1500字范文 > linux C语言access()函数（检查用户对文件的权限）（判断文件是否存在）

linux C语言access()函数（检查用户对文件的权限）（判断文件是否存在）

时间：2018-06-10 01:53:14

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man 2 access0601 不知道上面所说的pathname的符号链接取消引用是什么意思

man 2 access

ACCESS(2)Linux Programmer's Manual ACCESS(2)NAMEaccess, faccessat - check user's permissions for a file//检查用户对文件的权限SYNOPSIS//概要#include <unistd.h>int access(const char *pathname, int mode);#include <fcntl.h> /* Definition of AT_* constants */#include <unistd.h>int faccessat(int dirfd, const char *pathname, int mode, int flags);Feature Test Macro Requirements for glibc (see feature_test_macros(7)):faccessat():Since glibc 2.10:_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 09LBefore glibc 2.10:_ATFILE_SOURCEDESCRIPTIONaccess() checks whether the calling process can access the file pathname. If pathname is a symbolic link, it is dereferenced.//access() 检查调用进程是否可以访问文件路径名。 //如果 pathname 是符号链接，则"解引用"。The mode specifies the accessibility check(s) to be performed, and is either the value F_OK, or a mask consisting of the bitwise OR of one or more of R_OK,W_OK, and X_OK. F_OK tests for the existence of the file. R_OK, W_OK, and X_OK test whether the file exists and grants read, write, and execute permis‐sions, respectively.//模式指定要执行的可访问性检查，或者是值 F_OK，或者是由 R_OK、W_OK 和 X_OK 中的一个或多个按位或组成的掩码。 //F_OK 测试文件是否存在。 //R_OK、W_OK 和 X_OK 分别测试文件是否存在并授予读取、写入和执行权限。//示例：我从unistd.h和fcntl.h拷过来的/* Values for the second argument to access.These may be OR'd together. *///# define R_OK4/* Test for read permission. *///# define W_OK2/* Test for write permission. *///# define X_OK1/* Test for execute permission. *///# define F_OK0/* Test for existence. */The check is done using the calling process's real UID and GID, rather than the effective IDs as is done when actually attempting an operation (e.g.,open(2)) on the file. Similarly, for the root user, the check uses the set of permitted capabilities rather than the set of effective capabilities; andfor non-root users, the check uses an empty set of capabilities.//检查是使用调用进程的真实 UID 和 GID 完成的，而不是在实际尝试对文件进行操作（例如 open(2)）时使用的有效 ID。 //同样，对于 root 用户，检查使用的是允许的能力集合，而不是有效能力的集合；对于非 root 用户，检查使用一组空的功能。（什么是允许的能力集合和有效的能力集合？）//提问：linux C语言access()函数允许的能力集合和有效的能力集合指的是什么？//链接：/question/532523171This allows set-user-ID programs and capability-endowed programs to easily determine the invoking user's authority. In other words, access() does notanswer the "can I read/write/execute this file?" question. It answers a slightly different question: "(assuming I'm a setuid binary) can the user whoinvoked me read/write/execute this file?", which gives set-user-ID programs the possibility to prevent malicious users from causing them to read fileswhich users shouldn't be able to read.//这允许 set-user-ID 程序和具有能力的程序轻松确定调用用户的权限。 //换句话说，access() 没有回答“我可以读/写/执行这个文件吗？” 问题。 //它回答了一个稍微不同的问题：“（假设我是 setuid 二进制文件）调用我的用户可以读/写/执行此文件吗？”，这使 set-user-ID 程序有可能防止恶意用户导致它们读取用户不应该读取的文件。If the calling process is privileged (i.e., its real UID is zero), then an X_OK check is successful for a regular file if execute permission is enabled forany of the file owner, group, or other.faccessat()The faccessat() system call operates in exactly the same way as access(), except for the differences described here.If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than rela‐tive to the current working directory of the calling process, as is done by access() for a relative pathname).If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the callingprocess (like access()).If pathname is absolute, then dirfd is ignored.flags is constructed by ORing together zero or more of the following values:AT_EACCESSPerform access checks using the effective user and group IDs. By default, faccessat() uses the real IDs (like access()).AT_SYMLINK_NOFOLLOWIf pathname is a symbolic link, do not dereference it: instead return information about the link itself.See openat(2) for an explanation of the need for faccessat().RETURN VALUEOn success (all requested permissions granted, or mode is F_OK and the file exists), zero is returned. On error (at least one bit in mode asked for a per‐mission that is denied, or mode is F_OK and the file does not exist, or some other error occurred), -1 is returned, and errno is set appropriately.//成功时（授予所有请求的权限，或模式为 F_OK 且文件存在），返回零。 //出错时（mode 中的至少一位请求被拒绝的权限，或者 mode 是 F_OK 并且文件不存在，或者发生了其他错误），返回 -1，并适当地设置 errno。