果orig_start小于next_fd,那就从next_fd开始分配
-
- newfd = start;
- if (start max_fdset) {//max_fdset是描述符问题的位数,下面会具体讲解
- newfd = find_next_zero_bit(fdt->open_fds->fds_bits,
- fdt->max_fdset, start);//分配fd
- }
-
- error = -EMFILE;
- if (newfd >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//进行判断,分配的fd不能大于进程最大可以打开的文件数量
- goto out;
-
- error = expand_files(files, newfd);//文件描述符表的扩展,这个我们留在下一篇文章中详细讲解
- if (error
- goto out;
-
- /*
- * If we needed to expand the fs array we
- * might have blocked - try again.
- */
- if (error)
- goto repeat;
-
- /*
- * We reacquired files_lock, so we are safe as long as
- * we reacquire the fdtable pointer and use it while holding
- * the lock, no one can free it during that time.
- */
- fdt = files_fdtable(files);
- if (start <= fdt->next_fd)
- fdt->next_fd = newfd + 1;//更新next_fd值
-
- error = newfd;
-
- out:
- return error;
- }
max_fdset值的分析和rlim_cur差不多,最初的值时从父进程继承过来的。
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- linux/arch/arm/kernel/init_task.c
-
- struct task_struct init_task = INIT_TASK(init_task);
-
- #define INIT_TASK(tsk) \
- { \
- ...
- .files = &init_files, \
- ...
- }
init_files的定义如下:
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- static struct files_struct init_files = INIT_FILES;
-
- linux/init_task.h
-
- #define INIT_FDTABLE \
- { \
- .max_fds = NR_OPEN_DEFAULT, \
- .max_fdset = __FD_SETSIZE, \
- .next_fd = 0, \
- .fd = &init_files.fd_array[0], \
- .close_on_exec = &init_files.close_on_exec_init, \
- .open_fds = &init_files.open_fds_init, \
- .rcu = RCU_HEAD_INIT, \
- .free_files = NULL, \
- .next = NULL, \
- }
-
- #define NR_OPEN_DEFAULT BITS_PER_LONG
- #define __FD_SETSIZE 1024
-
- #define INIT_FILES \
- { \
- .count = ATOMIC_INIT(1), \
- .file_lock = SPIN_LOCK_UNLOCKED, \
- .fdt = &init_files.fdtab, \
- .fdtab = INIT_FDTABLE, \
- .close_on_exec_init = { { 0, } }, \
- .open_fds_init = { { 0, } }, \
- .fd_array = { NULL, } \
- }
BITS_PER_LONG是long型数据的字节数,即4*8=3,也就是说max_fds = 32。max_fdset为1024。max_fdset是进程打开文件描述符位图open_fds的大小。open_fds是fd_set的指针。
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- typedef __kernel_fd_set fd_set;
-
- #undef __NFDBITS
- #define __NFDBITS (8 * sizeof(unsigned long))
-
- #undef __FD_SETSIZE
- #define __FD_SETSIZE 1024
-
- #undef __FDSET_LONGS
- #define __FDSET_LONGS (__FD_SETSIZE/__NFDBITS)
-
- #undef __FDELT
- #define __FDELT(d) ((d) / __NFDBITS)
-
- #undef __FDMASK
- #define __FDMASK(d) (1UL <
-
- typedef struct {
- unsigned long fds_bits [__FDSET_LONGS];
- } __kernel_fd_set;
fds_bits是一个long型数组,共有32个元素,共有1024bit。
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