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ACCESS(P) ACCESS(P)
NAME
access - determine accessibility of a file
SYNOPSIS
#include <unistd.h>
int access(const char *path, int amode);
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 IEEE Std 1003.1-2001, Chapter 3, Defi-
nitions. If the process has appropriate privileges, an implementation may indicate
success for X_OK even if none of the execute file permission bits are set.
RETURN VALUE
If the requested access is permitted, access() succeeds and shall return 0; other-
wise, -1 shall be returned and errno shall be set to indicate the error.
ERRORS
The access() function 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 the path argument exceeds {PATH_MAX} or a pathname component
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 is not a directory.
EROFS Write access is requested for a file on a read-only file system.
The access() function 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
As a result of encountering a symbolic link in resolution of the path argu-
ment, the length of the substituted pathname string exceeded {PATH_MAX}.
ETXTBSY
Write access is requested for a pure procedure (shared text) file that is
being executed.
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 direc-
tory.
#include <unistd.h>
...
int result;
const char *filename = "/tmp/myfile";
result = access (filename, 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.
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 exe-
cutable.
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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001.
The sentence concerning appropriate privileges and execute permission bits reflects
the two possibilities implemented by historical implementations when checking supe-
ruser access for X_OK.
New implementations are discouraged from returning X_OK unless at least one execu-
tion permission bit is set.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod() , stat() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C)
2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The
Open Group. 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.opengroup.org/unix/online.html .
POSIX 2003 ACCESS(P)
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