getaccess(2)

NAME

getaccess — get a user's effective access rights to a file

SYNOPSIS

#include <sys/getaccess.h> 

int getaccess( 
     const char *path, 
     uid_t uid, 
     int ngroups, 
     const gid_t *gidset, 
     void *label, 
     void *privs 
); 

DESCRIPTION

getaccess() identifies the access rights (read, write, execute/search) a specific user ID has to an existing file. path points to a path name of a file. If the call succeeds, it returns a value of zero or greater, representing the specified user's effective access rights (modes) to the file. The rights are expressed as the logical OR of bits (R_OK, W_OK, and X_OK) whose values are defined in the header <unistd.h>. A return of zero means that access is denied.

The uid parameter is a user ID. Special values, defined in <sys/getaccess.h>, represent the calling process's effective, real, or saved user ID:

UID_EUID: Effective user ID.
UID_RUID: Real user ID.
UID_SUID: Saved user ID.

ngroups is the number of group IDs in gidset, not to exceed NGROUPS_MAX + 1 (NGROUPS_MAX is defined in <limits.h>). If the ngroups parameter is positive, the gidset parameter is an array of group ID values to use in the check. If ngroups is a recognized negative value, gidset is ignored. Special negative values of ngroups, defined in <sys/getaccess.h>, represent various combinations of the process's effective, real, or saved user ID and its supplementary groups list:

NGROUPS_EGID: Use process's effective group ID only.
NGROUPS_RGID: Use process's real group ID only.
NGROUPS_SGID: Use process's saved group ID only.
NGROUPS_SUPP: Use process's supplementary groups only.
NGROUPS_EGID_SUPP: Use process's effective group ID plus supplementary groups.
NGROUPS_RGID_SUPP: Use process's real group ID plus supplementary groups.
NGROUPS_SGID_SUPP: Use process's saved group ID plus supplementary groups.

The label and privs parameters are placeholders for future extensions. For now, the values of these parameters must be (void *) 0.

The access check rules for access control lists are described in acl(5). In addition, the W_OK bit is cleared for files on read-only file systems or shared-text programs being executed. Note that as in access(2), the X_OK bit is not turned off for shared-text programs open for writing because there is no easy way to know that a file open for writing is a shared-text program.

If the caller's user ID is 0, or if it is UID_EUID, UID_RUID, or UID_SUID (see <sys/getaccess.h>) and the process's respective user ID is 0, R_OK and W_OK are always set except when W_OK is cleared for files on read-only file systems or shared-text programs being executed. X_OK is set if and only if the file is not a regular file or the execute bit is set in any of the file's ACL entries.

getaccess() checks each directory component of path by first using the caller's effective user ID, effective group ID, and supplementary groups list, regardless of the user ID specified. An error occurs, distinct from ``no access allowed,'' if the caller cannot search the path to the file. (In this case it is inappropriate for the caller to learn anything about the file.)

Comparison of access(2) and getaccess(2)

The following table compares various attributes of access() and getaccess().

access()	getaccess()
checks all ACL entries	same
(HFS File Systems only)
uses real uid, real gid, and	uses specified uid and groups list;
supplementary groups list	macros available for typical values
checks specific mode value,	returns all mode bits, each on or off
returns succeed or fail
checks path to file using caller's effective IDs	same
W_OK false if shared-text file	same
currently being executed
`W_OK` false if file on	same
read-only file system
`X_OK` not modified for file	same
currently open for writing
`R_OK` and `W_OK` always true for superuser	same
(except as above)
`X_OK` always true for superuser	`X_OK` true for super-user if file is not a regular
	file `or` execute is set in any ACL entry

RETURN VALUE

Upon successful completion, getaccess() returns a non-negative value representing the access rights of the specified user to the specified file. If an error occurs, a value of -1 is returned and errno is set to indicate the error.

ERRORS

getaccess() fails if any of the following conditions are encountered:

[EACCES]: A component of the path prefix denies search permission to the caller.
[EFAULT]: path or gidset points outside the allocated address space of the process. The reliable detection of this error is implementation dependent.
[EINVAL]: ngroups is invalid; ngroups is either zero, an unrecognized negative value, or a value larger than NGROUPS + 1.
[EINVAL]: gidset contains an invalid group ID value.
[EINVAL]: The value of label or privs is not a null pointer.
[ELOOP]: Too many symbolic links were encountered in translating the path name.
[ENAMETOOLONG]: The length of the specified path name exceeds PATH_MAX bytes, or the length of a component of the path name exceeds NAME_MAX bytes while _POSIX_NO_TRUNC is in effect.
[ENOENT]: The named file does not exist (for example, path is null or a component of path does not exist).
[ENOTDIR]: A component of the path prefix is not a directory.
[EOPNOTSUPP]: getaccess() is not supported on some types of remote files.

EXAMPLES

The following call determines the caller's effective access rights to file ``test,'' and succeeds if the user has read access:

#include <unistd.h>
#include <sys/getaccess.h>

int mode;

mode = getaccess ("test", UID_EUID, NGROUPS_EGID_SUPP,
       (int *) 0, (void *) 0, (void *) 0);

if ((mode >= 0) && (mode & R_OK)) ...

Here is one way to test access rights to file /tmp/hold for user ID 23, group ID 109:

int gid = 109;
int mode;

mode = getaccess ("/tmp/hold", 23, 1, & gid,
       (void *) 0, (void *) 0);

Should the need arise, the following code builds a gidset that includes the process's effective group ID:

#include <limits.h>

int gidset [NGROUPS_MAX + 1];
int ngroups;

gidset [0] = getegid();
ngroups = 1 + getgroups (NGROUPS_MAX, & gidset [1]);

AUTHOR

getaccess() was developed by HP.

getaccess(2)

Technical documentation

» Table of Contents

» Index