Merge pull request #125 from elichai/manual-memory

Implement pre allocation context creation

depend/secp256k1/include/secp256k1.h

@@ -188,51 +188,6 @@ typedef int (*secp256k1_nonce_function)(
  */
 SECP256K1_API extern const secp256k1_context *secp256k1_context_no_precomp;
 
-/** Create a secp256k1 context object (in dynamically allocated memory).
- *
- *  This function uses malloc to allocate memory. It is guaranteed that malloc is
- *  called at most once for every call of this function. If you need to avoid dynamic
- *  memory allocation entirely, see the functions in secp256k1_preallocated.h.
- *
- *  Returns: a newly created context object.
- *  In:      flags: which parts of the context to initialize.
- *
- *  See also secp256k1_context_randomize.
- */
-SECP256K1_API secp256k1_context* secp256k1_context_create(
-    unsigned int flags
-) SECP256K1_WARN_UNUSED_RESULT;
-
-/** Copy a secp256k1 context object (into dynamically allocated memory).
- *
- *  This function uses malloc to allocate memory. It is guaranteed that malloc is
- *  called at most once for every call of this function. If you need to avoid dynamic
- *  memory allocation entirely, see the functions in secp256k1_preallocated.h.
- *
- *  Returns: a newly created context object.
- *  Args:    ctx: an existing context to copy (cannot be NULL)
- */
-SECP256K1_API secp256k1_context* secp256k1_context_clone(
-    const secp256k1_context* ctx
-) SECP256K1_ARG_NONNULL(1) SECP256K1_WARN_UNUSED_RESULT;
-
-/** Destroy a secp256k1 context object (created in dynamically allocated memory).
- *
- *  The context pointer may not be used afterwards.
- *
- *  The context to destroy must have been created using secp256k1_context_create
- *  or secp256k1_context_clone. If the context has instead been created using
- *  secp256k1_context_preallocated_create or secp256k1_context_preallocated_clone, the
- *  behaviour is undefined. In that case, secp256k1_context_preallocated_destroy must
- *  be used instead.
- *
- *  Args:   ctx: an existing context to destroy, constructed using
- *               secp256k1_context_create or secp256k1_context_clone
- */
-SECP256K1_API void secp256k1_context_destroy(
-    secp256k1_context* ctx
-);
-
 /** Set a callback function to be called when an illegal argument is passed to
  *  an API call. It will only trigger for violations that are mentioned
  *  explicitly in the header.
@@ -301,28 +256,6 @@ SECP256K1_API void secp256k1_context_set_error_callback(
     const void* data
 ) SECP256K1_ARG_NONNULL(1);
 
-/** Create a secp256k1 scratch space object.
- *
- *  Returns: a newly created scratch space.
- *  Args: ctx:  an existing context object (cannot be NULL)
- *  In:   size: amount of memory to be available as scratch space. Some extra
- *              (<100 bytes) will be allocated for extra accounting.
- */
-SECP256K1_API SECP256K1_WARN_UNUSED_RESULT secp256k1_scratch_space* secp256k1_scratch_space_create(
-    const secp256k1_context* ctx,
-    size_t size
-) SECP256K1_ARG_NONNULL(1);
-
-/** Destroy a secp256k1 scratch space.
- *
- *  The pointer may not be used afterwards.
- *  Args:       ctx: a secp256k1 context object.
- *          scratch: space to destroy
- */
-SECP256K1_API void secp256k1_scratch_space_destroy(
-    const secp256k1_context* ctx,
-    secp256k1_scratch_space* scratch
-) SECP256K1_ARG_NONNULL(1);
 
 /** Parse a variable-length public key into the pubkey object.
  *

depend/secp256k1/src/scratch.h

@@ -21,9 +21,7 @@ typedef struct secp256k1_scratch_space_struct {
     size_t max_size;
 } secp256k1_scratch;
 
-static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t max_size);
 
-static void secp256k1_scratch_destroy(const secp256k1_callback* error_callback, secp256k1_scratch* scratch);
 
 /** Returns an opaque object used to "checkpoint" a scratch space. Used
  *  with `secp256k1_scratch_apply_checkpoint` to undo allocations. */

depend/secp256k1/src/scratch_impl.h

@@ -10,31 +10,6 @@
 #include "util.h"
 #include "scratch.h"
 
