Crate solana_program

Expand description

The base library for all Solana on-chain Rust programs.

All Solana Rust programs that run on-chain will link to this crate, which acts as a standard library for Solana programs. Solana programs also link to the Rust standard library, though it is modified for the Solana runtime environment. While off-chain programs that interact with the Solana network can link to this crate, they typically instead use the solana-sdk crate, which reexports all modules from solana-program.

This library defines

macros for declaring the program entrypoint,
core data types,
logging macros,
serialization methods,
methods for cross-program instruction execution,
program IDs and instruction constructors for the system program and other native programs,
sysvar accessors.

Idiomatic examples of solana-program usage can be found in the Solana Program Library.

Defining a solana program

Solana program crates have some unique properties compared to typical Rust programs:

They are often compiled for both on-chain use and off-chain use. This is primarily because off-chain clients may need access to data types defined by the on-chain program.
They do not define a main function, but instead define their entrypoint with the entrypoint! macro.
They are compiled as the “cdylib” crate type for dynamic loading by the Solana runtime.
They run in a constrained VM environment, and while they do have access to the Rust standard library, many features of the standard library, particularly related to OS services, will fail at runtime, will silently do nothing, or are not defined. See the restrictions to the Rust standard library in the Solana documentation for more.

Because multiple crates that are linked together cannot all define program entrypoints (see the entrypoint! documentation) a common convention is to use a Cargo feature called no-entrypoint to allow the program entrypoint to be disabled.

The skeleton of a Solana program typically looks like:

#[cfg(not(feature = "no-entrypoint"))]
pub mod entrypoint {
    use solana_program::{
        account_info::AccountInfo,
        entrypoint,
        entrypoint::ProgramResult,
        pubkey::Pubkey,
    };

    entrypoint!(process_instruction);

    pub fn process_instruction(
        program_id: &Pubkey,
        accounts: &[AccountInfo],
        instruction_data: &[u8],
    ) -> ProgramResult {
        // Decode and dispatch instructions here.
        todo!()
    }
}

// Additional code goes here.

With a Cargo.toml file that contains

[lib]
crate-type = ["cdylib", "rlib"]

[features]
no-entrypoint = []

Note that a Solana program must specify its crate-type as “cdylib”, and “cdylib” crates will automatically be discovered and built by the cargo build-bpf command. Solana programs also often have crate-type “rlib” so they can be linked to other Rust crates.

On-chain vs. off-chain compilation targets

Solana programs run on the rbpf VM, which implements a variant of the eBPF instruction set. Because this crate can be compiled for both on-chain and off-chain execution, the environments of which are significantly different, it extensively uses conditional compilation to tailor its implementation to the environment. The cfg predicate used for identifying compilation for on-chain programs is target_os = "solana", as in this example from the solana-program codebase that logs a message via a syscall when run on-chain, and via a library call when offchain:

pub fn sol_log(message: &str) {
    #[cfg(target_os = "solana")]
    unsafe {
        sol_log_(message.as_ptr(), message.len() as u64);
    }

    #[cfg(not(target_os = "solana"))]
    program_stubs::sol_log(message);
}

This cfg pattern is suitable as well for user code that needs to work both on-chain and off-chain.

solana-program and solana-sdk were previously a single crate. Because of this history, and because of the dual-usage of solana-program for two different environments, it contains some features that are not available to on-chain programs at compile-time. It also contains some on-chain features that will fail in off-chain scenarios at runtime. This distinction is not well-reflected in the documentation.

For a more complete description of Solana’s implementation of eBPF and its limitations, see the main Solana documentation for on-chain programs.

Core data types

Pubkey — The address of a Solana account. Some account addresses are ed25519 public keys, with corresponding secret keys that are managed off-chain. Often, though, account addresses do not have corresponding secret keys — as with program derived addresses — or the secret key is not relevant to the operation of a program, and may have even been disposed of. As running Solana programs can not safely create or manage secret keys, the full Keypair is not defined in solana-program but in solana-sdk.
Hash — A cryptographic hash. Used to uniquely identify blocks, and also for general purpose hashing.
AccountInfo — A description of a single Solana account. All accounts that might be accessed by a program invocation are provided to the program entrypoint as AccountInfo.
Instruction — A directive telling the runtime to execute a program, passing it a set of accounts and program-specific data.
ProgramError and ProgramResult — The error type that all programs must return, reported to the runtime as a u64.
Sol — The Solana native token type, with conversions to and from lamports, the smallest fractional unit of SOL, in the native_token module.

Serialization

Within the Solana runtime, programs, and network, at least three different serialization formats are used, and solana-program provides access to those needed by programs.

