Add programming faq (#12545)
* Add programming faq * feedback and new content * nudge
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Jack May
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---
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title: "Programming FAQ"
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---
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When writing or interacting with Solana programs, there are common questions or
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challenges that often come up. Below are resources to help answer these
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questions. If not addressed here, the Solana
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[#developers](https://discord.gg/RxeGBH) Discord channel is a great resource.
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## CallDepth
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Cross-program invocations allow programs to invoke other programs directly but
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the depth is constrained currently to 1.
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## CallDepthExceeded
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Programs are constrained to run quickly, and to facilitate this, the program's
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call stack is limited to max depth. If this error is encountered, then the
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program itself or its dependent crate packages have exceeded the max stack
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depth.
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## Computational constraints
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To prevent a program from abusing computation resources, a cap is enforced
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during execution. The following operations incur a cost:
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- Executing BPF instructions
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- Calling system calls (logging, creating program addresses, ...)
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- Cross-program invocations incur a base cost and the cost of the program
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invoked.
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## Failure to compiler due to `rand` incompatibility
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Programs are constrained to run deterministically, so random numbers are not
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available. Sometimes a program may depend on a crate that depends itself on
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`rand` even if the program does not use any of the random number functionality.
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If a program depends on `rand`, the compilation will fail because there is not
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`get-random` support for Solana. To work around this dependency issue, add the
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following dependency to the program's `Cargo.toml`:
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```
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getrandom = { version = "0.1.14", features = ["dummy"] }
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```
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## Float Rust types
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Programs support a limited subset of Rust's float operations, though they
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are highly discouraged due to the overhead involved. If a program attempts to
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use a float operation that is not supported, the runtime will report an
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unresolved symbol error. Be sure to include integration tests against a local
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cluster to ensure the operation is supported.
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## Heap size
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Programs have access to a heap either directly in C or via the Rust `alloc`
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APIs. To facilitate fast allocations, a simple 32KB bump heap is utilized. The
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heap does not support `free` or `realloc` so use it wisely.
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## InvalidAccountData
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This program error can happen for a lot of reasons. Usually, it's caused by
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passing an account to the program that the program is not expecting, either in
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the wrong position in the instruction or an account not compatible with the
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instruction being executed.
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An implementation of a program might also cause this error when performing a
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cross-program instruction and forgetting to provide the account for the program
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that you are calling.
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## InvalidInstructionData
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This program error can occur while trying to deserialize the instruction, check
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that the structure passed in matches exactly the instruction. There may be some
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padding between fields. If the program implements the Rust `Pack` trait then ry
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packing and unpacking the instruction type `T` to determine the exact encoding
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the program expects:
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https://github.com/solana-labs/solana/blob/master/sdk/src/program_pack.rs
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## MissingRequiredSignature
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Some instructions require the account to be a signer; this error is returned if
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an account expected to be signed is not.
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An implementation of a program might also cause this error when performing a
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cross-program invocation that requires a signed program address, but the passed
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signer seeds passed to `invoke_signed` don't match the signer seeds used to
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create the program address (`create_program_address`).
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## Rust restrictions
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There are some Rust limitations since programs run in a resource-constrained,
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single-threaded environment, and must be deterministic:
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- No access to
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- std::fs
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- std::net
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- std::os
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- std::future
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- std::net
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- std::process
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- std::sync
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- std::task
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- std::thread
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- std::time
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- Limited access to:
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- std::os
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- rand or any crates that depend on it
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- Bincode is extremely computationally expensive in both cycles and call depth and should be avoided
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- String formatting should be avoided since it is also computational expensive
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- No support for `println!`, `print!`, the Solana SDK helpers in `src/log.rs`
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should be used instead
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## Stack size
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Solana programs compile down to Berkley Packet Filter instructions, which use
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stack frames instead of a variable stack pointer. Each stack frame is limited
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to 4KB. If a program violates that stack frame size, the compiler will report
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the overrun as a warning. The reason a warning is reported rather than an error
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is because some dependent crates may include functionality that violates the
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stack frame restrictions even if the program doesn't use that functionality. If
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the program violates the stack size at runtime, an `AccessViolation` error will
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be reported.
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