-static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t size) {
-    const size_t base_alloc = ((sizeof(secp256k1_scratch) + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
-    void *alloc = checked_malloc(error_callback, base_alloc + size);
-    secp256k1_scratch* ret = (secp256k1_scratch *)alloc;
-    if (ret != NULL) {
-        memset(ret, 0, sizeof(*ret));
-        memcpy(ret->magic, "scratch", 8);
-        ret->data = (void *) ((char *) alloc + base_alloc);
-        ret->max_size = size;
-    }
-    return ret;
-}
-
-static void secp256k1_scratch_destroy(const secp256k1_callback* error_callback, secp256k1_scratch* scratch) {
-    if (scratch != NULL) {
-        VERIFY_CHECK(scratch->alloc_size == 0); /* all checkpoints should be applied */
-        if (memcmp(scratch->magic, "scratch", 8) != 0) {
-            secp256k1_callback_call(error_callback, "invalid scratch space");
-            return;
-        }
-        memset(scratch->magic, 0, sizeof(scratch->magic));
-        free(scratch);
-    }
-}
-
 static size_t secp256k1_scratch_checkpoint(const secp256k1_callback* error_callback, const secp256k1_scratch* scratch) {
     if (memcmp(scratch->magic, "scratch", 8) != 0) {
         secp256k1_callback_call(error_callback, "invalid scratch space");

depend/secp256k1/src/secp256k1.c

@@ -136,17 +136,6 @@ secp256k1_context* secp256k1_context_preallocated_create(void* prealloc, unsigne
     return (secp256k1_context*) ret;
 }
 
-secp256k1_context* secp256k1_context_create(unsigned int flags) {
-    size_t const prealloc_size = secp256k1_context_preallocated_size(flags);
-    secp256k1_context* ctx = (secp256k1_context*)checked_malloc(&default_error_callback, prealloc_size);
-    if (EXPECT(secp256k1_context_preallocated_create(ctx, flags) == NULL, 0)) {
-        free(ctx);
-        return NULL;
-    }
-
-    return ctx;
-}
-
 secp256k1_context* secp256k1_context_preallocated_clone(const secp256k1_context* ctx, void* prealloc) {
     size_t prealloc_size;
     secp256k1_context* ret;
@@ -161,17 +150,6 @@ secp256k1_context* secp256k1_context_preallocated_clone(const secp256k1_context*
     return ret;
 }
 
-secp256k1_context* secp256k1_context_clone(const secp256k1_context* ctx) {
-    secp256k1_context* ret;
-    size_t prealloc_size;
-
-    VERIFY_CHECK(ctx != NULL);
-    prealloc_size = secp256k1_context_preallocated_clone_size(ctx);
-    ret = (secp256k1_context*)checked_malloc(&ctx->error_callback, prealloc_size);
-    ret = secp256k1_context_preallocated_clone(ctx, ret);
-    return ret;
-}
-
 void secp256k1_context_preallocated_destroy(secp256k1_context* ctx) {
     ARG_CHECK_NO_RETURN(ctx != secp256k1_context_no_precomp);
     if (ctx != NULL) {
@@ -180,13 +158,6 @@ void secp256k1_context_preallocated_destroy(secp256k1_context* ctx) {
     }
 }
 
-void secp256k1_context_destroy(secp256k1_context* ctx) {
-    if (ctx != NULL) {
-        secp256k1_context_preallocated_destroy(ctx);
-        free(ctx);
-    }
-}
-
 void secp256k1_context_set_illegal_callback(secp256k1_context* ctx, void (*fun)(const char* message, void* data), const void* data) {
     ARG_CHECK_NO_RETURN(ctx != secp256k1_context_no_precomp);
     if (fun == NULL) {
@@ -205,16 +176,6 @@ void secp256k1_context_set_error_callback(secp256k1_context* ctx, void (*fun)(co
     ctx->error_callback.data = data;
 }
 
