GNU Info
(libc.info)Testing File Access
Testing Permission to Access a File
-----------------------------------
In some situations it is desirable to allow programs to access files
or devices even if this is not possible with the permissions granted to
the user. One possible solution is to set the setuid-bit of the program
file. If such a program is started the _effective_ user ID of the
process is changed to that of the owner of the program file. So to
allow write access to files like `/etc/passwd', which normally can be
written only by the super-user, the modifying program will have to be
owned by `root' and the setuid-bit must be set.
But beside the files the program is intended to change the user
should not be allowed to access any file to which s/he would not have
access anyway. The program therefore must explicitly check whether _the
user_ would have the necessary access to a file, before it reads or
writes the file.
To do this, use the function `access', which checks for access
permission based on the process's _real_ user ID rather than the
effective user ID. (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)
There is another way you could check this access, which is easy to
describe, but very hard to use. This is to examine the file mode bits
and mimic the system's own access computation. This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have. Using `access' is simple and automatically does whatever is
appropriate for the system you are using.
`access' is _only_ only appropriate to use in setuid programs. A
non-setuid program will always use the effective ID rather than the
real ID.
The symbols in this section are declared in `unistd.h'.
- Function: int access (const char *FILENAME, int HOW)
The `access' function checks to see whether the file named by
FILENAME can be accessed in the way specified by the HOW argument.
The HOW argument either can be the bitwise OR of the flags
`R_OK', `W_OK', `X_OK', or the existence test `F_OK'.
This function uses the _real_ user and group IDs of the calling
process, rather than the _effective_ IDs, to check for access
permission. As a result, if you use the function from a `setuid'
or `setgid' program (Note: How Change Persona), it gives
information relative to the user who actually ran the program.
The return value is `0' if the access is permitted, and `-1'
otherwise. (In other words, treated as a predicate function,
`access' returns true if the requested access is _denied_.)
In addition to the usual file name errors (Note: File Name
Errors), the following `errno' error conditions are defined for
this function:
`EACCES'
The access specified by HOW is denied.
`ENOENT'
The file doesn't exist.
`EROFS'
Write permission was requested for a file on a read-only file
system.
These macros are defined in the header file `unistd.h' for use as
the HOW argument to the `access' function. The values are integer
constants.
- Macro: int R_OK
Flag meaning test for read permission.
- Macro: int W_OK
Flag meaning test for write permission.
- Macro: int X_OK
Flag meaning test for execute/search permission.
- Macro: int F_OK
Flag meaning test for existence of the file.
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