Logging with msg!

msg! writes human-readable lines to transaction logs. Useful for development and ops, but each call costs compute units - budget logs like any hot-path code.

Recipe

solana_program::msg!("swap in={} out={}", amount_in, amount_out);

When to reach for this:

• Debugging devnet failures.
• Correlating on-chain steps with support tickets.
• Temporary instrumentation during incident response.

Working Example

#[cfg(feature = "verbose_logs")]
macro_rules! vlog { ($($t:tt)*) => { solana_program::msg!($($t)*) }; }
#[cfg(not(feature = "verbose_logs"))]
macro_rules! vlog { ($($t:tt)*) => {}; }

What this demonstrates:

• Feature-gated logs for release builds.
• Macro no-ops when disabled.
• Production path stays quiet.

Deep Dive

CU Impact

Logs charge per syscall + bytes.

Binary Logs

sol_log_data for compact key/value.

Rust Notes

// Prefer fixed prefixes: "MYPROG:step1"

Gotchas

• Logs in tight loop - CU exceeded. Fix: Aggregate outside loop.
• PII in logs - Public chain data. Fix: Redact.
• Rely on logs for auth - Attacker sees same logs. Fix: State is truth.
• Format user strings - Injection irrelevant but costly. Fix: Log hashes or ids.
• Leave debug logs on mainnet - Noise and cost. Fix: Feature flags.

Alternatives

FAQs

Related

• Emitting Logs - Native angle
• Emitting Events - Structured
• Debugging Failed Transactions - Workflow

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.