-secp256k1_scratch_space* secp256k1_scratch_space_create(const secp256k1_context* ctx, size_t max_size) {
-    VERIFY_CHECK(ctx != NULL);
-    return secp256k1_scratch_create(&ctx->error_callback, max_size);
-}
-
-void secp256k1_scratch_space_destroy(const secp256k1_context *ctx, secp256k1_scratch_space* scratch) {
-    VERIFY_CHECK(ctx != NULL);
-    secp256k1_scratch_destroy(&ctx->error_callback, scratch);
-}
-
 static int secp256k1_pubkey_load(const secp256k1_context* ctx, secp256k1_ge* ge, const secp256k1_pubkey* pubkey) {
     if (sizeof(secp256k1_ge_storage) == 64) {
         /* When the secp256k1_ge_storage type is exactly 64 byte, use its

depend/secp256k1/src/util.h

@@ -68,22 +68,6 @@ static SECP256K1_INLINE void secp256k1_callback_call(const secp256k1_callback *
 #define VERIFY_SETUP(stmt)
 #endif
 
-static SECP256K1_INLINE void *checked_malloc(const secp256k1_callback* cb, size_t size) {
-    void *ret = malloc(size);
-    if (ret == NULL) {
-        secp256k1_callback_call(cb, "Out of memory");
-    }
-    return ret;
-}
-
-static SECP256K1_INLINE void *checked_realloc(const secp256k1_callback* cb, void *ptr, size_t size) {
-    void *ret = realloc(ptr, size);
-    if (ret == NULL) {
-        secp256k1_callback_call(cb, "Out of memory");
-    }
-    return ret;
-}
-
 #if defined(__BIGGEST_ALIGNMENT__)
 #define ALIGNMENT __BIGGEST_ALIGNMENT__
 #else

