If any directories are named on the command line, then those are processed in turn. If not, then the SSL_CERT_DIR environment variable is consulted; this shold be a colon-separated list of directories, like the Unix PATH variable. If that is not set then the default directory (installation-specific but often /usr/local/ssl/certs) is processed.
In order for a directory to be processed, the user must have write permissions on that directory, otherwise it will be skipped. The links created are of the form "HHHHHHHH.D", where each H is a hexadecimal character and D is a single decimal digit. When processing a directory, c_rehash will first remove all links that have a name in that syntax. If you have links in that format used for other purposes, they will be removed. To skip the removal step, use the -n flag. Hashes for CRL's look similar except the letter r appears after the period, like this: "HHHHHHHH.rD".
Multiple objects may have the same hash; they will be indicated by incrementing the D value. Duplicates are found by comparing the full SHA-1 fingerprint. A warning will be displayed if a duplicate is found.
A warning will also be displayed if there are files that cannot be parsed as either a certificate or a CRL.
The program uses the openssl program to compute the hashes and fingerprints. If not found in the user's PATH, then set the OPENSSL environment variable to the full pathname. Any program can be used, it will be invoked as follows for either a certificate or CRL:
$OPENSSL x509 -hash -fingerprint -noout -in FILENAME $OPENSSL crl -hash -fingerprint -noout -in FILENAME
where FILENAME is the filename. It must output the hash of the file on the first line, and the fingerprint on the second, optionally prefixed with some text and an equals sign.