An extern struct has in-memory layout guaranteed to match the C ABI for the target. This kind of struct should only be used for compatibility with the C ABI.
Every other use case should be solved with packed struct or normal struct. Unlike normal structs, packed structs have guaranteed in-memory layout:.
This means that a packed struct can participate in a bitCast or a ptrCast to reinterpret memory. This even works at comptime :. However, the pointer to a non-byte-aligned field has special properties and cannot be passed when a normal pointer is expected:.
In this case, the function bar cannot be called becuse the pointer to the non-ABI-aligned field mentions the bit offset, but the function expects an ABI-aligned pointer. Pointers to non-ABI-aligned fields share the same address as the other fields within their host integer:.
This can be observed with bitOffsetOf and byteOffsetOf :. Packed structs have 1-byte alignment. However if you have an overaligned pointer to a packed struct, Zig should correctly understand the alignment of fields.
However there is a bug :. When this bug is fixed, the above test in the documentation will unexpectedly pass, which will cause the test suite to fail, notifying the bug fixer to update these docs.
Using packed structs with volatile is problematic, and may be a compile error in the future. For details on this subscribe to this issue. TODO update these docs with a recommendation on how to use packed structs with MMIO the use case for volatile packed structs once this issue is resolved.
Don't worry, there will be a good solution for this use case in zig. Zig allows omitting the struct type of a literal. When the result is coerced , the struct literal will directly instantiate the result location, with no copy:. The struct type can be inferred. Here the result location does not include a type, and so Zig infers the type:.
Anonymous structs can be created without specifying field names, and are referred to as "tuples". The fields are implicitly named using numbers starting from 0. Because their names are integers, the "0" syntax must be used to access them. Names inside "" are always recognised as identifiers. Like arrays, tuples have a. They can also be iterated over with inline for. For a C-ABI-compatible enum, use extern enum :. Enum literals allow specifying the name of an enum field without specifying the enum type:.
It must specify a tag type and cannot consume every enumeration value. A bare union defines a set of possible types that a value can be as a list of fields. Only one field can be active at a time. The in-memory representation of bare unions is not guaranteed. Bare unions cannot be used to reinterpret memory. For that, use ptrCast , or use an extern union or a packed union which have guaranteed in-memory layout. Accessing the non-active field is safety-checked Undefined Behavior :.
In order to use switch with a union, it must be a Tagged union. To initialize a union when the tag is a comptime -known name, see unionInit. Unions can be declared with an enum tag type. This turns the union into a tagged union, which makes it eligible to use with switch expressions. Tagged unions coerce to their tag type: Type Coercion: unions and enums. Unions can be made to infer the enum tag type. Further, unions can have methods just like structs and enums.
An extern union has memory layout guaranteed to be compatible with the target C ABI. A packed union has well-defined in-memory layout and is eligible to be in a packed struct.
It can contain declarations the same as structs , unions , and enums. This is typically used for type safety when interacting with C code that does not expose struct details. Blocks are expressions. When labeled, break can be used to return a value from the block:. Because of this, when you read Zig code you can rely on an identifier always meaning the same thing, within the scope it is defined.
Note that you can, however use the same name if the scopes are separate:. When a switch expression does not have an else clause, it must exhaustively list all the possible values.
Failure to do so is a compile error:. Enum Literals can be useful to use with switch to avoid repetitively specifying enum or union types:. A while loop is used to repeatedly execute an expression until some condition is no longer true. Use break to exit a while loop early. Use continue to jump back to the beginning of the loop.
While loops support a continue expression which is executed when the loop is continued. The continue keyword respects this expression. While loops are expressions.
The result of the expression is the result of the else clause of a while loop, which is executed when the condition of the while loop is tested as false. This is the result of the while expression. When you break from a while loop, the else branch is not evaluated. When a while loop is labeled, it can be referenced from a break or continue from within a nested loop:.
Just like if expressions, while loops can take an optional as the condition and capture the payload. When null is encountered the loop exits.
When the x syntax is present on a while expression, the while condition must have an Optional Type. The else branch is allowed on optional iteration. In this case, it will be executed on the first null value encountered.
Just like if expressions, while loops can take an error union as the condition and capture the payload or the error code. When the condition results in an error code the else branch is evaluated and the loop is finished. When the else x syntax is present on a while expression, the while condition must have an Error Union Type. While loops can be inlined.
