ACCESS(P) POSIX Programmer's Manual ACCESS(P)NAMEaccess - 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 individu‐
ally, as described in the Base Definitions volume of
IEEE Std 1003.1-2001, Chapter 3, Definitions. If the process has appro‐
priate 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 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 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 descrip‐
tion may be valid; for example, if a system has extended access con‐
trols.
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 cor‐
rectly 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 IEEE Std 1003.1-2001 now allows access() to
behave in the desired way because several implementations have cor‐
rected the problem. It was also argued that problem (2) is more easily
solved by using open(), chdir(), or one of the exec functions as appro‐
priate 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 implemen‐
tations when checking superuser access for X_OK.
New implementations are discouraged from returning X_OK unless at least
one execution permission bit is set.
FUTURE DIRECTIONS
None.
SEE ALSOchmod() , 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 .
IEEE/The Open Group 2003 ACCESS(P)
