text: #ifndef SECP256K1_ECDH_H
  #define SECP256K1_ECDH_H
  
  #include "secp256k1.h"
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  /** A pointer to a function that hashes an EC point to obtain an ECDH secret
   *
   *  Returns: 1 if the point was successfully hashed.
   *           0 will cause rustsecp256k1_v0_4_0_ecdh to fail and return 0.
   *           Other return values are not allowed, and the behaviour of
   *           rustsecp256k1_v0_4_0_ecdh is undefined for other return values.
   *  Out:     output:     pointer to an array to be filled by the function
   *  In:      x32:        pointer to a 32-byte x coordinate
   *           y32:        pointer to a 32-byte y coordinate
   *           data:       arbitrary data pointer that is passed through
   */
  typedef int (*rustsecp256k1_v0_4_0_ecdh_hash_function)(
    unsigned char *output,
    const unsigned char *x32,
    const unsigned char *y32,
    void *data
  );
  
  /** An implementation of SHA256 hash function that applies to compressed public key.
   * Populates the output parameter with 32 bytes. */
  SECP256K1_API extern const rustsecp256k1_v0_4_0_ecdh_hash_function rustsecp256k1_v0_4_0_ecdh_hash_function_sha256;
  
  /** A default ECDH hash function (currently equal to rustsecp256k1_v0_4_0_ecdh_hash_function_sha256).
   * Populates the output parameter with 32 bytes. */
  SECP256K1_API extern const rustsecp256k1_v0_4_0_ecdh_hash_function rustsecp256k1_v0_4_0_ecdh_hash_function_default;
  
  /** Compute an EC Diffie-Hellman secret in constant time
   *
   *  Returns: 1: exponentiation was successful
   *           0: scalar was invalid (zero or overflow) or hashfp returned 0
   *  Args:    ctx:        pointer to a context object (cannot be NULL)
   *  Out:     output:     pointer to an array to be filled by hashfp
   *  In:      pubkey:     a pointer to a rustsecp256k1_v0_4_0_pubkey containing an
   *                       initialized public key
   *           seckey:     a 32-byte scalar with which to multiply the point
   *           hashfp:     pointer to a hash function. If NULL, rustsecp256k1_v0_4_0_ecdh_hash_function_sha256 is used
   *                       (in which case, 32 bytes will be written to output)
   *           data:       arbitrary data pointer that is passed through to hashfp
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int rustsecp256k1_v0_4_0_ecdh(
    const rustsecp256k1_v0_4_0_context* ctx,
    unsigned char *output,
    const rustsecp256k1_v0_4_0_pubkey *pubkey,
    const unsigned char *seckey,
    rustsecp256k1_v0_4_0_ecdh_hash_function hashfp,
    void *data
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
  
  #ifdef __cplusplus
  }
  #endif
  
  #endif /* SECP256K1_ECDH_H */
  