This causes the loop to be unrolled, which allows the code to do some things which only work at compile time, such as use types as first class values. It is recommended to use inline loops only for one of these reasons:. When a for loop is labeled, it can be referenced from a break or continue from within a nested loop:.
For loops can be inlined. The capture value and iterator value of inlined for loops are compile-time known. In Debug and ReleaseSafe mode, and when using zig test , unreachable emits a call to panic with the message reached unreachable code.
In ReleaseFast mode, the optimizer uses the assumption that unreachable code will never be hit to perform optimizations. However, zig test even in ReleaseFast mode still emits unreachable as calls to panic.
When resolving types together, such as if clauses or switch prongs, the noreturn type is compatible with every other type. Another use case for noreturn is the exit function:. Primitive types such as Integers and Floats passed as parameters are copied, and then the copy is available in the function body. This is called "passing by value". Copying a primitive type is essentially free and typically involves nothing more than setting a register. Structs, unions, and arrays can sometimes be more efficiently passed as a reference, since a copy could be arbitrarily expensive depending on the size.
When these types are passed as parameters, Zig may choose to copy and pass by value, or pass by reference, whichever way Zig decides will be faster. This is made possible, in part, by the fact that parameters are immutable. Function parameters can be declared with anytype in place of the type. In this case the parameter types will be inferred when the function is called.
Use TypeOf and typeInfo to get information about the inferred type. An error set is like an enum. However, each error name across the entire compilation gets assigned an unsigned integer greater than 0. You are allowed to declare the same error name more than once, and if you do, it gets assigned the same integer value.
The number of unique error values across the entire compilation should determine the size of the error set type. However right now it is hard coded to be a u See There is a shortcut for declaring an error set with only 1 value, and then getting that value:.
This is the error set that contains all errors in the entire compilation unit. It is a superset of all other error sets and a subset of none of them. You can coerce any error set to the global one, and you can explicitly cast an error of the global error set to a non-global one.
This inserts a language-level assert to make sure the error value is in fact in the destination error set. The global error set should generally be avoided because it prevents the compiler from knowing what errors are possible at compile-time. Knowing the error set at compile-time is better for generated documentation and helpful error messages, such as forgetting a possible error value in a switch. An error set type and normal type can be combined with the!
You are likely to use an error union type more often than an error set type by itself. Notice the return type is! This means that the function either returns an unsigned 64 bit integer, or an error. We left off the error set to the left of the! Within the function definition, you can see some return statements that return an error, and at the bottom a return statement that returns a u Both types coerce to anyerror!
What it looks like to use this function varies depending on what you're trying to do. One of the following:. If you want to provide a default value, you can use the catch binary operator:. In this code, number will be equal to the successfully parsed string, or a default value of The type of the right hand side of the binary catch operator must match the unwrapped error union type, or be of type noreturn.
Let's say you wanted to return the error if you got one, otherwise continue with the function logic:. There is a shortcut for this. The try expression:. If it is an error, it returns from the current function with the same error. Otherwise, the expression results in the unwrapped value. Maybe you know with complete certainty that an expression will never be an error.
In this case you can do this:. Here we know for sure that "" will parse successfully. So we put the unreachable value on the right hand side. So, while we're debugging the application, if there was a surprise error here, the application would crash appropriately. Finally, you may want to take a different action for every situation. For that, we combine the if and switch expression:.
The other component to error handling is defer statements. In addition to an unconditional defer , Zig has errdefer , which evaluates the deferred expression on block exit path if and only if the function returned with an error from the block. The neat thing about this is that you get robust error handling without the verbosity and cognitive overhead of trying to make sure every exit path is covered. The deallocation code is always directly following the allocation code.
An error union is created with the! You can use compile-time reflection to access the child type of an error union:.
Use the operator to merge two error sets together. The resulting error set contains the errors of both error sets. Doc comments from the left-hand side override doc comments from the right-hand side. In this example, the doc comments for C.
PathNotFound is A doc comment. This is especially useful for functions which return different error sets depending on comptime branches. Because many functions in Zig return a possible error, Zig supports inferring the error set.
To infer the error set for a function, use this syntax:. When a function has an inferred error set, that function becomes generic and thus it becomes trickier to do certain things with it, such as obtain a function pointer, or have an error set that is consistent across different build targets.
Additionally, inferred error sets are incompatible with recursion. In these situations, it is recommended to use an explicit error set. You can generally start with an empty error set and let compile errors guide you toward completing the set. Error Return Traces show all the points in the code that an error was returned to the calling function.