ERRORSaccess() and faccessat() shall fail if:EACCES The requested access would be denied to the file, or search permission is denied for one of the directories in the path prefix of pathname. (Seealso path_resolution(7).)ELOOP Too many symbolic links were encountered in resolving pathname.ENAMETOOLONGpathname is too long.ENOENT A component of pathname does not exist or is a dangling symbolic link.ENOTDIRA component used as a directory in pathname is not, in fact, a directory.EROFS Write permission was requested for a file on a read-only filesystem.access() and faccessat() may fail if:EFAULT pathname points outside your accessible address space.EINVAL mode was incorrectly specified.EIO An I/O error occurred.ENOMEM Insufficient kernel memory was available.ETXTBSYWrite access was requested to an executable which is being executed.The following additional errors can occur for faccessat():EBADF dirfd is not a valid file descriptor.EINVAL Invalid flag specified in flags.ENOTDIRpathname is relative and dirfd is a file descriptor referring to a file other than a directory.VERSIONSfaccessat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.CONFORMING TOaccess(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-.faccessat(): POSIX.1-.NOTESWarning: Using these calls to check if a user is authorized to, for example, open a file before actually doing so using open(2) creates a security hole,because the user might exploit the short time interval between checking and opening the file to manipulate it. For this reason, the use of this systemcall should be avoided. (In the example just described, a safer alternative would be to temporarily switch the process's effective user ID to the real IDand then call open(2).)access() always dereferences symbolic links. If you need to check the permissions on a symbolic link, use faccessat(2) with the flag AT_SYMLINK_NOFOLLOW.These calls return an error if any of the access types in mode is denied, even if some of the other access types in mode are permitted.If the calling process has appropriate privileges (i.e., is superuser), POSIX.1-2001 permits an implementation to indicate success for an X_OK check evenif none of the execute file permission bits are set. Linux does not do this.A file is accessible only if the permissions on each of the directories in the path prefix of pathname grant search (i.e., execute) access. If any direc‐tory is inaccessible, then the access() call will fail, regardless of the permissions on the file itself.Only access bits are checked, not the file type or contents. Therefore, if a directory is found to be writable, it probably means that files can be cre‐ated in the directory, and not that the directory can be written as a file. Similarly, a DOS file may be found to be "executable," but the execve(2) callwill still fail.These calls may not work correctly on NFSv2 filesystems with UID mapping enabled, because UID mapping is done on the server and hidden from the client,which checks permissions. (NFS versions 3 and higher perform the check on the server.) Similar problems can occur to FUSE mounts.C library/kernel differencesThe raw faccessat() system call takes only the first three arguments. The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are actually implemented within theglibc wrapper function for faccessat(). If either of these flags is specified, then the wrapper function employs fstatat(2) to determine access permis‐sions.Glibc notesOn older kernels where faccessat() is unavailable (and when the AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are not specified), the glibc wrapper functionfalls back to the use of access(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that cor‐responds to the dirfd argument.BUGSIn kernel 2.4 (and earlier) there is some strangeness in the handling of X_OK tests for superuser. If all categories of execute permission are disabledfor a nondirectory file, then the only access() test that returns -1 is when mode is specified as just X_OK; if R_OK or W_OK is also specified in mode,then access() returns 0 for such files. Early 2.6 kernels (up to and including 2.6.3) also behaved in the same way as kernel 2.4.In kernels before 2.6.20, these calls ignored the effect of the MS_NOEXEC flag if it was used to mount(2) the underlying filesystem. Since kernel 2.6.20,the MS_NOEXEC flag is honored.SEE ALSOchmod(2), chown(2), open(2), setgid(2), setuid(2), stat(2), euidaccess(3), credentials(7), path_resolution(7), symlink(7)COLOPHONThis page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest versionof this page, can be found at /doc/man-pages/.Linux -07-23 ACCESS(2)Manual page access(2) line 123/164 (END) (press h for help or q to quit)

太多了，先放着，以后再看，大概知道它是用来检查文件的就好了，，，