ACCESS(3) Library Functions Manual ACCESS(3)

access faccessat — determine accessibility of a file relative to directory file descriptor

#include <unistd.h>

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

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 XBD V1_chap04(7), 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>:

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 ().

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

These functions shall fail if:

Permission bits of the file mode do not permit the requested access, or search permission is denied on a component of the path prefix.
A loop exists in symbolic links encountered during resolution of the path argument.
The length of a component of a pathname is longer than {NAME_MAX}.
A component of path does not name an existing file or path is an empty string.
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.
Write access is requested for a file on a read-only file system.

The faccessat () function shall fail if:

fd was not opened with O_SEARCH and the permissions of the directory underlying fd do not permit directory searches.
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.
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:

The value of the amode argument is invalid.
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
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}.
Write access is requested for a pure procedure (shared text) file that is being executed.

The faccessat () function may fail if:

The value of the flag argument is not valid.

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

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


result = access (pathname, F_OK);

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 ().

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(3), chdir(3), or one of the 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.

None.

chmod(3), fstatat(3)

XBD V1_chap04(7), <fcntl.h>, <unistd.h>

First released in Issue 1. Derived from Issue 1 of the SVID.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

  • The [ELOOP] mandatory error condition is added.
  • A second [ENAMETOOLONG] is added as an optional error condition.
  • The [ETXTBSY] optional error condition is added.

The following changes were made to align with the IEEE P1003.1a draft standard:

  • The [ELOOP] optional error condition is added.

Austin Group Interpretations 1003.1-2001 #046 and #143 are applied.

The faccessat () function is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

Changes are made to allow a directory to be opened for searching.

The [ENOTDIR] error condition is clarified to cover the condition where the last component of a pathname exists but is not a directory or a symbolic link to a directory.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0019 [461], XSH/TC1-2008/0020 [324], XSH/TC1-2008/0021 [278], XSH/TC1-2008/0022 [278], and XSH/TC1-2008/0023 [291] are applied.

January 1, 2013 posix.fail