Emitting Logs

msg! and sol_log_data write to transaction logs consumed by explorers, indexers, and simulations. Use sparingly - each log spends compute units.

Recipe

solana_program::msg!("vault deposit {}", amount);

When to reach for this:

• Debugging failing instructions in devnet.
• Correlating on-chain steps with off-chain analytics.
• Temporary instrumentation during audit fixes.

Working Example

pub fn deposit(amount: u64) -> ProgramResult {
    solana_program::msg!("deposit_start amount={}", amount);
    if amount == 0 {
        solana_program::msg!("deposit_reject zero");
        return Err(ProgramError::InvalidArgument);
    }
    solana_program::msg!("deposit_ok");
    Ok(())
}

What this demonstrates:

• Structured prefix tokens help grep logs.
• Reject path logs before error return.
• Remove or gate verbose logs before mainnet.

Deep Dive

CU Cost

Logs scale with string length and count.

Alternatives to Logs

• Custom errors for users
• Anchor events for indexers

Rust Notes

solana_program::log::sol_log_data(&[b"evt", &amount.to_le_bytes()]);

Gotchas

• Logs in loops - CU explosion. Fix: Log summary only.
• Sensitive data - Logs are public. Fix: Never log secrets or full PII.
• Relying on logs for control flow - Logs are not consensus inputs. Fix: State must encode truth.
• Format panics - Bad format is dev error. Fix: Use simple literals in prod.
• Production noise - Hard to index. Fix: Prefer events with schema.

Alternatives

FAQs

Related

• Logging with msg! - CU-aware logging
• Emitting Events - Structured events
• Debugging Failed Transactions - Reading output

Stack versions: This page was written for Agave 4.1.1, Solana CLI 3.0.10, Anchor 0.32.1, anchor-lang 0.32.1, Rust 1.91.1, @solana/kit 7.0.0, Surfpool 0.12.0, and LiteSVM 0.6.x.