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To call a C variadic function from Rust, declare its exact C signature in an extern block and call it inside unsafe only when the fixed parameters and every variadic argument satisfy the C API’s contract. To define a variadic function in Rust for C callers, use an unsafe extern "C" or extern "C-unwind" function, then read its call-scoped VaList only when you know the next argument exists and its type is compatible. Neither the ABI nor Rust’s unsafe syntax checks the argument count or types for you.
Calling a C variadic function from Rust
A C variadic function has fixed parameters followed by .... Your Rust declaration must match the C header’s fixed parameters, return type, calling convention, and symbol. Put the ellipsis last, and do not treat the declaration as a substitute for checking the platform API.
use core::ffi::{c_char, c_int};
unsafe extern "C" {
unsafe fn printf(format: *const c_char, ...) -> c_int;
}In Rust 2024, extern blocks must be marked unsafe; writing unsafe extern explicitly also makes the responsibility clear in earlier editions. An individual foreign function declaration may be marked unsafe as shown. The declaration is illustrative: use the prototype and linked symbol from the target platform’s headers and bindings. Rust’s extern-block rules describe the syntax and edition requirement.
Call the function in an unsafe block only after verifying the format and values meet the function’s contract:
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// SAFETY: the format is NUL-terminated and %d matches the c_int argument.
unsafe {
printf(c"value=%dn".as_ptr(), 42 as c_int);
}The c"..." C string literal syntax depends on the Rust version. On versions without it, construct a NUL-terminated string using an appropriate method and pass a pointer valid for the duration of the call. A function such as printf requires its fixed format argument; calling it with no arguments is not made valid by its variadic signature.
What the caller must verify
- The Rust declaration uses the C API’s actual ABI, fixed parameter types, return type, and symbol for the target.
- Every required argument is present, and the variadic argument sequence matches the API’s count, format, sentinel, or other convention.
- Each supplied value has the ABI type the callee expects after C’s default argument promotions. Values narrower than
c_intare promoted toc_int; floating-point values narrower thanc_doubleare promoted toc_double. - Pointers and any referenced data remain valid for the call, and comply with the API’s mutability and lifetime requirements.
C’s ellipsis does not carry runtime type metadata that checks a format string or argument count. A missing argument or a mismatch between the callee’s interpretation and the passed ABI type can cause undefined behavior or make the call unsound. An unsafe block records that the programmer is taking responsibility for these conditions; it does not validate the declaration or inspect the values.
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Defining a C variadic function in Rust
Rust can define a function with a C variadic tail on supported target architectures. The function must be unsafe, use extern "C" or extern "C-unwind", and put ... last. In the definition, the named variadic parameter is treated as a VaList:
use core::ffi::{c_int, VaList};
/// # Safety
/// The caller must pass exactly `count` promoted c_int arguments.
unsafe extern "C" fn sum(count: c_int, mut args: ...) -> c_int {
read_sum(count, args.as_va_list())
}
unsafe fn read_sum(count: c_int, mut args: VaList<'_>) -> c_int {
let mut total = 0;
for _ in 0..count {
// SAFETY: the caller contract guarantees count c_int arguments remain.
total += unsafe { args.next_arg::<c_int>() };
}
total
}This example’s contract is deliberately narrow: the caller must provide a nonnegative count that accurately describes the number of trailing arguments, and each trailing value must be passed as a promoted c_int. If the count is negative or larger than the actual number of arguments, the reads are invalid. A production API should document the expected count, promoted ABI types, and any sentinel or stopping rule that determines when reading ends.
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The Rust Reference specifies variadic definitions and target support. Do not assume a target is supported unless it is listed there, and confirm support for the project’s Rust version and target before relying on a definition. For the list-reading API and its lifetime rules, see the [standard library documentation for VaList](https://doc.rust-lang.org/std/ffi/struct.VaList.html).
Reading the argument list safely
VaList::next_arg::<T>() is unsafe because the callee must know that another argument exists and that its actual type is compatible with T. Documented compatibility includes the same type, integer types of the same size when the value is representable in both types, and compatible pointer types. Reading past the provided arguments or requesting an incompatible type is not rescued by a format string or the Rust type parameter.
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Account for C’s default argument promotions when selecting T: a narrow integer argument arrives promoted to c_int, and a narrower floating-point argument arrives promoted to c_double. Read the type actually passed, not merely the source-level type the caller started with.
Keep VaList within the call
A VaList is tied to the variadic call’s lifetime. Do not store it for later use or return it beyond that scope. If a helper needs to traverse an independent copy, use the list’s clone operation, which corresponds to C’s va_copy. Dropping the list corresponds to va_end; use Rust’s provided API rather than inventing a Rust representation of C’s va_list.
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ABI, edition, and target checks
- Match the ABI: use an explicit C ABI for a C interface. Choose
"C-unwind"only when unwinding across the boundary is part of the intended contract; otherwise use the API’s documented ABI. - Check the Rust edition: Rust 2024 requires
unsafeon extern blocks. Keep declarations consistent with the edition and compiler version used by the project. - Check target support: Rust variadic definitions are supported only on documented target architectures. Verify the Rust Reference for the version and target you ship; support for one target does not establish support for another.
- Review the whole boundary: confirm the linked C declaration, Rust declaration or definition, caller’s promoted argument types, and callee’s reading rules all agree.
Rust’s ABI documentation describes the available ABI strings. These rules concern a low-level interface: if the count, types, or ABI cannot be stated and maintained reliably, prefer a small fixed-arity C wrapper over exposing an unconstrained variadic boundary.
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