✏ 编辑: BN_CTX_start.3

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.\" ========================================================================
.\"
.IX Title "BN_CTX_start 3"
.TH BN_CTX_start 3 "2019-12-20" "1.0.2u" "OpenSSL"
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.nh
.SH "NAME"
BN_CTX_start, BN_CTX_get, BN_CTX_end \- use temporary BIGNUM variables
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
These functions are used to obtain temporary \fB\s-1BIGNUM\s0\fR variables from
a \fB\s-1BN_CTX\s0\fR (which can been created by using \fIBN_CTX_new\fR\|(3))
in order to save the overhead of repeatedly creating and
freeing \fB\s-1BIGNUM\s0\fRs in functions that are called from inside a loop.
.PP
A function must call \fIBN_CTX_start()\fR first. Then, \fIBN_CTX_get()\fR may be
called repeatedly to obtain temporary \fB\s-1BIGNUM\s0\fRs. All \fIBN_CTX_get()\fR
calls must be made before calling any other functions that use the
\&\fBctx\fR as an argument.
.PP
Finally, \fIBN_CTX_end()\fR must be called before returning from the function.
When \fIBN_CTX_end()\fR is called, the \fB\s-1BIGNUM\s0\fR pointers obtained from
\&\fIBN_CTX_get()\fR become invalid.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIBN_CTX_start()\fR and \fIBN_CTX_end()\fR return no values.
.PP
\&\fIBN_CTX_get()\fR returns a pointer to the \fB\s-1BIGNUM\s0\fR, or \fB\s-1NULL\s0\fR on error.
Once \fIBN_CTX_get()\fR has failed, the subsequent calls will return \fB\s-1NULL\s0\fR
as well, so it is sufficient to check the return value of the last
\&\fIBN_CTX_get()\fR call. In case of an error, an error code is set, which
can be obtained by \fIERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIBN_CTX_new\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIBN_CTX_start()\fR, \fIBN_CTX_get()\fR and \fIBN_CTX_end()\fR were added in OpenSSL 0.9.5.

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