This set of API is obsoleted as of ATS v9.0.0, and will be removed with ATS v10.0.0! For details of the new APIs, see TSUserArgs.

#include <ts/ts.h>
TSReturnCode TSHttpTxnArgIndexReserve(const char *name, const char *description, int *arg_idx)
TSReturnCode TSHttpTxnArgIndexNameLookup(const char *name, int *arg__idx, const char **description)
TSReturnCode TSHttpTxnArgIndexLookup(int arg_idx, const char **name, const char **description)
TSReturnCode TSHttpSsnArgIndexReserve(const char *name, const char *description, int *arg_idx)
TSReturnCode TSHttpSsnArgIndexNameLookup(const char *name, int *arg__idx, const char **description)
TSReturnCode TSHttpSsnArgIndexLookup(int arg_idx, const char **name, const char **description)
void TSHttpTxnArgSet(TSHttpTxn txnp, int arg_idx, void *arg)
void *TSHttpTxnArgGet(TSHttpTxn txnp, int arg_idx)
void TSHttpSsnArgSet(TSHttpSsn ssnp, int arg_idx, void *arg)
void *TSHttpSsnArgGet(TSHttpSsn ssnp, int arg_idx)


Traffic Server sessions and transactions provide a fixed array of void pointers that can be used by plugins to store information. This can be used to avoid creating a per session or transaction continuations to hold data, or to communicate between plugins as the values in the array are visible to any plugin which can access the session or transaction. The array values are opaque to Traffic Server and it will not dereference nor release them. Plugins are responsible for cleaning up any resources pointed to by the values or, if the values are simply values, there is no need for the plugin to remove them after the session or transaction has completed.

To avoid collisions between plugins a plugin should first reserve an index in the array. A transaction based plugin argument is reserved by calling TSHttpTxnArgIndexReserve(). A session base plugin argument is reserved by calling TSHttpSsnArgIndexReserve(). Both functions have the arguments


An identifying name for the plugin that reserved the index. Required.


An optional description of the use of the arg. This can be nullptr.


A pointer to an int. If an index is successfully reserved, the int pointed at by this is set to the reserved index. It is not modified if the call is unsuccessful.

The functions return TS_SUCCESS if an index was reserved, TS_ERROR if not (most likely because all of the indices have already been reserved). Generally this will be a file or library scope global which is set at plugin initialization. This function is used in the example remap plugin example/plugins/c-api/remap/remap.cc. The index is stored in the plugin global arg_index. Transaction and session plugin argument indices are reserved independently.

To look up the owner of a reserved index use TSHttpTxnArgIndexNameLookup() or TSHttpSsnArgIndexNameLookup() for transaction and session plugin argument respectively. If name is found as an owner, the function returns TS_SUCCESS and arg_index is updated with the index reserved under that name. If description is not NULL then the character pointer to which it points will be updated to point at the description for that reserved index. This enables communication between plugins where plugin “A” reserves an index under a well known name and plugin “B” locates the index by looking it up under that name.

The owner of a reserved index can be found with TSHttpTxnArgIndexLookup() or TSHttpSsnArgIndexLookup() for transaction and session plugin arguments respectively. If arg_index is reserved then the function returns TS_SUCCESS and the pointers referred to by name and description are updated. name must point at a valid character pointer but description can be NULL in which case it is ignored.

Manipulating the array is simple. TSHttpTxnArgSet() sets the array slot at arg_idx for the transaction txnp to the value arg. Note this sets the value only for the specific transaction. Similarly TSHttpSsnArgSet() sets the value for a session argument. The values can be retrieved with TSHttpTxnArgGet() for transactions and TSHttpSsnArgGet() for sessions, which return the specified value. Values that have not been set are NULL.


Session arguments persist for the entire session, which means potentially across all transactions in that session.


Following arg index reservations is conventional, it is not enforced.