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ACCESS

Section: POSIX Programmer's Manual (3P)
Updated: 2013
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PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

access, faccessat --- determine accessibility of a file relative to directory file descriptor

SYNOPSIS

#include <unistd.h>

int access(const char *path, int amode);
int faccessat(int fd, const char *path, int amode, int flag);

DESCRIPTION

The access() function shall check the file named by the pathname pointed to by the path argument for accessibility according to the bit pattern contained in amode, using the real user ID in place of the effective user ID and the real group ID in place of the effective group ID.

The value of amode is either the bitwise-inclusive OR of the access permissions to be checked (R_OK, W_OK, X_OK) or the existence test (F_OK).

If any access permissions are checked, each shall be checked individually, as described in the Base Definitions volume of POSIX.1-2008, Section 4.4, File Access Permissions, except that where that description refers to execute permission for a process with appropriate privileges, an implementation may indicate success for X_OK even if execute permission is not granted to any user.

The faccessat() function shall be equivalent to the access() function, except in the case where path specifies a relative path. In this case the file whose accessibility is to be determined shall be located relative to the directory associated with the file descriptor fd instead of the current working directory. If the file descriptor was opened without O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the file descriptor was opened with O_SEARCH, the function shall not perform the check.

If faccessat() is passed the special value AT_FDCWD in the fd parameter, the current working directory shall be used and the behavior shall be identical to a call to access().

Values for flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

AT_EACCESS: The checks for accessibility are performed using the effective user and group IDs instead of the real user and group ID as required in a call to access().

RETURN VALUE

Upon successful completion, these functions shall return 0. Otherwise, these functions shall return -1 and set errno to indicate the error.

ERRORS

These functions shall fail if:

EACCES: Permission bits of the file mode do not permit the requested access, or search permission is denied on a component of the path prefix.
ELOOP: A loop exists in symbolic links encountered during resolution of the path argument.
ENAMETOOLONG: The length of a component of a pathname is longer than {NAME_MAX}.
ENOENT: A component of path does not name an existing file or path is an empty string.
ENOTDIR: A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the path argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.
EROFS: Write access is requested for a file on a read-only file system.

The faccessat() function shall fail if:

EACCES: fd was not opened with O_SEARCH and the permissions of the directory underlying fd do not permit directory searches.
EBADF: The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor open for reading or searching.
ENOTDIR: The path argument is not an absolute path and fd is a file descriptor associated with a non-directory file.

These functions may fail if:

EINVAL: The value of the amode argument is invalid.
ELOOP: More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
ENAMETOOLONG: The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.
ETXTBSY: Write access is requested for a pure procedure (shared text) file that is being executed.

The faccessat() function may fail if:

EINVAL: The value of the flag argument is not valid.

The following sections are informative.

EXAMPLES

Testing for the Existence of a File

The following example tests whether a file named myfile exists in the /tmp directory.


#include <unistd.h>
...
int result;
const char *pathname = "/tmp/myfile";

result = access (pathname, F_OK);

APPLICATION USAGE

Additional values of amode other than the set defined in the description may be valid; for example, if a system has extended access controls.

The use of the AT_EACCESS value for flag enables functionality not available in access().

RATIONALE

In early proposals, some inadequacies in the access() function led to the creation of an eaccess() function because:

1.: Historical implementations of access() do not test file access correctly when the process' real user ID is superuser. In particular, they always return zero when testing execute permissions without regard to whether the file is executable.
2.: The superuser has complete access to all files on a system. As a consequence, programs started by the superuser and switched to the effective user ID with lesser privileges cannot use access() to test their file access permissions.

However, the historical model of eaccess() does not resolve problem (1), so this volume of POSIX.1-2008 now allows access() to behave in the desired way because several implementations have corrected the problem. It was also argued that problem (2) is more easily solved by using open(), chdir(), or one of the exec functions as appropriate and responding to the error, rather than creating a new function that would not be as reliable. Therefore, eaccess() is not included in this volume of POSIX.1-2008.

The sentence concerning appropriate privileges and execute permission bits reflects the two possibilities implemented by historical implementations when checking superuser access for X_OK.

New implementations are discouraged from returning X_OK unless at least one execution permission bit is set.

The purpose of the faccessat() function is to enable the checking of the accessibility of files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to access(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the faccessat() function it can be guaranteed that the file tested for accessibility is located relative to the desired directory.

FUTURE DIRECTIONS

None.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

Index

Testing for the Existence of a File

APPLICATION USAGE

RATIONALE

FUTURE DIRECTIONS

SEE ALSO

COPYRIGHT

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Time: 14:28:45 GMT, February 25, 2017