Note: Sections 2.0 and 3.0 of this article were added in response to comments on Hacker News and Reddit.
Since its inception on 2000-05-29, SQLite has been implemented in generic C. C was and continues to be the best language for implementing a software library like SQLite. There are no plans to recode SQLite in any other programming language at this time.
The reasons why C is the best language to implement SQLite include:
C is a great language for writing fast code. C is sometimes described as "portable assembly language". It enables to developers to code as close to the underlying hardware as possible while still remaining portable across platforms.
Other programming languages sometimes claim to be "as fast as C". But no other language claims to be faster than C for general-purpose programming, because none are.
Nearly all systems have the ability to call libraries written in C. This is not true of other implementation languages.
So, for example, Android applications written in Java are able to invoke SQLite (through an adaptor). Maybe it would have been more convenient for Android if SQLite had been coded in Java as that would make the interface simpler. However, on iPhone applications are coded in Objective-C or Swift, neither of which have the ability to call libraries written in Java. Thus, SQLite would be unusable on iPhones had it been written in Java.
Libraries written in C do not have a huge run-time dependency. In its minimum configuration, SQLite requires only the following routines from the standard C library:
In a more complete build, SQLite also uses library routines like malloc() and free() and operating system interfaces for opening, reading, writing, and closing files. But even then, the number of dependencies is very small. Other "modern" language, in contrast, often require multi-megabyte runtimes loaded with thousands and thousands of interfaces.
The C language is old and boring. It is a well-known and well-understood language. This is exactly what one wants when developing a module like SQLite. Writing a small, fast, and reliable database engine is hard enough as it is without the implementation language changing out from under you with each update to the implementation language specification.
Some programmers cannot imagine developing a complex system like SQLite in a language that is not "object oriented". So why is SQLite not coded in C++ or Java?
Libraries written in C++ or Java can generally only be used by applications written in the same language. It is difficult to get an application written in Haskell or Java to invoke a library written in C++. On the other hand, libraries written in C are callable from any programming language.
Object-Oriented is a design pattern, not a programming language. You can do object-oriented programming in any language you want, including assembly language. Some languages (ex: C++ or Java) make object-oriented easier. But you can still do object-oriented programming in languages like C.
Object-oriented is not the only valid design pattern. Many programmers have been taught to think purely in terms of objects. And, to be fair, objects are often a good way to decompose a problem. But objects are not the only way, and are not always the best way to decompose a problem. Sometimes good old procedural code is easier to write, easier to maintain and understand, and faster than object-oriented code.
When SQLite was first being developed, Java was a young and immature language. C++ was older, but was undergoing such growing pains that it was difficult to find any two C++ compilers that worked the same way. So C was definitely a better choice back when SQLite was first being developed. The situation is less stark now, but there is little to no benefit in recoding SQLite at this point.
There has lately been a lot of interest in "safe" programming languages like Rust or Go in which it is impossible, or is at least difficult, to make common programming errors like memory leaks or array overruns. So the question often arises as to why SQLite is not coded in a "safe" language.
None of the safe programming languages existed for the first 10 years of SQLite's existence. SQLite could be recoded in Go or Rust, but doing so would probably introduce far more bugs than would be fixed, and it seems also likely to result in slower code.
Safe programming languages solve the easy problems: memory leaks, use-after-free errors, array overruns, etc. Safe languages provide no help beyond ordinary C code in solving the rather more difficult problem of computing a correct answer to an SQL statement.
Safe languages are often touted for helping to prevent security vulnerabilities. True enough, but SQLite is not a particularly security-sensitive library. If an application is running untrusted and unverified SQL, then it already has much bigger security issues (SQL injection) that no "safe" language will fix.
It is true that applications sometimes import complete binary SQLite database files from untrusted sources, and such imports could present a possible attack vector. However, those code paths in SQLite are limited and are extremely well tested. And pre-validation routines are available to applications that want to read untrusted databases that can help detect possible attacks prior to use.
Some "safe" languages (ex: Go) dislike the use of assert(). But the use of assert() is a vital part of keeping SQLite maintainable. The lack of assert() in Go is a show-stopper as far as the developers of SQLite are concerned. See the The Use Of assert() In SQLite article for additional information.
Safe languages insert additional machine branches to do things like verify that array accesses are in-bounds. In correct code, those branches are never taken. That means that the machine code cannot be 100% branch tested, which is an important component of SQLite's quality strategy.
Safe languages usually want to abort if they encounter an out-of-memory (OOM) situation. SQLite is designed to recover gracefully from an OOM. It is unclear how this could be accomplished in the current crop of safe languages.
All of the existing safe languages are new. The developers of SQLite applaud the efforts of computer language researchers in trying to develop languages that are easier to program safely. We encourage these efforts to continue. Be we ourselves are more interested in old and boring languages when it comes to implementing SQLite.
All that said, it is possible that SQLite might one day be recoded in Rust. Recoding SQLite in Go is unlikely since Go hates assert(). But Rust is a possibility. Some preconditions that must occur before SQLite is recoded in Rust include:
If you are a "rustacean" and feel that Rust already meets the preconditions listed above, and that SQLite should be recoded in Rust, then you are welcomed and encouraged to contact the SQLite developers privately and argue your case.