src/context.rs

@@ -0,0 +1,228 @@
+use core::marker::PhantomData;
+use ffi;
+use types::{c_uint, c_void};
+use Error;
+use Secp256k1;
+
+#[cfg(feature = "std")]
+pub use self::std_only::*;
+
+/// A trait for all kinds of Context's that Lets you define the exact flags and a function to deallocate memory.
+/// * DO NOT * implement it for your own types.
+pub unsafe trait Context {
+    /// Flags for the ffi.
+    const FLAGS: c_uint;
+    /// A constant description of the context.
+    const DESCRIPTION: &'static str;
+    /// A function to deallocate the memory when the context is dropped.
+    fn deallocate(ptr: *mut [u8]);
+}
+
+/// Marker trait for indicating that an instance of `Secp256k1` can be used for signing.
+pub trait Signing: Context {}
+
+/// Marker trait for indicating that an instance of `Secp256k1` can be used for verification.
+pub trait Verification: Context {}
+
+/// Represents the set of capabilities needed for signing with a user preallocated memory.
+pub struct SignOnlyPreallocated<'buf> {
+    phantom: PhantomData<&'buf ()>,
+}
+
+/// Represents the set of capabilities needed for verification with a user preallocated memory.
+pub struct VerifyOnlyPreallocated<'buf> {
+    phantom: PhantomData<&'buf ()>,
+}
+
+/// Represents the set of all capabilities with a user preallocated memory.
+pub struct AllPreallocated<'buf> {
+    phantom: PhantomData<&'buf ()>,
+}
+
+#[cfg(feature = "std")]
+mod std_only {
+    use super::*;
+
+    /// Represents the set of capabilities needed for signing.
+    pub enum SignOnly {}
+
+    /// Represents the set of capabilities needed for verification.
+    pub enum VerifyOnly {}
+
+    /// Represents the set of all capabilities.
+    pub enum All {}
+
+    impl Signing for SignOnly {}
+    impl Signing for All {}
+
+    impl Verification for VerifyOnly {}
+    impl Verification for All {}
+
+    unsafe impl Context for SignOnly {
+        const FLAGS: c_uint = ffi::SECP256K1_START_SIGN;
+        const DESCRIPTION: &'static str = "signing only";
+
+        fn deallocate(ptr: *mut [u8]) {
+            let _ = unsafe { Box::from_raw(ptr) };
+        }
+    }
+
+    unsafe impl Context for VerifyOnly {
+        const FLAGS: c_uint = ffi::SECP256K1_START_VERIFY;
+        const DESCRIPTION: &'static str = "verification only";
+
+        fn deallocate(ptr: *mut [u8]) {
+            let _ = unsafe { Box::from_raw(ptr) };
+        }
+    }
+
+    unsafe impl Context for All {
+        const FLAGS: c_uint = VerifyOnly::FLAGS | SignOnly::FLAGS;
+        const DESCRIPTION: &'static str = "all capabilities";
+
+        fn deallocate(ptr: *mut [u8]) {
+            let _ = unsafe { Box::from_raw(ptr) };
+        }
+    }
+
+    impl<C: Context> Secp256k1<C> {
+        /// Lets you create a context in a generic manner(sign/verify/all)
+        pub fn gen_new() -> Secp256k1<C> {
+            let buf = vec![0u8; Self::preallocate_size_gen()].into_boxed_slice();
+            let ptr = Box::into_raw(buf);
+            Secp256k1 {
+                ctx: unsafe { ffi::secp256k1_context_preallocated_create(ptr as *mut c_void, C::FLAGS) },
+                phantom: PhantomData,
+                buf: ptr,
+            }
+        }
+    }
+
+    impl Secp256k1<All> {
+        /// Creates a new Secp256k1 context with all capabilities
+        pub fn new() -> Secp256k1<All> {
+            Secp256k1::gen_new()
+        }
+    }
+
+    impl Secp256k1<SignOnly> {
+        /// Creates a new Secp256k1 context that can only be used for signing
+        pub fn signing_only() -> Secp256k1<SignOnly> {
+            Secp256k1::gen_new()
+        }
+    }
+
+    impl Secp256k1<VerifyOnly> {
+        /// Creates a new Secp256k1 context that can only be used for verification
+        pub fn verification_only() -> Secp256k1<VerifyOnly> {
+            Secp256k1::gen_new()
+        }
+    }
+
+    impl Default for Secp256k1<All> {
+        fn default() -> Self {
+            Self::new()
+        }
+    }
+
+    impl<C: Context> Clone for Secp256k1<C> {
+        fn clone(&self) -> Secp256k1<C> {
+            let clone_size = unsafe {ffi::secp256k1_context_preallocated_clone_size(self.ctx)};
+            let ptr_buf = Box::into_raw(vec![0u8; clone_size].into_boxed_slice());
+            Secp256k1 {
+                ctx: unsafe { ffi::secp256k1_context_preallocated_clone(self.ctx, ptr_buf as *mut c_void) },
+                phantom: PhantomData,
+                buf: ptr_buf,
+            }
+        }
+    }
+
+}
+
+impl<'buf> Signing for SignOnlyPreallocated<'buf> {}
+impl<'buf> Signing for AllPreallocated<'buf> {}
+
+impl<'buf> Verification for VerifyOnlyPreallocated<'buf> {}
+impl<'buf> Verification for AllPreallocated<'buf> {}
+
+unsafe impl<'buf> Context for SignOnlyPreallocated<'buf> {
+    const FLAGS: c_uint = ffi::SECP256K1_START_SIGN;
+    const DESCRIPTION: &'static str = "signing only";
+
+    fn deallocate(_ptr: *mut [u8]) {
+        // Allocated by the user
+    }
+}
+
+unsafe impl<'buf> Context for VerifyOnlyPreallocated<'buf> {
+    const FLAGS: c_uint = ffi::SECP256K1_START_VERIFY;
+    const DESCRIPTION: &'static str = "verification only";
+
+    fn deallocate(_ptr: *mut [u8]) {
+        // Allocated by the user
+    }
+}
+
+unsafe impl<'buf> Context for AllPreallocated<'buf> {
+    const FLAGS: c_uint = SignOnlyPreallocated::FLAGS | VerifyOnlyPreallocated::FLAGS;
+    const DESCRIPTION: &'static str = "all capabilities";
+
+    fn deallocate(_ptr: *mut [u8]) {
+        // Allocated by the user
+    }
+}
+
+impl<'buf, C: Context + 'buf> Secp256k1<C> {
+    /// Lets you create a context with preallocated buffer in a generic manner(sign/verify/all)
+    pub fn preallocated_gen_new(buf: &'buf mut [u8]) -> Result<Secp256k1<C>, Error> {
+        if buf.len() < Self::preallocate_size_gen() {
+            return Err(Error::NotEnoughMemory);
+        }
+        Ok(Secp256k1 {
+            ctx: unsafe {
+                ffi::secp256k1_context_preallocated_create(
+                    buf.as_mut_ptr() as *mut c_void,
+                    C::FLAGS)
+            },
+            phantom: PhantomData,
+            buf: buf as *mut [u8],
+        })
+    }
+}
+
+impl<'buf> Secp256k1<AllPreallocated<'buf>> {
+    /// Creates a new Secp256k1 context with all capabilities
+    pub fn preallocated_new(buf: &'buf mut [u8]) -> Result<Secp256k1<AllPreallocated<'buf>>, Error> {
+        Secp256k1::preallocated_gen_new(buf)
+    }
+    /// Uses the ffi `secp256k1_context_preallocated_size` to check the memory size needed for a context
+    pub fn preallocate_size() -> usize {
+        Self::preallocate_size_gen()
+    }
+}
+
+impl<'buf> Secp256k1<SignOnlyPreallocated<'buf>> {
+    /// Creates a new Secp256k1 context that can only be used for signing
+    pub fn preallocated_signing_only(buf: &'buf mut [u8]) -> Result<Secp256k1<SignOnlyPreallocated<'buf>>, Error> {
+        Secp256k1::preallocated_gen_new(buf)
+    }
+
+    /// Uses the ffi `secp256k1_context_preallocated_size` to check the memory size needed for the context
+    #[inline]
+    pub fn preallocate_signing_size() -> usize {
+        Self::preallocate_size_gen()
+    }
+}
+
+impl<'buf> Secp256k1<VerifyOnlyPreallocated<'buf>> {
+    /// Creates a new Secp256k1 context that can only be used for verification
+    pub fn preallocated_verification_only(buf: &'buf mut [u8]) -> Result<Secp256k1<VerifyOnlyPreallocated<'buf>>, Error> {
+        Secp256k1::preallocated_gen_new(buf)
+    }
+
+    /// Uses the ffi `secp256k1_context_preallocated_size` to check the memory size needed for the context
+    #[inline]
+    pub fn preallocate_verification_size() -> usize {
+        Self::preallocate_size_gen()
+    }
+}

