access(2)

1ACCESS(2)                  Linux Programmer's Manual                 ACCESS(2)
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NAME

6       access - check user's permissions for a file
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SYNOPSIS

9       #include <unistd.h>
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11       int access(const char *pathname, int mode);
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DESCRIPTION

14       access()  checks whether the process would be allowed to read, write or
15       test for existence of the file (or other file system object) whose name
16       is  pathname.   If  pathname is a symbolic link permissions of the file
17       referred to by this symbolic link are tested.
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19       mode is a mask consisting of one or more of R_OK, W_OK, X_OK and F_OK.
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21       R_OK, W_OK and X_OK request checking whether the file  exists  and  has
22       read,  write and execute permissions, respectively.  F_OK just requests
23       checking for the existence of the file.
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25       The tests depend on the permissions of the directories occurring in the
26       path  to  the  file,  as  given  in pathname, and on the permissions of
27       directories and files referred to by symbolic links encountered on  the
28       way.
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30       The check is done with the process's real UID and GID, rather than with
31       the effective IDs as is done when  actually  attempting  an  operation.
32       This  is to allow set-user-ID programs to easily determine the invoking
33       user's authority.
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35       Only access bits are checked, not the file type  or  contents.   There‐
36       fore,  if a directory is found to be "writable," it probably means that
37       files can be created in the directory, and not that the  directory  can
38       be  written  as a file.  Similarly, a DOS file may be found to be "exe‐
39       cutable," but the execve(2) call will still fail.
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41       If the process has appropriate privileges, an implementation may  indi‐
42       cate  success for X_OK even if none of the execute file permission bits
43       are set.
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RETURN VALUE

46       On success (all requested permissions granted), zero is  returned.   On
47       error  (at least one bit in mode asked for a permission that is denied,
48       or some other error occurred), -1 is returned, and errno is set  appro‐
49       priately.
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ERRORS

52       access() shall fail if:
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54       EACCES The  requested access would be denied to the file or search per‐
55              mission is denied for one of the directories in the path  prefix
56              of pathname.  (See also path_resolution(2).)
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58       ELOOP  Too many symbolic links were encountered in resolving pathname.
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60       ENAMETOOLONG
61              pathname is too long.
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63       ENOENT A component of pathname does not exist or is a dangling symbolic
64              link.
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66       ENOTDIR
67              A component used as a directory in pathname is not, in  fact,  a
68              directory.
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70       EROFS  Write  permission  was  requested  for  a  file  on  a read-only
71              filesystem.
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73       access() may fail if:
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75       EFAULT pathname points outside your accessible address space.
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77       EINVAL mode was incorrectly specified.
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79       EIO    An I/O error occurred.
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81       ENOMEM Insufficient kernel memory was available.
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83       ETXTBSY
84              Write access was requested to an executable which is being  exe‐
85              cuted.
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LINUX NOTES

88       In  kernels before 2.6.20, access() ignored the effect of the MS_NOEXEC
89       flag if it was used to mount(2) the underlying file system.  Since ker‐
90       nel 2.6.20, access() honours this flag.
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RESTRICTIONS

93       access()  returns  an error if any of the access types in the requested
94       call fails, even if other types might be successful.
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96       access() may not work correctly on NFS file systems  with  UID  mapping
97       enabled,  because UID mapping is done on the server and hidden from the
98       client, which checks permissions.
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100       Using access() to check if a user is authorized to  e.g.  open  a  file
101       before actually doing so using open(2) creates a security hole, because
102       the user might exploit the short time  interval  between  checking  and
103       opening the file to manipulate it.
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CONFORMING TO

106       SVr4, 4.3BSD, POSIX.1-2001.
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