mbedtls中sha256的使用特性

/*
* signv_sha256_calcu calculate the sha256 value upon one continous memory aera, there is no restriction on how many bytes the data are.
* The SHA-256 method is a standard and the value can be calculated by mbedtls library.
*/
int sha256_calcu(const unsigned char *input, size_t len, unsigned char hash_value[])
{
int ret = 0;
// unsigned char *buf;
mbedtls_sha256_context ctx;

// buf = mbedtls_calloc( 1024, sizeof(unsigned char) );

mbedtls_sha256_init( &ctx );

do
{
/* SHA-256 */
if( ( ret = mbedtls_sha256_starts_ret( &ctx, 0 ) ) != 0 )
{
mbedtls_sha256_free( &ctx );
// mbedtls_free( buf );

break;
}

ret = mbedtls_sha256_update_ret( &ctx, input, len);
if( ret != 0 )
{
mbedtls_sha256_free( &ctx );
// mbedtls_free( buf );

break;
}

if( ( ret = mbedtls_sha256_finish_ret( &ctx, hash_value ) ) != 0 )
{
mbedtls_sha256_free( &ctx );
// mbedtls_free( buf );

break;
}

mbedtls_sha256_free( &ctx );
// mbedtls_free( buf );

}while(0);

return( ret );
}