D. J. Bernstein
Data structures and program structures

The cdb-reading library interface


You can read records in a constant database from file descriptor fd as follows:
  1. Use cdb_init to place information about fd into a struct cdb variable c.
  2. Carry out any number of searches, as described below.
  3. Use cdb_free to remove any memory map that might have been reserved by cdb_init.
Each search works as follows:
  1. Use cdb_find to search for a record under key k. If cdb_find returns 0, the database does not contain that key; stop. If cdb_find returns -1, there was a read error; abort.
  2. Use cdb_datalen to find the number of bytes of data in this record. Allocate a pointer d to a region of memory large enough to hold the data. If not enough memory is available, abort.
  3. Use cdb_read with cdb_datapos to read the data. If cdb_read returns -1, there was a read error; abort.
  4. Do something with the data, and then free the allocated region of memory.
There may be several records under a single key. You can use cdb_findnext to find the next record under this key.


     #include <cdb.h>

     result = cdb_read(&c,d,dlen,dpos);

     result = cdb_findnext(&c,k,klen);
     result = cdb_find(&c,k,klen);

     dpos = cdb_datapos(&c);
     dlen = cdb_datalen(&c);

     static struct cdb c;
     int fd;

     char *d;
     unsigned int dlen;
     uint32 dpos;

     char *k;
     unsigned int klen;
     int result;

A struct cdb variable such as c is either unallocated or allocated. If it is allocated, it holds information about a constant database:

c must be initialized to zero, meaning unallocated.

cdb_free unallocates c if c is allocated. Otherwise it leaves c alone. cdb_free does not close fd.

cdb_init allocates c to hold information about a constant database read by descriptor fd. You may call cdb_init repeatedly; if c is already allocated, cdb_init unallocates it first.

cdb_read reads dlen bytes into d from byte position dpos in the database. You must allocate c before calling cdb_read. Normally cdb_read returns 0. If the database file is shorter than dpos+dlen bytes, or if there is a disk read error, cdb_read returns -1, setting errno appropriately.

cdb_findstart prepares c to search for the first record under a new key. You must allocate c before calling cdb_findstart, and you must call cdb_findstart before calling cdb_findnext.

cdb_findnext looks for the nth record under key k in the database, where n is the number of calls to cdb_findnext after the most recent call to cdb_findstart. If it finds the record, cdb_findnext returns 1; if there are exactly n-1 such records, cdb_findnext returns 0; if there are fewer than n-1 such records, the behavior of cdb_findnext is undefined; if there is a database format error or disk error, cdb_findnext returns -1, setting errno appropriately. Each call to cdb_findnext (before another call to cdb_findstart) must use the same k and klen.

If cdb_findnext returns 1, it arranges for cdb_datapos to return the starting byte position of the data in the record, and for cdb_datalen to return the number of bytes of data in the record. Otherwise the results of cdb_datapos and cdb_datalen are undefined.

cdb_find is the same as cdb_findstart followed by cdb_findnext: it finds the first record under key k.

Beware that these functions may rely on non-atomic operations on the fd ofile, such as seeking to a particular position and then reading. Do not attempt two simultaneous database reads using a single ofile.