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.
The
accessfunction 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 flagsR_OK,W_OK,X_OK, or the existence testF_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
setuidorsetgidprogram (see How Change Persona), it gives information relative to the user who actually ran the program.The return value is
0if the access is permitted, and-1otherwise. (In other words, treated as a predicate function,accessreturns true if the requested access is denied.)In addition to the usual file name errors (see File Name Errors), the following
errnoerror 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.