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, Definitions. If the pro-
cess 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; otherwise, -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 per-
mission 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 argument, 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 directory.
#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 pro-
cess' 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
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 reli-
able. 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 superuser access
for X_OK.
New implementations are discouraged from returning X_OK unless at least one execution per-
mission 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
POSIX 2003 ACCESS(P)
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