Undefined Behavior

    When a safety check fails, Zig crashes with a stack trace, like this:

    test.zig

    Shell

    2. 1/1 test "safety check"... thread 797490 panic: reached unreachable code
    3. /home/andy/Downloads/zig/docgen_tmp/test.zig:2:5: 0x2079da in test "safety check" (test)
    4.     unreachable;
    5.     ^
    6. /home/andy/Downloads/zig/lib/std/special/test_runner.zig:80:28: 0x22f3e3 in std.special.main (test)
    7.         } else test_fn.func();
    8.                            ^
    9. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x227ddc in std.start.callMain (test)
    10.             root.main();
    11.                      ^
    12. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2091fe in std.start.callMainWithArgs (test)
    13.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    14.            ^
    15. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x208296 in std.start.posixCallMainAndExit (test)
    16.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    17.                 ^
    18. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x2080a2 in std.start._start (test)
    19.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    20.     ^
    21. error: the following test command crashed:
    22. docgen_tmp/zig-cache/o/8e4964e05ddaf866e77360a551b3c05b/test /home/andy/Downloads/zig/build-release/zig

    At compile-time:

    test.zig

    1. comptime {
    2.     assert(false);
    3. }
    4. fn assert(ok: bool) void {
    5.     if (!ok) unreachable; // assertion failure
    6. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:5:14: error: reached unreachable code
    3.     if (!ok) unreachable; // assertion failure
    4.              ^
    5. ./docgen_tmp/test.zig:2:11: note: called from here
    6.     assert(false);
    7.           ^
    8. ./docgen_tmp/test.zig:1:10: note: called from here
    9. comptime {
    10.          ^
    11. ./docgen_tmp/test.zig:2:11: note: referenced here
    12.     assert(false);
    13.           ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     std.debug.assert(false);
    5. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 797551 panic: reached unreachable code
    5. /home/andy/Downloads/zig/lib/std/debug.zig:224:14: 0x204e3b in std.debug.assert (test)
    6.     if (!ok) unreachable; // assertion failure
    7.              ^
    8. /home/andy/Downloads/zig/docgen_tmp/test.zig:4:21: 0x22ce2a in main (test)
    9.     std.debug.assert(false);
    10.                     ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225bbc in std.start.callMain (test)
    12.             root.main();
    13.                      ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x206fbe in std.start.callMainWithArgs (test)
    15.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    16.            ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206056 in std.start.posixCallMainAndExit (test)
    18.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    19.                 ^
    20. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205e62 in std.start._start (test)
    21.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    22.     ^
    23. (process terminated by signal)

    Index out of Bounds

    At compile-time:

    test.zig

    1. comptime {
    2.     const array: [5]u8 = "hello".*;
    3.     const garbage = array[5];
    4.     _ = garbage;
    5. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:26: error: index 5 outside array of size 5
    3.     const garbage = array[5];
    4.                          ^

    At runtime:

    test.zig

    1. pub fn main() void {
    2.     var x = foo("hello");
    3.     _ = x;
    4. }
    6. fn foo(x: []const u8) u8 {
    7.     return x[5];
    8. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 797611 panic: index out of bounds
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:7:13: 0x2344e9 in foo (test)
    6.     return x[5];
    7.             ^
    8. /home/andy/Downloads/zig/docgen_tmp/test.zig:2:16: 0x22ce36 in main (test)
    9.     var x = foo("hello");
    10.                ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225bbc in std.start.callMain (test)
    12.             root.main();
    13.                      ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x206fbe in std.start.callMainWithArgs (test)
    15.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    16.            ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206056 in std.start.posixCallMainAndExit (test)
    18.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    19.                 ^
    20. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205e62 in std.start._start (test)
    21.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    22.     ^
    23. (process terminated by signal)

    Cast Negative Number to Unsigned Integer

    At compile-time:

    test.zig

    1. comptime {
    2.     const value: i32 = -1;
    3.     const unsigned = @intCast(u32, value);
    4.     _ = unsigned;
    5. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:22: error: attempt to cast negative value to unsigned integer
    3.     const unsigned = @intCast(u32, value);
    4.                      ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var value: i32 = -1;
    5.     var unsigned = @intCast(u32, value);
    6.     std.debug.print("value: {}\n", .{unsigned});
    7. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 797672 panic: attempt to cast negative value to unsigned integer
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:20: 0x22cf27 in main (test)
    6.     var unsigned = @intCast(u32, value);
    7.                    ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    To obtain the maximum value of an unsigned integer, use std.math.maxInt.