In user-written Solana program code, serialization is primarily used for accessing AccountInfo data and Instruction data, both of which are program-specific binary data. Every program is free to decide their own serialization format, but data received from other sources — sysvars for example — must be deserialized using the methods indicated by the documentation for that data or data type.

The three serialization formats in use in Solana are:

Borsh, a compact and well-specified format developed by the NEAR project, suitable for use in protocol definitions and for archival storage. It has a Rust implementation and a JavaScript implementation and is recommended for all purposes.

Users need to import the borsh crate themselves — it is not re-exported by solana-program, though this crate provides several useful utilities in its borsh module that are not available in the borsh library.

The Instruction::new_with_borsh function creates an Instruction by serializing a value with borsh.
Bincode, a compact serialization format that implements the Serde Rust APIs. As it does not have a specification nor a JavaScript implementation, and uses more CPU than borsh, it is not recommend for new code.

Many system program and native program instructions are serialized with bincode, and it is used for other purposes in the runtime. In these cases Rust programmers are generally not directly exposed to the encoding format as it is hidden behind APIs.

The Instruction::new_with_bincode function creates an Instruction by serializing a value with bincode.
Pack, a Solana-specific serialization API that is used by many older programs in the Solana Program Library to define their account format. It is difficult to implement and does not define a language-independent serialization format. It is not generally recommended for new code.

Developers should carefully consider the CPU cost of serialization, balanced against the need for correctness and ease of use: off-the-shelf serialization formats tend to be more expensive than carefully hand-written application-specific formats; but application-specific formats are more difficult to ensure the correctness of, and to provide multi-language implementations for. It is not uncommon for programs to pack and unpack their data with hand-written code.

Cross-program instruction execution

Solana programs may call other programs, termed cross-program invocation (CPI), with the invoke and invoke_signed functions. When calling another program the caller must provide the Instruction to be invoked, as well as the AccountInfo for every account required by the instruction. Because the only way for a program to acquire AccountInfo values is by receiving them from the runtime at the program entrypoint, any account required by the callee program must transitively be required by the caller program, and provided by its caller.

A simple example of transferring lamports via CPI:

use solana_program::{
    account_info::{next_account_info, AccountInfo},
    entrypoint,
    entrypoint::ProgramResult,
    program::invoke,
    pubkey::Pubkey,
    system_instruction,
    system_program,
};

entrypoint!(process_instruction);

fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    let account_info_iter = &mut accounts.iter();

    let payer = next_account_info(account_info_iter)?;
    let recipient = next_account_info(account_info_iter)?;
    // The system program is a required account to invoke a system
    // instruction, even though we don't use it directly.
    let system_account = next_account_info(account_info_iter)?;

    assert!(payer.is_writable);
    assert!(payer.is_signer);
    assert!(recipient.is_writable);
    assert!(system_program::check_id(system_account.key));

    let lamports = 1000000;

    invoke(
        &system_instruction::transfer(payer.key, recipient.key, lamports),
        &[payer.clone(), recipient.clone(), system_account.clone()],
    )
}

Solana also includes a mechanism to let programs control and sign for accounts without needing to protect a corresponding secret key, called program derived addresses. PDAs are derived with the Pubkey::find_program_address function. With a PDA, a program can call invoke_signed to call another program while virtually “signing” for the PDA.

A simple example of creating an account for a PDA:

use solana_program::{
    account_info::{next_account_info, AccountInfo},
    entrypoint,
    entrypoint::ProgramResult,
    program::invoke_signed,
    pubkey::Pubkey,
    system_instruction,
    system_program,
};

entrypoint!(process_instruction);

fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    let account_info_iter = &mut accounts.iter();
    let payer = next_account_info(account_info_iter)?;
    let vault_pda = next_account_info(account_info_iter)?;
    let system_program = next_account_info(account_info_iter)?;

    assert!(payer.is_writable);
    assert!(payer.is_signer);
    assert!(vault_pda.is_writable);
    assert_eq!(vault_pda.owner, &system_program::ID);
    assert!(system_program::check_id(system_program.key));

    let vault_bump_seed = instruction_data[0];
    let vault_seeds = &[b"vault", payer.key.as_ref(), &[vault_bump_seed]];
    let expected_vault_pda = Pubkey::create_program_address(vault_seeds, program_id)?;

    assert_eq!(vault_pda.key, &expected_vault_pda);

    let lamports = 10000000;
    let vault_size = 16;

    invoke_signed(
        &system_instruction::create_account(
            &payer.key,
            &vault_pda.key,
            lamports,
            vault_size,
            &program_id,
        ),
        &[
            payer.clone(),
            vault_pda.clone(),
        ],
        &[
            &[
                b"vault",
                payer.key.as_ref(),
                &[vault_bump_seed],
            ],
        ]
    )?;
    Ok(())
}

Native programs

Some solana programs are native programs, running native machine code that is distributed with the runtime, with well-known program IDs.