src/ffi.rs

@@ -142,11 +142,15 @@ extern "C" {
     pub static secp256k1_context_no_precomp: *const Context;
 
     // Contexts
-    pub fn secp256k1_context_create(flags: c_uint) -> *mut Context;
+    pub fn secp256k1_context_preallocated_size(flags: c_uint) -> usize;
 
-    pub fn secp256k1_context_clone(cx: *mut Context) -> *mut Context;
+    pub fn secp256k1_context_preallocated_create(prealloc: *mut c_void, flags: c_uint) -> *mut Context;
 
-    pub fn secp256k1_context_destroy(cx: *mut Context);
+    pub fn secp256k1_context_preallocated_destroy(cx: *mut Context);
+
+    pub fn secp256k1_context_preallocated_clone_size(cx: *const Context) -> usize;
+
+    pub fn secp256k1_context_preallocated_clone(cx: *const Context, prealloc: *mut c_void) -> *mut Context;
 
     pub fn secp256k1_context_randomize(cx: *mut Context,
                                        seed32: *const c_uchar)
@@ -320,7 +324,7 @@ mod fuzz_dummy {
     extern crate std;
     use types::*;
     use ffi::*;
-    use self::std::ptr;
+    use self::std::{ptr, mem};
     use self::std::boxed::Box;
 
     extern "C" {
@@ -331,20 +335,31 @@ mod fuzz_dummy {
 
     // Contexts
     /// Creates a dummy context, tracking flags to ensure proper calling semantics
-    pub unsafe fn secp256k1_context_create(flags: c_uint) -> *mut Context {
+    pub unsafe fn secp256k1_context_preallocated_create(_ptr: *mut c_void, flags: c_uint) -> *mut Context {
         let b = Box::new(Context(flags as i32));
         Box::into_raw(b)
     }
 
-    /// Copies a dummy context
-    pub unsafe fn secp256k1_context_clone(cx: *mut Context) -> *mut Context {
-        let b = Box::new(Context((*cx).0));
-        Box::into_raw(b)
+    /// Return dummy size of context struct.
+    pub unsafe fn secp256k1_context_preallocated_size(_flags: c_uint) -> usize {
+        mem::size_of::<Context>()
     }
 
