From bd38c417fc9ba367b3cfcd49d29b52412ef04251 Mon Sep 17 00:00:00 2001 From: Andrew Kelley Date: Fri, 11 Apr 2025 14:16:57 -0700 Subject: [PATCH] langref: reword some packed struct text and example --- doc/langref.html.in | 47 +++++++++++++++--------------- doc/langref/packed_struct_mmio.zig | 17 ++++++----- 2 files changed, 34 insertions(+), 30 deletions(-) diff --git a/doc/langref.html.in b/doc/langref.html.in index 5075458aad..102bc8aeb9 100644 --- a/doc/langref.html.in +++ b/doc/langref.html.in @@ -1649,6 +1649,7 @@ unwrapped == 1234{#endsyntax#}
  • {#link|Floats#}
  • {#link|bool|Primitive Types#}
  • {#link|type|Primitive Types#}
    • +
  • {#link|packed struct#}
    • @@ -2224,27 +2225,24 @@ or {#header_open|packed struct#}

    - Unlike normal structs, {#syntax#}packed{#endsyntax#} structs have guaranteed in-memory layout: + {#syntax#}packed{#endsyntax#} structs, like {#syntax#}enum{#endsyntax#}, are based on the concept + of interpreting integers differently. All packed structs have a backing integer, + which is implicitly determined by the total bit count of fields, or explicitly specified. + Packed structs have well-defined memory layout - exactly the same ABI as their backing integer. +

    +

    + Each field of a packed struct is interpreted as a logical sequence of bits, arranged from + least to most significant. Allowed field types:

    This means that a {#syntax#}packed struct{#endsyntax#} can participate @@ -2252,9 +2250,11 @@ or This even works at {#link|comptime#}:

    {#code|test_packed_structs.zig#} -

    - The backing integer can be inferred or explicitly provided. When inferred, it will be unsigned. When explicitly provided, its bit width will be enforced at compile time to exactly match the total bit width of the fields: + The backing integer can be inferred or explicitly provided. When + inferred, it will be unsigned. When explicitly provided, its bit width + will be enforced at compile time to exactly match the total bit width of + the fields:

    {#code|test_missized_packed_struct.zig#} @@ -2296,17 +2296,18 @@ or

    Equating packed structs results in a comparison of the backing integer, - and only works for the `==` and `!=` operators. + and only works for the {#syntax#}=={#endsyntax#} and {#syntax#}!={#endsyntax#} {#link|Operators#}.

    {#code|test_packed_struct_equality.zig#}

    - Packed structs can be used to interact with memory-mapped input-output (MMIO), which is - common in embedded applications. A pointer of the correct alignment and address to a packed struct - can be constructed to faciltiate manipulation of bit-packed registers without arduous bitshifting. - - {#code|packed_struct_mmio.zig#} + Field access and assignment can be understood as shorthand for bitshifts + on the backing integer. These operations are not {#link|atomic|Atomics#}, + so beware using field access syntax when combined with memory-mapped + input-output (MMIO). Instead of field access on {#link|volatile#} {#link|Pointers#}, + construct a fully-formed new value first, then write that value to the volatile pointer.

    + {#code|packed_struct_mmio.zig#} {#header_close#} {#header_open|Struct Naming#} diff --git a/doc/langref/packed_struct_mmio.zig b/doc/langref/packed_struct_mmio.zig index 79236a76ea..18b57f83b2 100644 --- a/doc/langref/packed_struct_mmio.zig +++ b/doc/langref/packed_struct_mmio.zig @@ -1,16 +1,19 @@ -pub const GPIORegister = packed struct(u8) { +pub const GpioRegister = packed struct(u8) { GPIO0: bool, GPIO1: bool, GPIO2: bool, GPIO3: bool, - _reserved: u4 = 0, + reserved: u4 = 0, }; -/// Write a new state to the memory-mapped IO. -pub fn writeToGPIO(new_states: GPIORegister) void { - const gpio_register_address = 0x0123; - const raw_ptr: *align(1) volatile GPIORegister = @ptrFromInt(gpio_register_address); - raw_ptr.* = new_states; +const gpio: *volatile GpioRegister = @ptrFromInt(0x0123); + +pub fn writeToGpio(new_states: GpioRegister) void { + // Example of what not to do: + // BAD! gpio.GPIO0 = true; BAD! + + // Instead, do this: + gpio.* = new_states; } // syntax