Manpage of ACCESS
ACCESSSection: Linux Programmer's Manual (2)
NAMEaccess, faccessat - check user's permissions for a file
#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)):
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
DESCRIPTIONaccess() checks whether the calling process can access the file pathname. If pathnameis a symbolic link, it is dereferenced.
The modespecifies 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_OKtests for the existence of the file. R_OK, W_OK, and X_OKtest whether the file exists and grants read, write, and execute permissions, respectively.
The check is done using the calling process's realUID 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; and for non-root users, the check uses an empty set of capabilities.
This allows set-user-ID programs and capability-endowed programs to easily determine the invoking user's authority. In other words, access() does not answer the "can I read/write/execute this file?" question. It answers a slightly different question: "(assuming I'm a setuid binary) can the user who invoked meread/write/execute this file?", which gives set-user-ID programs the possibility to prevent malicious users from causing them to read files which users shouldn't be able to read.
faccessat()The faccessat() system call operates in exactly the same way as access(), except for the differences described here.
If the pathname given in pathnameis relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd(rather than relative to the current working directory of the calling process, as is done by access() for a relative pathname).
If pathnameis relative and dirfdis the special value AT_FDCWD, then pathnameis interpreted relative to the current working directory of the calling process (like access()).
If pathnameis absolute, then dirfdis ignored.
flagsis constructed by ORing together zero or more of the following values:
- Perform access checks using the effective user and group IDs. By default, faccessat() uses the real IDs (like access()).
- If pathnameis 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 modeis F_OKand the file exists), zero is returned. On error (at least one bit in modeasked for a permission that is denied, or modeis F_OKand the file does not exist, or some other error occurred), -1 is returned, and errnois set appropriately.
ERRORSaccess() and faccessat() shall fail if:
- 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. (See also path_resolution(7).)
- Too many symbolic links were encountered in resolving pathname.
- pathnameis too long.
- A component of pathnamedoes not exist or is a dangling symbolic link.
- A component used as a directory in pathnameis not, in fact, a directory.
- Write permission was requested for a file on a read-only filesystem.
access() and faccessat() may fail if:
- pathnamepoints outside your accessible address space.
- modewas incorrectly specified.
- An I/O error occurred.
- Insufficient kernel memory was available.
- Write access was requested to an executable which is being executed.
The following additional errors can occur for faccessat():
- dirfdis not a valid file descriptor.
- Invalid flag specified in flags.
- pathnameis relative and dirfdis 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.1-2008.
Warning: 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 system call 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 ID and 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 modeis denied, even if some of the other access types in modeare permitted.
If the calling process has appropriate privileges (i.e., is superuser), POSIX.1-2001 permits an implementation to indicate success for an X_OKcheck even if 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 pathnamegrant search (i.e., execute) access. If any directory 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 execve(2) call will still fail.. Therefore, if a directory is found to be writable, it probably means that files can be created 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
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_EACCESSand AT_SYMLINK_NOFOLLOWflags are actually implemented within the glibc wrapper function for faccessat(). If either of these flags is specified, then the wrapper function employs fstatat(2) to determine access permissions.
Glibc notesOn older kernels where faccessat() is unavailable (and when the AT_EACCESSand AT_SYMLINK_NOFOLLOWflags are not specified), the glibc wrapper function falls back to the use of access(). When pathnameis a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fdthat corresponds to the dirfdargument.
BUGSIn kernel 2.4 (and earlier) there is some strangeness in the handling of X_OKtests for superuser. If all categories of execute permission are disabled for a nondirectory file, then the only access() test that returns -1 is when modeis specified as just X_OK; if R_OKor W_OKis 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_NOEXECflag if it was used to mount(2) the underlying filesystem. Since kernel 2.6.20, the MS_NOEXECflag is honored.
SEE ALSOchmod(2), chown(2), open(2), setgid(2), setuid(2), stat(2), euidaccess(3), credentials(7), path_resolution(7), symlink(7)
- RETURN VALUE
- CONFORMING TO
- SEE ALSO
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