    Cast Truncates Data

    At compile-time:

    test.zig

    1. comptime {
    2.     const spartan_count: u16 = 300;
    3.     const byte = @intCast(u8, spartan_count);
    4.     _ = byte;
    5. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:18: error: cast from 'u16' to 'u8' truncates bits
    3.     const byte = @intCast(u8, spartan_count);
    4.                  ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var spartan_count: u16 = 300;
    5.     const byte = @intCast(u8, spartan_count);
    6.     std.debug.print("value: {}\n", .{byte});
    7. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 797733 panic: integer cast truncated bits
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:18: 0x22cf2c in main (test)
    6.     const byte = @intCast(u8, spartan_count);
    7.                  ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    To truncate bits, use .

    The following operators can cause integer overflow:

    • + (addition)
    • - (subtraction)
    • - (negation)
    • * (multiplication)
    • / (division)
    • @divTrunc (division)
    • (division)
    • @divExact (division)

    Example with addition at compile-time:

    test.zig

    1. comptime {
    2.     var byte: u8 = 255;
    3.     byte += 1;
    4. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:10: error: operation caused overflow
    3.     byte += 1;
    4.          ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var byte: u8 = 255;
    5.     byte += 1;
    6.     std.debug.print("value: {}\n", .{byte});
    7. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 797794 panic: integer overflow
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:10: 0x22cf0f in main (test)
    6.     byte += 1;
    7.          ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    These functions provided by the standard library return possible errors.

    • @import("std").math.add
    • @import("std").math.sub
    • @import("std").math.mul
    • @import("std").math.divTrunc
    • @import("std").math.divFloor
    • @import("std").math.divExact
    • @import("std").math.shl

    Example of catching an overflow for addition:

    test.zig

    1. const math = @import("std").math;
    2. const print = @import("std").debug.print;
    3. pub fn main() !void {
    4.     var byte: u8 = 255;
    6.     byte = if (math.add(u8, byte, 1)) |result| result else |err| {
    7.         print("unable to add one: {s}\n", .{@errorName(err)});
    8.         return err;
    9.     };
    11.     print("result: {}\n", .{byte});
    12. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. unable to add one: Overflow
    5. error: Overflow
    6. /home/andy/Downloads/zig/lib/std/math.zig:463:5: 0x234a3b in std.math.add (test)
    7.     return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
    8.     ^
    9. /home/andy/Downloads/zig/docgen_tmp/test.zig:8:9: 0x22d294 in main (test)
    10.         return err;
    11.         ^

    These builtins return a bool of whether or not overflow occurred, as well as returning the overflowed bits:

    Example of :

    test.zig

    1. const print = @import("std").debug.print;
    2. pub fn main() void {
    3.     var byte: u8 = 255;
    5.     var result: u8 = undefined;
    6.     if (@addWithOverflow(u8, byte, 10, &result)) {
    7.         print("overflowed result: {}\n", .{result});
    8.     } else {
    9.         print("result: {}\n", .{result});
    10.     }
    11. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. overflowed result: 9

    These operations have guaranteed wraparound semantics.

    • +% (wraparound addition)
    • -% (wraparound negation)
    • *% (wraparound multiplication)

    wraparound_semantics.zig

    1. const std = @import("std");
    2. const expect = std.testing.expect;
    3. const minInt = std.math.minInt;
    4. const maxInt = std.math.maxInt;
    6. test "wraparound addition and subtraction" {
    7.     const x: i32 = maxInt(i32);
    8.     const min_val = x +% 1;
    9.     try expect(min_val == minInt(i32));
    10.     const max_val = min_val -% 1;
    11.     try expect(max_val == maxInt(i32));
    12. }

    Shell

    1. $ zig test wraparound_semantics.zig
    2. 1/1 test "wraparound addition and subtraction"... OK
    3. All 1 tests passed.