-    /// Frees a dummy context
-    pub unsafe fn secp256k1_context_destroy(cx: *mut Context) {
-        Box::from_raw(cx);
+    /// Return dummy size of context struct.
+    pub unsafe fn secp256k1_context_preallocated_clone_size(cx: *mut Context) -> usize {
+        mem::size_of::<Context>()
+    }
+
+    /// Copies a dummy context
+    pub unsafe fn secp256k1_context_preallocated_clone(cx: *const Context, prealloc: *mut c_void) -> *mut Context {
+        let ret = prealloc as *mut Context;
+        *ret = (*cx).clone();
+        ret
+    }
+
+    /// "Destroys" a dummy context
+    pub unsafe fn secp256k1_context_preallocated_destroy(cx: *mut Context) {
+        (*cx).0 = 0;
     }
 
     /// Asserts that cx is properly initialized

src/lib.rs

@@ -148,6 +148,7 @@ use core::{fmt, ptr, str};
 #[macro_use]
 mod macros;
 mod types;
+mod context;
 pub mod constants;
 pub mod ecdh;
 pub mod ffi;
@@ -157,6 +158,7 @@ pub mod recovery;
 
 pub use key::SecretKey;
 pub use key::PublicKey;
+pub use context::*;
 use core::marker::PhantomData;
 use core::ops::Deref;
 
@@ -463,6 +465,9 @@ pub enum Error {
     InvalidRecoveryId,
     /// Invalid tweak for add_*_assign or mul_*_assign
     InvalidTweak,
+    /// Didn't pass enough memory to context creation with preallocated memory
+    NotEnoughMemory,
+
 }
 
 impl Error {
@@ -475,6 +480,7 @@ impl Error {
             Error::InvalidSecretKey => "secp: malformed or out-of-range secret key",
             Error::InvalidRecoveryId => "secp: bad recovery id",
             Error::InvalidTweak => "secp: bad tweak",
+            Error::NotEnoughMemory => "secp: not enough memory allocated",
         }
     }
 }
@@ -491,48 +497,21 @@ impl std::error::Error for Error {
     fn description(&self) -> &str { self.as_str() }
 }
 
-/// Marker trait for indicating that an instance of `Secp256k1` can be used for signing.
-pub trait Signing {}
-
-/// Marker trait for indicating that an instance of `Secp256k1` can be used for verification.
-pub trait Verification {}
-
-/// Represents the set of capabilities needed for signing.
-pub struct SignOnly {}
-
-/// Represents the set of capabilities needed for verification.
-pub struct VerifyOnly {}
-
-/// Represents the set of all capabilities.
-pub struct All {}
-
-impl Signing for SignOnly {}
-impl Signing for All {}
-
-impl Verification for VerifyOnly {}
-impl Verification for All {}
 
 /// The secp256k1 engine, used to execute all signature operations
-pub struct Secp256k1<C> {
+pub struct Secp256k1<C: Context> {
     ctx: *mut ffi::Context,
-    phantom: PhantomData<C>
+    phantom: PhantomData<C>,
+    buf: *mut [u8],
 }
 
 // The underlying secp context does not contain any references to memory it does not own
-unsafe impl<C> Send for Secp256k1<C> {}
+unsafe impl<C: Context> Send for Secp256k1<C> {}
 // The API does not permit any mutation of `Secp256k1` objects except through `&mut` references
-unsafe impl<C> Sync for Secp256k1<C> {}
+unsafe impl<C: Context> Sync for Secp256k1<C> {}
 
-impl<C> Clone for Secp256k1<C> {
-    fn clone(&self) -> Secp256k1<C> {
-        Secp256k1 {
-            ctx: unsafe { ffi::secp256k1_context_clone(self.ctx) },
-            phantom: self.phantom
-        }
-    }
-}
 
-impl<C> PartialEq for Secp256k1<C> {
+impl<C: Context> PartialEq for Secp256k1<C> {
     fn eq(&self, _other: &Secp256k1<C>) -> bool { true }
 }
 
