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< th colspan = "3" align = "center" > DB-> exists()< / th >
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The DB Handle
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< h2 class = "title" style = "clear: both" > < a id = "dbexists" > < / a > DB-> exists()< / h2 >
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< pre class = "programlisting" > #include < db.h>
int
DB-> exists(DB *db, DB_TXN *txnid, DBT *key, u_int32_t flags); < / pre >
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The < code class = "methodname" > DB-> exists()< / code > method returns whether the specified key appears in the
database.
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The < code class = "methodname" > DB-> exists()< / code > method will return < a href = "../../programmer_reference/program_errorret.html#program_errorret.DB_NOTFOUND" class = "olink" > DB_NOTFOUND< / a > if the specified key
is not in the database. The < code class = "methodname" > DB-> exists()< / code > method will return
< a href = "../../programmer_reference/program_errorret.html#program_errorret.DB_KEYEMPTY" class = "olink" > DB_KEYEMPTY< / a > if the database
is a Queue or Recno database and the specified key exists, but was never explicitly created
by the application or was later deleted.
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< h3 class = "title" > < a id = "id3675183" > < / a > Parameters< / h3 >
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< h4 class = "title" > < a id = "id3675588" > < / a > txnid< / h4 >
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If the operation is part of an application-specified transaction, the < span class = "bold" > < strong > txnid< / strong > < / span > parameter is a transaction handle returned from
< a class = "xref" href = "txnbegin.html" title = "DB_ENV->txn_begin()" > DB_ENV-> txn_begin()< / a > ; if the operation is part of a
Berkeley DB Concurrent Data Store group, the < span class = "bold" > < strong > txnid< / strong > < / span >
parameter is a handle returned from
< a class = "xref" href = "envcdsgroup_begin.html" title = "DB_ENV->cdsgroup_begin()" > DB_ENV-> cdsgroup_begin()< / a > ; otherwise NULL.
If no transaction handle is specified, but the
operation occurs in a transactional database, the operation will be
implicitly transaction protected.
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< h4 class = "title" > < a id = "id3675144" > < / a > key< / h4 >
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The key < a class = "link" href = "dbt.html" title = "Chapter > DBT< / a > operated on.
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< h4 class = "title" > < a id = "id3674787" > < / a > flags< / h4 >
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The < span class = "bold" > < strong > flags< / strong > < / span > parameter must be set to zero or
by bitwise inclusively < span class = "bold" > < strong > OR< / strong > < / span > 'ing together one
or more of the following values:
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< p > < a id = "exists_DB_READ_COMMITTED" > < / a >
< code class = "literal" > DB_READ_COMMITTED< / code >
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Configure a transactional read operation to have degree 2 isolation
(the read is not repeatable).
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< p > < a id = "exists_DB_READ_UNCOMMITTED" > < / a >
< code class = "literal" > DB_READ_UNCOMMITTED< / code >
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Configure a transactional read operation to have degree 1 isolation,
reading modified but not yet committed data. Silently ignored if the
< a class = "link" href = "dbopen.html#dbopen_DB_READ_UNCOMMITTED" > DB_READ_UNCOMMITTED< / a >
flag was not specified when the underlying database was opened.
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< p > < a id = "exists_DB_RMW" > < / a >
< code class = "literal" > DB_RMW< / code >
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Acquire write locks instead of read locks when doing the read, if locking is
configured. Setting this flag can eliminate deadlock during a read-modify-write
cycle by acquiring the write lock during the read part of the cycle so that
another thread of control acquiring a read lock for the same item, in its own
read-modify-write cycle, will not result in deadlock.
< / p >
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Because the < code class = "methodname" > DB-> exists()< / code > method will not hold locks
across Berkeley DB calls in non-transactional operations, the
< a class = "link" href = "dbcget.html#dbcget_DB_RMW" > DB_RMW< / a > flag to the
< code class = "methodname" > DB-> exists()< / code > call is meaningful only in the presence
of transactions.
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< a class = "link" href = "db.html" title = "Chapter > DB< / a >
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< h3 class = "title" > < a id = "id3675646" > < / a > See Also< / h3 >
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< a class = "xref" href = "db.html#dblist" title = "Database and Related Methods" > Database and Related Methods< / a >
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