    test.zig

    1. comptime {
    2.     const x = @shlExact(@as(u8, 0b01010101), 2);
    4. }

    Shell

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var x: u8 = 0b01010101;
    5.     var y = @shlExact(x, 2);
    6.     std.debug.print("value: {}\n", .{y});
    7. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 797961 panic: left shift overflowed bits
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:13: 0x22cf5b in main (test)
    6.     var y = @shlExact(x, 2);
    7.             ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225cac in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2070ae in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206146 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f52 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     const x = @shrExact(@as(u8, 0b10101010), 2);
    3.     _ = x;
    4. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:2:15: error: exact shift shifted out 1 bits
    3.     const x = @shrExact(@as(u8, 0b10101010), 2);
    4.               ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var x: u8 = 0b10101010;
    5.     var y = @shrExact(x, 2);
    6.     std.debug.print("value: {}\n", .{y});
    7. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798021 panic: right shift overflowed bits
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:13: 0x22cf6b in main (test)
    6.     var y = @shrExact(x, 2);
    7.             ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225cbc in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2070be in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206156 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f62 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     const a: i32 = 1;
    3.     const b: i32 = 0;
    4.     const c = a / b;
    5.     _ = c;
    6. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:4:17: error: division by zero
    3.     const c = a / b;
    4.                 ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var a: u32 = 1;
    5.     var b: u32 = 0;
    6.     var c = a / b;
    7.     std.debug.print("value: {}\n", .{c});
    8. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798081 panic: division by zero
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:6:15: 0x22cf19 in main (test)
    6.     var c = a / b;
    7.               ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     const a: i32 = 10;
    3.     const b: i32 = 0;
    4.     const c = a % b;
    5.     _ = c;
    6. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:4:17: error: division by zero
    3.     const c = a % b;
    4.                 ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var a: u32 = 10;
    5.     var b: u32 = 0;
    6.     var c = a % b;
    7.     std.debug.print("value: {}\n", .{c});
    8. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798142 panic: remainder division by zero or negative value
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:6:15: 0x22cf3b in main (test)
    6.     var c = a % b;
    7.               ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     const a: u32 = 10;
    3.     const b: u32 = 3;
    4.     const c = @divExact(a, b);
    5.     _ = c;
    6. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:4:15: error: exact division had a remainder
    3.     const c = @divExact(a, b);
    4.               ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var a: u32 = 10;
    5.     var b: u32 = 3;
    6.     var c = @divExact(a, b);
    7.     std.debug.print("value: {}\n", .{c});
    8. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798202 panic: exact division produced remainder
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:6:13: 0x22cf5d in main (test)
    6.     var c = @divExact(a, b);
    7.             ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     const optional_number: ?i32 = null;
    3.     const number = optional_number.?;
    4.     _ = number;
    5. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:35: error: unable to unwrap null
    3.     const number = optional_number.?;
    4.                                   ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var optional_number: ?i32 = null;
    5.     var number = optional_number.?;
    6.     std.debug.print("value: {}\n", .{number});
    7. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798262 panic: attempt to use null value
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:33: 0x22cf0c in main (test)
    6.     var number = optional_number.?;
    7.                                 ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c7c in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20707e in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206116 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f22 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    One way to avoid this crash is to test for null instead of assuming non-null, with the if expression:

    test.zig

    1. const print = @import("std").debug.print;
    2. pub fn main() void {
    3.     const optional_number: ?i32 = null;
    5.     if (optional_number) |number| {
    6.         print("got number: {}\n", .{number});
    7.     } else {
    8.         print("it's null\n", .{});
    9.     }
    10. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. it's null

    See also:

    Attempt to Unwrap Error

    At compile-time:

    test.zig

    1. comptime {
    2.     const number = getNumberOrFail() catch unreachable;
    3.     _ = number;
    4. }
    6. fn getNumberOrFail() !i32 {
    7.     return error.UnableToReturnNumber;
    8. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:2:38: error: caught unexpected error 'UnableToReturnNumber'
    3.     const number = getNumberOrFail() catch unreachable;
    4.                                      ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     const number = getNumberOrFail() catch unreachable;
    5.     std.debug.print("value: {}\n", .{number});
    6. }
    8. fn getNumberOrFail() !i32 {
    9.     return error.UnableToReturnNumber;
    10. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798359 panic: attempt to unwrap error: UnableToReturnNumber
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:9:5: 0x234644 in getNumberOrFail (test)
    6.     return error.UnableToReturnNumber;
    7.     ^
    8. ???:?:?: 0x20ce72 in ??? (???)
    9. /home/andy/Downloads/zig/docgen_tmp/test.zig:4:38: 0x22cf6b in main (test)
    10.     const number = getNumberOrFail() catch unreachable;
    11.                                      ^
    12. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225c9c in std.start.callMain (test)
    13.             root.main();
    14.                      ^
    15. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x20709e in std.start.callMainWithArgs (test)
    16.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    17.            ^
    18. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206136 in std.start.posixCallMainAndExit (test)
    19.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    20.                 ^
    21. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f42 in std.start._start (test)
    22.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    23.     ^
    24. (process terminated by signal)

    One way to avoid this crash is to test for an error instead of assuming a successful result, with the if expression:

    test.zig

    1. $ zig build-exe test.zig
    3. $ ./test

    See also:

    At compile-time:

    test.zig

    1. comptime {
    2.     const err = error.AnError;
    3.     const number = @errorToInt(err) + 10;
    4.     const invalid_err = @intToError(number);
    5.     _ = invalid_err;
    6. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:4:25: error: integer value 11 represents no error
    3.     const invalid_err = @intToError(number);

    At runtime:

    test.zig

    1. const std = @import("std");
    3. pub fn main() void {
    4.     var err = error.AnError;
    5.     var number = @errorToInt(err) + 500;
    6.     var invalid_err = @intToError(number);
    7.     std.debug.print("value: {}\n", .{invalid_err});
    8. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798455 panic: invalid error code
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:6:23: 0x22cf93 in main (test)
    6.     var invalid_err = @intToError(number);
    7.                       ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225ccc in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2070ce in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206166 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f72 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. const Foo = enum {
    2.     a,
    3.     b,
    4.     c,
    5. };
    6. comptime {
    7.     const a: u2 = 3;
    8.     const b = @intToEnum(Foo, a);
    9.     _ = b;
    10. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:8:15: error: enum 'Foo' has no tag matching integer value 3
    3.     const b = @intToEnum(Foo, a);
    4.               ^
    5. ./docgen_tmp/test.zig:1:13: note: 'Foo' declared here
    6. const Foo = enum {
    7.             ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. const Foo = enum {
    4.     a,
    5.     b,
    6.     c,
    7. };
    9. pub fn main() void {
    10.     var a: u2 = 3;
    11.     var b = @intToEnum(Foo, a);
    12.     std.debug.print("value: {s}\n", .{@tagName(b)});
    13. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798517 panic: invalid enum value
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:11:13: 0x22cf89 in main (test)
    6.     var b = @intToEnum(Foo, a);
    7.             ^
    8. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225cec in std.start.callMain (test)
    9.             root.main();
    10.                      ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2070ee in std.start.callMainWithArgs (test)
    12.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    13.            ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206186 in std.start.posixCallMainAndExit (test)
    15.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    16.                 ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f92 in std.start._start (test)
    18.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    19.     ^
    20. (process terminated by signal)

    At compile-time:

    test.zig

    1. const Set1 = error{
    2.     A,
    3.     B,
    4. };
    5. const Set2 = error{
    6.     A,
    7.     C,
    8. };
    9. comptime {
    10.     _ = @errSetCast(Set2, Set1.B);
    11. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:10:9: error: error.B not a member of error set 'Set2'
    3.     _ = @errSetCast(Set2, Set1.B);
    4.         ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. const Set1 = error{
    4.     A,
    5.     B,
    6. };
    7. const Set2 = error{
    8.     A,
    9.     C,
    10. };
    11. pub fn main() void {
    12.     foo(Set1.B);
    13. }
    14. fn foo(set1: Set1) void {
    15.     const x = @errSetCast(Set2, set1);
    16.     std.debug.print("value: {}\n", .{x});
    17. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798577 panic: invalid error code
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:15:15: 0x234636 in foo (test)
    6.     const x = @errSetCast(Set2, set1);
    7.               ^
    8. /home/andy/Downloads/zig/docgen_tmp/test.zig:12:8: 0x22cf5d in main (test)
    9.     foo(Set1.B);
    10.        ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x225cec in std.start.callMain (test)
    12.             root.main();
    13.                      ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2070ee in std.start.callMainWithArgs (test)
    15.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    16.            ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206186 in std.start.posixCallMainAndExit (test)
    18.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    19.                 ^
    20. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f92 in std.start._start (test)
    21.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    22.     ^
    23. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     const ptr = @intToPtr(*align(1) i32, 0x1);
    3.     const aligned = @alignCast(4, ptr);
    4.     _ = aligned;
    5. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes
    3.     const aligned = @alignCast(4, ptr);
    4.                                   ^
    5. ./docgen_tmp/test.zig:3:21: note: referenced here
    6.     const aligned = @alignCast(4, ptr);
    7.                     ^

    At runtime:

    test.zig

    1. const mem = @import("std").mem;
    2. pub fn main() !void {
    3.     var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
    4.     const bytes = mem.sliceAsBytes(array[0..]);
    5.     if (foo(bytes) != 0x11111111) return error.Wrong;
    6. }
    7. fn foo(bytes: []u8) u32 {
    8.     const slice4 = bytes[1..5];
    9.     const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
    10.     return int_slice[0];
    11. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798638 panic: incorrect alignment
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:9:59: 0x2348ac in foo (test)
    6.     const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
    7.                                                           ^
    8. /home/andy/Downloads/zig/docgen_tmp/test.zig:5:12: 0x22d0c5 in main (test)
    9.     if (foo(bytes) != 0x11111111) return error.Wrong;
    10.            ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:553:37: 0x225cfa in std.start.callMain (test)
    12.             const result = root.main() catch |err| {
    13.                                     ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x2070ce in std.start.callMainWithArgs (test)
    15.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    16.            ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x206166 in std.start.posixCallMainAndExit (test)
    18.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    19.                 ^
    20. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x205f72 in std.start._start (test)
    21.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    22.     ^
    23. (process terminated by signal)