This makes it practical to use try everywhere and then still be able to know what happened if an error ends up bubbling all the way out of your application.
You can see that the final error bubbled up was PermissionDenied , but the original error that started this whole thing was FileNotFound. In the bar function, the code handles the original error code, and then returns another one, from the switch statement. Error Return Traces make this clear, whereas a stack trace would look like this:. Here, the stack trace does not explain how the control flow in bar got to the hello call.
One would have to open a debugger or further instrument the application in order to find out. The error return trace, on the other hand, shows exactly how the error bubbled up. This debugging feature makes it easier to iterate quickly on code that robustly handles all error conditions.
This means that Zig developers will naturally find themselves writing correct, robust code in order to increase their development pace. For the case when no errors are returned, the cost is a single memory write operation, only in the first non-failable function in the call graph that calls a failable function, i. This is to initialize this struct in the stack memory:.
Here, N is the maximum function call depth as determined by call graph analysis. Recursion is ignored and counts for 2. A pointer to StackTrace is passed as a secret parameter to every function that can return an error, but it's always the first parameter, so it can likely sit in a register and stay there.
That's it for the path when no errors occur. It's practically free in terms of performance. When generating the code for a function that returns an error, just before the return statement only for the return statements that return errors , Zig generates a call to this function:. The cost is 2 math operations plus some memory reads and writes.
The memory accessed is constrained and should remain cached for the duration of the error return bubbling. As for code size cost, 1 function call before a return statement is no big deal. What is a return statement in code without error return tracing can become a jump instruction in code with error return tracing. One area that Zig provides safety without compromising efficiency or readability is with the optional type.
The question mark symbolizes the optional type. You can convert a type to an optional type by putting a question mark in front of it, like this:. Instead of integers, let's talk about pointers.
Null references are the source of many runtime exceptions, and even stand accused of being the worst mistake of computer science. Instead, you can use an optional pointer. This secretly compiles down to a normal pointer, since we know we can use 0 as the null value for the optional type.
But the compiler can check your work and make sure you don't assign null to something that can't be null. Typically the downside of not having null is that it makes the code more verbose to write.
But, let's compare some equivalent C code and Zig code. Here, Zig is at least as convenient, if not more, than C. The orelse keyword unwrapped the optional type and therefore ptr is guaranteed to be non-null everywhere it is used in the function.
Once again, the notable thing here is that inside the if block, foo is no longer an optional pointer, it is a pointer, which cannot be null. The optimizer can sometimes make better decisions knowing that pointer arguments cannot be null. An optional is created by putting? You can use compile-time reflection to access the child type of an optional:. Just like undefined , null has its own type, and the only way to use it is to cast it to a different type:.
An optional pointer is guaranteed to be the same size as a pointer. The null of the optional is guaranteed to be address 0. A type cast converts a value of one type to another. Zig has Type Coercion for conversions that are known to be completely safe and unambiguous, and Explicit Casts for conversions that one would not want to happen on accident.
There is also a third kind of type conversion called Peer Type Resolution for the case when a result type must be decided given multiple operand types. Type coercions are only allowed when it is completely unambiguous how to get from one type to another, and the transformation is guaranteed to be safe. There is one exception, which is C Pointers. Values which have the same representation at runtime can be cast to increase the strictness of the qualifiers, no matter how nested the qualifiers are:.
Integers coerce to integer types which can represent every value of the old type, and likewise Floats coerce to float types which can represent every value of the old type. A compiler error is appropriate because this ambiguous expression leaves the compiler two choices about the coercion. The payload type of Optionals , as well as null , coerce to the optional type. When a number is comptime -known to be representable in the destination type, it may be coerced:.
Tagged unions can be coerced to enums, and enums can be coerced to tagged unions when they are comptime -known to be a field of the union that has only one possible value, such as void :.
Zero Bit Types may be coerced to single-item Pointers , regardless of const. Explicit casts are performed via Builtin Functions. Some explicit casts are safe; some are not. Some explicit casts perform language-level assertions; some do not. Some explicit casts are no-ops at runtime; some are not. This kind of type resolution chooses a type that all peer types can coerce into. Here are some examples:. These types can only ever have one possible value, and thus require 0 bits to represent.