Some native programs can be invoked by other programs, but some can only be executed as “top-level” instructions included by off-chain clients in a Transaction.

This crate defines the program IDs for most native programs. Even though some native programs cannot be invoked by other programs, a Solana program may need access to their program IDs. For example, a program may need to verify that an ed25519 signature verification instruction was included in the same transaction as its own instruction. For many native programs, this crate also defines enums that represent the instructions they process, and constructors for building the instructions.

Locations of program IDs and instruction constructors are noted in the list below, as well as whether they are invokable by other programs.

While some native programs have been active since the genesis block, others are activated dynamically after a specific slot, and some are not yet active. This documentation does not distinguish which native programs are active on any particular network. The solana feature status CLI command can help in determining active features.

Native programs important to Solana program authors include:

System Program: Creates new accounts, allocates account data, assigns accounts to owning programs, transfers lamports from System Program owned accounts and pays transaction fees.
- ID: solana_program::system_program
- Instruction: solana_program::system_instruction
- Invokable by programs? yes
Compute Budget Program: Requests additional CPU or memory resources for a transaction. This program does nothing when called from another program.
- ID: solana_sdk::compute_budget
- Instruction: solana_sdk::compute_budget
- Invokable by programs? no
ed25519 Program: Verifies an ed25519 signature.
- ID: solana_program::ed25519_program
- Instruction: solana_sdk::ed25519_instruction
- Invokable by programs? no
secp256k1 Program: Verifies secp256k1 public key recovery operations.
- ID: solana_program::secp256k1_program
- Instruction: solana_sdk::secp256k1_instruction
- Invokable by programs? no
BPF Loader: Deploys, and executes immutable programs on the chain.
- ID: solana_program::bpf_loader
- Instruction: solana_program::loader_instruction
- Invokable by programs? yes
Upgradable BPF Loader: Deploys, upgrades, and executes upgradable programs on the chain.
- ID: solana_program::bpf_loader_upgradeable
- Instruction: solana_program::loader_upgradeable_instruction
- Invokable by programs? yes
Deprecated BPF Loader: Deploys, and executes immutable programs on the chain.
- ID: solana_program::bpf_loader_deprecated
- Instruction: solana_program::loader_instruction
- Invokable by programs? yes

Sysvars

Sysvars are special accounts that contain dynamically-updated data about the network cluster, the blockchain history, and the executing transaction.

The program IDs for sysvars are defined in the sysvar module, and simple sysvars implement the Sysvar::get method, which loads a sysvar directly from the runtime, as in this example that logs the clock sysvar:

use solana_program::{
    account_info::AccountInfo,
    clock,
    entrypoint::ProgramResult,
    msg,
    pubkey::Pubkey,
    sysvar::Sysvar,
};

fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    let clock = clock::Clock::get()?;
    msg!("clock: {:#?}", clock);
    Ok(())
}

Since Solana sysvars are accounts, if the AccountInfo is provided to the program, then the program can deserialize the sysvar with Sysvar::from_account_info to access its data, as in this example that again logs the clock sysvar.

use solana_program::{
    account_info::{next_account_info, AccountInfo},
    clock,
    entrypoint::ProgramResult,
    msg,
    pubkey::Pubkey,
    sysvar::Sysvar,
};

fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    let account_info_iter = &mut accounts.iter();
    let clock_account = next_account_info(account_info_iter)?;
    let clock = clock::Clock::from_account_info(&clock_account)?;
    msg!("clock: {:#?}", clock);
    Ok(())
}

When possible, programs should prefer to call Sysvar::get instead of deserializing with Sysvar::from_account_info, as the latter imposes extra overhead of deserialization while also requiring the sysvar account address be passed to the program, wasting the limited space available to transactions. Deserializing sysvars that can instead be retrieved with Sysvar::get should be only be considered for compatibility with older programs that pass around sysvar accounts.

Some sysvars are too large to deserialize within a program, and Sysvar::from_account_info returns an error. Some sysvars are too large to deserialize within a program, and attempting to will exhaust the program’s compute budget. Some sysvars do not implement Sysvar::get and return an error. Some sysvars have custom deserializers that do not implement the Sysvar trait. These cases are documented in the modules for individual sysvars.

For more details see the Solana documentation on sysvars.