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1.Dd $Mdocdate: February 23 2015 $
2.Dt BN_CTX_START 3
3.Os
4.Sh NAME
5.Nm BN_CTX_start ,
6.Nm BN_CTX_get ,
7.Nm BN_CTX_end
8.Nd use temporary BIGNUM variables
9.Sh SYNOPSIS
10.In openssl/bn.h
11.Ft void
12.Fo BN_CTX_start
13.Fa "BN_CTX *ctx"
14.Fc
15.Ft BIGNUM *
16.Fo BN_CTX_get
17.Fa "BN_CTX *ctx"
18.Fc
19.Ft void
20.Fo BN_CTX_end
21.Fa "BN_CTX *ctx"
22.Fc
23.Sh DESCRIPTION
24These functions are used to obtain temporary
25.Vt BIGNUM
26variables from a
27.Vt BN_CTX
28(which can been created by using
29.Xr BN_CTX_new 3 )
30in order to save the overhead of repeatedly creating and freeing
31.Vt BIGNUM Ns s
32in functions that are called from inside a loop.
33.Pp
34A function must call
35.Fn BN_CTX_start
36first.
37Then,
38.Fn BN_CTX_get
39may be called repeatedly to obtain temporary
40.Vt BIGNUM Ns s.
41All
42.Fn BN_CTX_get
43calls must be made before calling any other functions that use the
44.Fa ctx
45as an argument.
46.Pp
47Finally,
48.Fn BN_CTX_end
49must be called before returning from the function.
50When
51.Fn BN_CTX_end
52is called, the
53.Vt BIGNUM
54pointers obtained from
55.Fn BN_CTX_get
56become invalid.
57.Sh RETURN VALUES
58.Fn BN_CTX_start
59and
60.Fn BN_CTX_end
61return no values.
62.Pp
63.Fn BN_CTX_get
64returns a pointer to the
65.Vt BIGNUM ,
66or
67.Dv NULL
68on error.
69Once
70.Fn BN_CTX_get
71has failed, the subsequent calls will return
72.Dv NULL
73as well, so it is sufficient to check the return value of the last
74.Fn BN_CTX_get
75call.
76In case of an error, an error code is set, which can be obtained by
77.Xr ERR_get_error 3 .
78.Sh SEE ALSO
79.Xr BN_CTX_new 3
80.Sh HISTORY
81.Fn BN_CTX_start ,
82.Fn BN_CTX_get ,
83and
84.Fn BN_CTX_end
85were added in OpenSSL 0.9.5.