    At compile-time:

    test.zig

    1. comptime {
    2.     var f = Foo{ .int = 42 };
    3.     f.float = 12.34;
    4. }
    6. const Foo = union {
    7.     float: f32,
    8.     int: u32,
    9. };

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:6: error: accessing union field 'float' while field 'int' is set
    3.     f.float = 12.34;
    4.      ^

    At runtime:

    test.zig

    1. const std = @import("std");
    3. const Foo = union {
    4.     float: f32,
    5.     int: u32,
    6. };
    8. pub fn main() void {
    9.     var f = Foo{ .int = 42 };
    10.     bar(&f);
    11. }
    13. fn bar(f: *Foo) void {
    14.     f.float = 12.34;
    15.     std.debug.print("value: {}\n", .{f.float});
    16. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. thread 798698 panic: access of inactive union field
    5. /home/andy/Downloads/zig/docgen_tmp/test.zig:14:6: 0x241eea in bar (test)
    6.     f.float = 12.34;
    7.      ^
    8. /home/andy/Downloads/zig/docgen_tmp/test.zig:10:8: 0x23a80c in main (test)
    9.     bar(&f);
    10.        ^
    11. /home/andy/Downloads/zig/lib/std/start.zig:543:22: 0x23358c in std.start.callMain (test)
    12.             root.main();
    13.                      ^
    14. /home/andy/Downloads/zig/lib/std/start.zig:495:12: 0x21498e in std.start.callMainWithArgs (test)
    15.     return @call(.{ .modifier = .always_inline }, callMain, .{});
    16.            ^
    17. /home/andy/Downloads/zig/lib/std/start.zig:409:17: 0x213a26 in std.start.posixCallMainAndExit (test)
    18.     std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
    19.                 ^
    20. /home/andy/Downloads/zig/lib/std/start.zig:322:5: 0x213832 in std.start._start (test)
    21.     @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    22.     ^
    23. (process terminated by signal)

    This safety is not available for extern or packed unions.

    To change the active field of a union, assign the entire union, like this:

    test.zig

    1. const std = @import("std");
    3. const Foo = union {
    4.     float: f32,
    5.     int: u32,
    6. };
    8. pub fn main() void {
    9.     var f = Foo{ .int = 42 };
    10.     bar(&f);
    11. }
    13. fn bar(f: *Foo) void {
    14.     f.* = Foo{ .float = 12.34 };
    15.     std.debug.print("value: {}\n", .{f.float});
    16. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. value: 1.23400001e+01

    To change the active field of a union when a meaningful value for the field is not known, use undefined, like this:

    test.zig

    1. const std = @import("std");
    3. const Foo = union {
    4.     float: f32,
    5.     int: u32,
    6. };
    8. pub fn main() void {
    9.     var f = Foo{ .int = 42 };
    10.     f = Foo{ .float = undefined };
    11.     bar(&f);
    12.     std.debug.print("value: {}\n", .{f.float});
    13. }
    15. fn bar(f: *Foo) void {
    16.     f.float = 12.34;
    17. }

    Shell

    1. $ zig build-exe test.zig
    3. $ ./test
    4. value: 1.23400001e+01

    See also:

    Out of Bounds Float to Integer Cast

    TODO

    This happens when casting a pointer with the address 0 to a pointer which may not have the address 0. For example, , Optional Pointers, and pointers allow address zero, but normal Pointers do not.

    At compile-time:

    test.zig

    1. comptime {
    2.     const opt_ptr: ?*i32 = null;
    3.     const ptr = @ptrCast(*i32, opt_ptr);
    4.     _ = ptr;
    5. }

    Shell

    1. $ zig test test.zig
    2. ./docgen_tmp/test.zig:3:17: error: null pointer casted to type '*i32'
    3.     const ptr = @ptrCast(*i32, opt_ptr);
    4.                 ^

    At runtime:

    test.zig

    1. pub fn main() void {
    2.     var opt_ptr: ?*i32 = null;
    3.     var ptr = @ptrCast(*i32, opt_ptr);
    4. }

    Shell