[#BN_mod_sub]
= BN_mod_sub
:mrdocs:

Compute `r` = (`a` ‐ `b)` mod `m.`

== Synopsis

Declared in `<openssl/bn.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
int
BN_mod_sub(
    xref:BIGNUM.adoc[BIGNUM]* r,
    xref:BIGNUM.adoc[BIGNUM] const* a,
    xref:BIGNUM.adoc[BIGNUM] const* b,
    xref:BIGNUM.adoc[BIGNUM] const* m,
    xref:BN_CTX.adoc[BN_CTX]* ctx);
----

== Return Value

1 on success, or 0 on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *r*
| Destination BIGNUM.
| *a*
| Minuend.
| *b*
| Subtrahend.
| *m*
| Modulus (must be positive).
| *ctx*
| Optional BN_CTX for temporaries, or NULL.
|===


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