@@ -566,60 +545,22 @@ impl Deref for SerializedSignature {
 
 impl Eq for SerializedSignature {}
 
-impl<C> Eq for Secp256k1<C> { }
+impl<C: Context> Eq for Secp256k1<C> { }
 
-impl<C> Drop for Secp256k1<C> {
+impl<C: Context> Drop for Secp256k1<C> {
     fn drop(&mut self) {
-        unsafe { ffi::secp256k1_context_destroy(self.ctx); }
+        unsafe { ffi::secp256k1_context_preallocated_destroy(self.ctx) };
+        C::deallocate(self.buf);
     }
 }
 
-impl fmt::Debug for Secp256k1<SignOnly> {
+impl<C: Context> fmt::Debug for Secp256k1<C> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "<secp256k1 context {:?}, signing only>", self.ctx)
+        write!(f, "<secp256k1 context {:?}, {}>", self.ctx, C::DESCRIPTION)
     }
 }
 
-impl fmt::Debug for Secp256k1<VerifyOnly> {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "<secp256k1 context {:?}, verification only>", self.ctx)
-    }
-}
-
-impl fmt::Debug for Secp256k1<All> {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "<secp256k1 context {:?}, all capabilities>", self.ctx)
-    }
-}
-
-impl Secp256k1<All> {
-    /// Creates a new Secp256k1 context with all capabilities
-    pub fn new() -> Secp256k1<All> {
-        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_SIGN | ffi::SECP256K1_START_VERIFY) }, phantom: PhantomData }
-    }
-}
-
-impl Default for Secp256k1<All> {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-impl Secp256k1<SignOnly> {
-    /// Creates a new Secp256k1 context that can only be used for signing
-    pub fn signing_only() -> Secp256k1<SignOnly> {
-        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_SIGN) }, phantom: PhantomData }
-    }
-}
-
-impl Secp256k1<VerifyOnly> {
-    /// Creates a new Secp256k1 context that can only be used for verification
-    pub fn verification_only() -> Secp256k1<VerifyOnly> {
-        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_VERIFY) }, phantom: PhantomData }
-    }
-}
-
-impl<C> Secp256k1<C> {
+impl<C: Context> Secp256k1<C> {
 
     /// Getter for the raw pointer to the underlying secp256k1 context. This
     /// shouldn't be needed with normal usage of the library. It enables
@@ -629,6 +570,11 @@ impl<C> Secp256k1<C> {
         &self.ctx
     }
 
+    /// Returns the required memory for a preallocated context buffer in a generic manner(sign/verify/all)
+    pub fn preallocate_size_gen() -> usize {
+        unsafe { ffi::secp256k1_context_preallocated_size(C::FLAGS) }
+    }
+
     /// (Re)randomizes the Secp256k1 context for cheap sidechannel resistance;
     /// see comment in libsecp256k1 commit d2275795f by Gregory Maxwell. Requires
     /// compilation with "rand" feature.
@@ -750,6 +696,30 @@ mod tests {
         });
     }
 
+    #[test]
+    fn test_preallocation() {
+        let mut buf_ful = vec![0u8; Secp256k1::preallocate_size()];
+        let mut buf_sign = vec![0u8; Secp256k1::preallocate_signing_size()];
+        let mut buf_vfy = vec![0u8; Secp256k1::preallocate_verification_size()];
+//
+        let full = Secp256k1::preallocated_new(&mut buf_ful).unwrap();
+        let sign = Secp256k1::preallocated_signing_only(&mut buf_sign).unwrap();
+        let vrfy = Secp256k1::preallocated_verification_only(&mut buf_vfy).unwrap();
+
+//        drop(buf_vfy); // The buffer can't get dropped before the context.
+//        println!("{:?}", buf_ful[5]); // Can't even read the data thanks to the borrow checker.
+
+        let (sk, pk) = full.generate_keypair(&mut thread_rng());
+        let msg = Message::from_slice(&[2u8; 32]).unwrap();
+        // Try signing
+        assert_eq!(sign.sign(&msg, &sk), full.sign(&msg, &sk));
+        let sig = full.sign(&msg, &sk);
+
+        // Try verifying
+        assert!(vrfy.verify(&msg, &sig, &pk).is_ok());
+        assert!(full.verify(&msg, &sig, &pk).is_ok());
+    }
+
     #[test]
     fn capabilities() {
         let sign = Secp256k1::signing_only();