Code that makes use of these types is not included in the final generated code:. When this turns into machine code, there is no code generated in the body of entry , even in Debug mode. These assembly instructions do not have any code associated with the void values - they only perform the function call prologue and epilog. Note that this is different from using a dummy value for the hash map value. By using void as the type of the value, the hash map entry type has no value field, and thus the hash map takes up less space.
Further, all the code that deals with storing and loading the value is deleted, as seen above. Expressions of type void are the only ones whose value can be ignored. For example:. However, if the expression has type void , there will be no error.
The type being pointed to can only ever be one value; therefore loads and stores are never generated. Instead of the above pattern, it is generally recommended to explicitly alias individual declarations. However, usingnamespace has an important use case when organizing the public API of a file or package. For example, one might have c. The above example demonstrates using pub to qualify the usingnamespace additionally makes the imported declarations pub.
This can be used to forward declarations, giving precise control over what declarations a given file exposes. Zig places importance on the concept of whether an expression is known at compile-time. There are a few different places this concept is used, and these building blocks are used to keep the language small, readable, and powerful. In Zig, types are first-class citizens. They can be assigned to variables, passed as parameters to functions, and returned from functions.
However, they can only be used in expressions which are known at compile-time , which is why the parameter T in the above snippet must be marked with comptime. This is an error because the programmer attempted to pass a value only known at run-time to a function which expects a value known at compile-time. Another way to get an error is if we pass a type that violates the type checker when the function is analyzed.
This is what it means to have compile-time duck typing. On the flip side, inside the function definition with the comptime parameter, the value is known at compile-time.
This means that we actually could make this work for the bool type if we wanted to:. This works because Zig implicitly inlines if expressions when the condition is known at compile-time, and the compiler guarantees that it will skip analysis of the branch not taken. This means that the actual function generated for max in this situation looks like this:.
All the code that dealt with compile-time known values is eliminated and we are left with only the necessary run-time code to accomplish the task. This works the same way for switch expressions - they are implicitly inlined when the target expression is compile-time known. In Zig, the programmer can label variables as comptime.
This guarantees to the compiler that every load and store of the variable is performed at compile-time. Any violation of this results in a compile error. This combined with the fact that we can inline loops allows us to write a function which is partially evaluated at compile-time and partially at run-time. This example is a bit contrived, because the compile-time evaluation component is unnecessary; this code would work fine if it was all done at run-time.
But it does end up generating different code. Note that this happens even in a debug build; in a release build these generated functions still pass through rigorous LLVM optimizations. The important thing to note, however, is not that this is a way to write more optimized code, but that it is a way to make sure that what should happen at compile-time, does happen at compile-time.
This catches more errors and as demonstrated later in this article, allows expressiveness that in other languages requires using macros, generated code, or a preprocessor to accomplish. In Zig, it matters whether a given expression is known at compile-time or run-time. A programmer can use a comptime expression to guarantee that the expression will be evaluated at compile-time.
If this cannot be accomplished, the compiler will emit an error. It doesn't make sense that a program could call exit or any other external function at compile-time, so this is a compile error. However, a comptime expression does much more than sometimes cause a compile error. This means that a programmer can create a function which is called both at compile-time and run-time, with no modification to the function required.
Imagine if we had forgotten the base case of the recursive function and tried to run the tests:. The compiler produces an error which is a stack trace from trying to evaluate the function at compile-time. Luckily, we used an unsigned integer, and so when we tried to subtract 1 from 0, it triggered undefined behavior, which is always a compile error if the compiler knows it happened. But what would have happened if we used a signed integer? The compiler noticed that evaluating this function at compile-time took a long time, and thus emitted a compile error and gave up.
If the programmer wants to increase the budget for compile-time computation, they can use a built-in function called setEvalBranchQuota to change the default number to something else. What if we fix the base case, but put the wrong value in the expect line?
What happened is Zig started interpreting the expect function with the parameter ok set to false. When the interpreter hit panic it emitted a compile error because a panic during compile causes a compile error if it is detected at compile-time. At container level outside of any function , all expressions are implicitly comptime expressions. This means that we can use functions to initialize complex static data. When we compile this program, Zig generates the constants with the answer pre-computed.
Note that we did not have to do anything special with the syntax of these functions. For example, we could call the sum function as is with a slice of numbers whose length and values were only known at run-time.
Zig uses these capabilities to implement generic data structures without introducing any special-case syntax. If you followed along so far, you may already know how to create a generic data structure. Here is an example of a generic List data structure, that we will instantiate with the type i In Zig we refer to the type as List i That's it.
