# Need help with /bitcoin-core/secp256k1
**URL:** https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215
**Category:** New to Julia
**Created:** [January 7, 2018, 7:10am UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215 "2018-01-07T07:10:00Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@1112](https://discourse.julialang.org/u/1112)
#### Post date: [January 7, 2018, 7:10am UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/1 "2018-01-07T07:10:00Z")
I want to create a public key for private key 7
There is such code
```julia
struct secp256k1_pubkey
data::NTuple{64, Cuchar}
end
pubkey = Ref{secp256k1_pubkey}()
private_key = 7
pub = ccall((:secp256k1_ec_pubkey_create, "/usr/local/lib/libsecp256k1.so"), Int, (Pair{Int64, Int64}, Ref{secp256k1_pubkey}, Int32), secp256k1_ctx, pubkey, private_key)
```
in return, I get
[libsecp256k1] illegal argument: pubkey != NULL
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [January 7, 2018, 8:12am UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/2 "2018-01-07T08:12:28Z")
Do you have source for that library? Can you give a link to it?
---
### Author:  [@1112](https://discourse.julialang.org/u/1112)
#### Post date: [January 7, 2018, 8:15am UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/3 "2018-01-07T08:15:31Z")
ScottPJones, hello!
Oh sure
[https://github.com/bitcoin-core/secp256k1](https://github.com/bitcoin-core/secp256k1)
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [January 7, 2018, 8:36am UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/4 "2018-01-07T08:36:14Z")
It looks like you don’t have the calling sequence correct:
The function wants 3 pointers:
```nohighlight
int secp256k1_ec_pubkey_create(const secp256k1_context* ctx, secp256k1_pubkey *pubkey, const unsigned char *seckey) {
```
I don’t know why you are trying to pass a Julia type for the first argument, and `seckey` needs to be a pointer to a `Vector{UInt8}`, you are passing an `Int`, which is likely to cause a core-dump.
Also, it returns a `Cint`, not an `Int` (`Cint` will be 32-bits, `Int` will be 32 or 64 on Julia, 64 for 64-bit platforms)
---
### Author:  [@1112](https://discourse.julialang.org/u/1112)
#### Post date: [January 7, 2018, 8:54am UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/5 "2018-01-07T08:54:09Z")
Can you demonstrate the code?
If I change the type, I get a type conversion error.
```julia
int secp256k1_ec_pubkey_create(const secp256k1_context* ctx, secp256k1_pubkey *pubkey, const unsigned char *seckey) {
secp256k1_gej pj;
secp256k1_ge p;
secp256k1_scalar sec;
int overflow;
int ret = 0;
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(pubkey != NULL);
memset(pubkey, 0, sizeof(*pubkey));
ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
ARG_CHECK(seckey != NULL);
secp256k1_scalar_set_b32(&sec, seckey, &overflow);
ret = (!overflow) & (!secp256k1_scalar_is_zero(&sec));
if (ret) {
secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &pj, &sec);
secp256k1_ge_set_gej(&p, &pj);
secp256k1_pubkey_save(pubkey, &p);
}
secp256k1_scalar_clear(&sec);
return ret;
}
```
on the first argument the function does not swear
VERIFY\_CHECK (ctx! = NULL); here we pass
and here
ARG\_CHECK (pubkey! = NULL); we get an error
---
### Author:  [@1112](https://discourse.julialang.org/u/1112)
#### Post date: [January 7, 2018, 1:28pm UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/6 "2018-01-07T13:28:27Z")
Tell me, do not you exist the standard library of Julia for the ecdsa?
---
### Author:  [@dfdx](https://discourse.julialang.org/u/dfdx)
#### Post date: [January 7, 2018, 2:06pm UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/7 "2018-01-07T14:06:25Z")
> ARG\_CHECK (pubkey! = NULL); we get an error
This means that you have passed `NULL` for `pubkey`. Let’s see what is `pubkey` in your code:
> pubkey = Ref{secp256k1\_pubkey}()
Well, `Ref` without arguments is that same as `Ref(0)`, so your `pubkey` is indeed `NULL`, while `secp256k1_ec_pubkey_create` clearly needs it to be a pointer to a pre-allocated array of 64 bytes. Try something like this:
```julia
pubkey = Vector{Cuchar}(64) # pre-allocate buffer
private_key = 7
pub = ccall((:secp256k1_ec_pubkey_create, "/usr/local/lib/libsecp256k1.so"), Cint, (Pair{Int64, Int64}, Ptr{Cuchar}, Int32), secp256k1_ctx, pubkey, private_key)
```
Note that I also replaced `Ref{secp256k1_pubkey}` with `Ptr{Cuchar}`. When you invoke `ccall`, Julia should(\*) take a pointer to array data and pass it to the function, so when it’s finished this array will contain the populated data.
(\* - “should” because I always forget all the conversions between Julia and C types, but a quick test will confirm it).
---
### Author:  [@1112](https://discourse.julialang.org/u/1112)
#### Post date: [January 7, 2018, 2:40pm UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/8 "2018-01-07T14:40:43Z")
dfdx, thanks, but the mistake is same
[libsecp256k1] illegal argument: pubkey != NULL
---
### Author:  [@1112](https://discourse.julialang.org/u/1112)
#### Post date: [January 7, 2018, 2:58pm UTC](https://discourse.julialang.org/t/need-help-with-bitcoin-core-secp256k1/8215/9 "2018-01-07T14:58:56Z")
```julia
pubkey = Vector{Cuchar}(64)
println(pubkey)
UInt8[0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]
```
```julia
struct secp256k1_pubkey
data::NTuple{64, Cuchar}
end
pubkey = Ref{secp256k1_pubkey}()
println(pubkey)
Base.RefValue{secp256k1_pubkey}(secp256k1_pubkey((0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00)))
UInt8[0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]
```