[#BN_usub]
= BN_usub
:mrdocs:

Subtract unsigned BIGNUMs: `r` = `a` ‐ `b` (requires `a` >= `b).`

== Synopsis

Declared in `<openssl/bn.h>`

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

== Return Value

1 on success, or 0 if `a` < `b` or on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *r*
| Destination difference (non‐negative).
| *a*
| Minuend.
| *b*
| Subtrahend; must not exceed `a.`
|===


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