It's a function that returns an anonymous struct. For the purposes of error messages and debugging, Zig infers the name "List i32 " from the function name and parameters invoked when creating the anonymous struct.
To keep the language small and uniform, all aggregate types in Zig are anonymous. To give a type a name, we assign it to a constant:. This works because all top level declarations are order-independent, and as long as there isn't an actual infinite regression, values can refer to themselves, directly or indirectly. In this case, Node refers to itself as a pointer, which is not actually an infinite regression, so it works fine. This is a proof of concept implementation; the actual function in the standard library has more formatting capabilities.
Note that this is not hard-coded into the Zig compiler; this is userland code in the standard library. When this function is analyzed from our example code above, Zig partially evaluates the function and emits a function that actually looks like this:.
Zig doesn't care whether the format argument is a string literal, only that it is a compile-time known value that can be coerced to a [] const u8 :. Zig does not special case string formatting in the compiler and instead exposes enough power to accomplish this task in userland. It does so without introducing another language on top of Zig, such as a macro language or a preprocessor language.
It's Zig all the way down. For some use cases, it may be necessary to directly control the machine code generated by Zig programs, rather than relying on Zig's code generation.
For these cases, one can use inline assembly. This is due to technical constraints; assembly parsing is provided by LLVM and its support for Intel syntax is buggy and not well tested. Some day Zig may have its own assembler. This would allow it to integrate more seamlessly into the language, as well as be compatible with the popular NASM syntax.
This documentation section will be updated before 1. Output constraints are still considered to be unstable in Zig, and so LLVM documentation and GCC documentation must be used to understand the semantics. Note that some breaking changes to output constraints are planned with issue Input constraints are still considered to be unstable in Zig, and so LLVM documentation and GCC documentation must be used to understand the semantics. Note that some breaking changes to input constraints are planned with issue Clobbers are the set of registers whose values will not be preserved by the execution of the assembly code.
These do not include output or input registers. The special clobber value of "memory" means that the assembly causes writes to arbitrary undeclared memory locations - not only the memory pointed to by a declared indirect output.
Failure to declare the full set of clobbers for a given inline assembly expression is unchecked Undefined Behavior. When an assembly expression occurs in a container level comptime block, this is global assembly.
This kind of assembly has different rules than inline assembly. First, volatile is not valid because all global assembly is unconditionally included.
Second, there are no inputs, outputs, or clobbers. All global assembly is concatenated verbatim into one long string and assembled together. When a function is called, a frame is pushed to the stack, the function runs until it reaches a return statement, and then the frame is popped from the stack. At the callsite, the following code does not run until the function returns.
An async function is a function whose callsite is split into an async initiation, followed by an await completion. Its frame is provided explicitly by the caller, and it can be suspended and resumed any number of times. Zig infers that a function is async when it observes that the function contains a suspension point. Async functions can be called the same as normal functions.
A function call of an async function is a suspend point. At any point, a function may suspend itself. This causes control flow to return to the callsite in the case of the first suspension , or resumer in the case of subsequent suspensions. In the same way that each allocation should have a corresponding free, Each suspend should have a corresponding resume. A suspend block allows a function to put a pointer to its own frame somewhere, for example into an event loop, even if that action will perform a resume operation on a different thread.
Upon entering a suspend block, the async function is already considered suspended, and can be resumed. For example, if you started another kernel thread, and had that thread call resume on the frame pointer provided by the frame , the new thread would begin executing after the suspend block, while the old thread continued executing the suspend block. However, the async function can be directly resumed from the suspend block, in which case it never returns to its resumer and continues executing.
In the same way that every suspend has a matching resume , every async has a matching await. The await keyword is used to coordinate with an async function's return statement. There is a common misconception that await resumes the target function. In this case a new target was also backported from trunk. There is a list of changes that may break backward compatibility. You can also have a look at the FPC 3. Downloads are available at the download section. Some links might be stale but will be updated in the coming days.
If you have trouble using FTP due to recent browser updates, try the sourceforge mirror. June 19th, FPC version 3. This version is a major new release and contains bugfixes and updates packages, new features and new targets. Due to the age of the FPC 3. June 8, Today FPC celebrates its 25th birthday! Version 3. Hit the download link and select a mirror close to you to download your copy. The development releases have version numbers 3.
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