BN_CTX_start

- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- SEE ALSO
- HISTORY

# NAME

BN_CTX_start, BN_CTX_get, BN_CTX_end - use temporary BIGNUM variables

# SYNOPSIS

     #include <openssl/bn.h>

     void BN_CTX_start(BN_CTX *ctx);

     BIGNUM *BN_CTX_get(BN_CTX *ctx);

     void BN_CTX_end(BN_CTX *ctx);

# DESCRIPTION

These functions are used to obtain temporary BIGNUM variables from a BN_CTX (which can been created by using BN_CTX_new(3)) in order to save the overhead of repeatedly creating and freeing BIGNUMs in functions that are called from inside a loop.

A function must call `BN_CTX_start()` first. Then, `BN_CTX_get()` may be called repeatedly to obtain temporary BIGNUMs. All `BN_CTX_get()` calls must be made before calling any other functions that use the ctx as an argument.

Finally, `BN_CTX_end()` must be called before returning from the function. When `BN_CTX_end()` is called, the BIGNUM pointers obtained from `BN_CTX_get()` become invalid.

# RETURN VALUES

`BN_CTX_start()` and `BN_CTX_end()` return no values.

`BN_CTX_get()` returns a pointer to the BIGNUM, or NULL on error. Once `BN_CTX_get()` has failed, the subsequent calls will return NULL as well, so it is sufficient to check the return value of the last `BN_CTX_get()` call. In case of an error, an error code is set, which can be obtained by ERR_get_error(3).

# SEE ALSO

BN_CTX_new(3)

# HISTORY

`BN_CTX_start()`, `BN_CTX_get()` and `BN_CTX_end()` were added in OpenSSL 0.9.5.

BN_CTX_start