Unix学习笔记------实例练习---Day02_access函数文件访问权限测试

电玩女神 2022-08-10 03:56 243阅读 0赞

Day02\_access函数文件访问权限测试

：：：：：：：：：：

/\*

access()函数，文件访问权限测试，

能测试这个进程是否有权限访问这个文件。

access函数原型：

：：：：：：：：：：：：

access - check real user鈥檚permissions for a file

SYNOPSIS

\#include <unistd.h>

int access(const char \*pathname, int mode);

DESCRIPTION

access() checks whether the calling process can access the filepathname. If pathname is a symboliclink, it is dereferenced.

The mode specifies the accessibility check(s) to beperformed, and is either the value F\_OK, or a mask consisting of the bitwise ORof one or more of R\_OK, W\_OK, and

X\_OK. F\_OK tests for theexistence of the file. R\_OK, W\_OK, andX\_OK test whether the file exists and grants read, write, and executepermissions, respectively.

The check is done using the calling process鈥檚 real UIDand GID, rather than the effective IDs as is done when actually attempting anoperation (e.g., open(2)) on the

file. This allows set-user-IDprograms to easily determine the invoking user鈥檚 authority.

If the calling process is privileged (i.e., its real UID is zero), then an X\_OK checkis successful for a regular file if execute permission is enabled for any ofthe

file owner, group, or other.

RETURN VALUE

On success (all requested permissions granted), zero is returned. On error (at least one bit in mode askedfor a permission that is denied, or some other error

occurred), -1 is returned, and errno is set appropriately.

ERRORS

access() shall fail if:

EACCES The requested access wouldbe denied to the file, or search permission is denied for one of thedirectories in the path prefix of pathname. (See also path\_reso-

lution(7).)

ELOOP Too many symbolic linkswere encountered in resolving pathname.

ENAMETOOLONG

pathname is too long.

ENOENT A component of pathname does not exist or is a dangling symboliclink.

ENOTDIR

A component used as a directoryin pathname is not, in fact, a directory.

EROFS Write permission wasrequested for a file on a read-only file system.

access() may fail if:

EFAULT pathname points outside your accessible address space.

EINVAL mode was incorrectly specified.

EIO An I/O error occurred.