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This program now successfully restricts access to files owned by the real user of the program, by matching the file owner's user and group IDs to the processes real user and group IDs. This solution can be used verify that the owner of the file is the one the program expects, reducing opportunities for attackers to replace configuration files with malicious ones, for example.
Alternatively, the same solution could be implemented using the C99 fopen() function to open the file and the POSIX fileno() function to convert the FILE object pointer to a file descriptor.
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