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Add fmt_print and fmt_println to C API #4789

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1df8db8
Add FMT_FORMAT_ARGS(fmt, ...) macro to C API to simplify user-defined…
Ferdi265 Mar 20, 2026
9bae2b6
Add fmt_print and fmt_println to C API
Ferdi265 Mar 20, 2026
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24 changes: 19 additions & 5 deletions include/fmt/fmt-c.h
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@

#include <stdbool.h> // bool
#include <stddef.h> // size_t
#include <stdio.h> // FILE

#ifdef __cplusplus
extern "C" {
Expand Down Expand Up @@ -46,6 +47,10 @@ enum { fmt_error = -1, fmt_error_invalid_arg = -2 };

int fmt_vformat(char* buffer, size_t size, const char* fmt, const fmt_arg* args,
size_t num_args);
int fmt_vprint(FILE* stream, const char* fmt, const fmt_arg* args,
size_t num_args);
int fmt_vprintln(FILE* stream, const char* fmt, const fmt_arg* args,
size_t num_args);

#ifdef __cplusplus
}
Expand Down Expand Up @@ -181,11 +186,20 @@ typedef enum {} fmt_signed_char;
(fmt_arg[]) { FMT_MAP(FMT_MAKE_ARG, ##__VA_ARGS__) }
# define FMT_EXPAND(v) v

# define fmt_format(buffer, size, fmt, ...) \
fmt_vformat((buffer), (size), (fmt), \
FMT_EXPAND(FMT_VA_SELECT(FMT_MAKE_NULL, FMT_MAKE_ARGLIST, \
##__VA_ARGS__)(__VA_ARGS__)), \
FMT_NARG(, ##__VA_ARGS__))
# define FMT_FORMAT_ARGS(fmt, ...) \
(fmt), \
FMT_EXPAND(FMT_VA_SELECT(FMT_MAKE_NULL, FMT_MAKE_ARGLIST, \
##__VA_ARGS__)(__VA_ARGS__)), \
FMT_NARG(, ##__VA_ARGS__)

# define fmt_format(buffer, size, fmt, ...) \
fmt_vformat((buffer), (size), FMT_FORMAT_ARGS((fmt), ##__VA_ARGS__))

# define fmt_print(stream, fmt, ...) \
fmt_vprint((stream), FMT_FORMAT_ARGS((fmt), ##__VA_ARGS__))

# define fmt_println(stream, fmt, ...) \
fmt_vprintln((stream), FMT_FORMAT_ARGS((fmt), ##__VA_ARGS__))

#endif // __cplusplus

Expand Down
49 changes: 41 additions & 8 deletions src/fmt-c.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -8,13 +8,14 @@
#include "fmt/fmt-c.h"

#include <fmt/base.h>
#include <utility>

extern "C" int fmt_vformat(char* buffer, size_t size, const char* fmt,
const fmt_arg* args, size_t num_args) {
constexpr size_t max_args = 16;
if (num_args > max_args) return fmt_error_invalid_arg;
constexpr size_t max_c_format_args = 16;
static int parse_c_format_args(
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@vitaut vitaut Jun 1, 2026

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It's not parsing, maybe rename to convert?

fmt::basic_format_arg<fmt::format_context>* format_args,
const fmt_arg* args, size_t num_args) {
if (num_args > max_c_format_args) return fmt_error_invalid_arg;

fmt::basic_format_arg<fmt::format_context> format_args[max_args];
for (size_t i = 0; i < num_args; ++i) {
switch (args[i].type) {
case fmt_int: format_args[i] = args[i].value.int_value; break;
Expand All @@ -31,12 +32,44 @@ extern "C" int fmt_vformat(char* buffer, size_t size, const char* fmt,
default: return fmt_error_invalid_arg;
}
}
return 0;
}

template <typename F>
static int fmt_c_wrapper(const char* fmt, const fmt_arg* args, size_t num_args,
F&& f) {
Comment on lines +39 to +40
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@vitaut vitaut Jun 1, 2026

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I don't think this is needed.

fmt::basic_format_arg<fmt::format_context> format_args[max_c_format_args];
int error = parse_c_format_args(format_args, args, num_args);
if (error != 0) return error;

FMT_TRY {
auto result = fmt::vformat_to_n(
buffer, size, fmt,
return std::forward<F>(f)(
fmt::format_args(format_args, static_cast<int>(num_args)));
return static_cast<int>(result.size);
}
FMT_CATCH(...) {}
return fmt_error;
}

extern "C" int fmt_vformat(char* buffer, size_t size, const char* fmt,
const fmt_arg* args, size_t num_args) {
return fmt_c_wrapper(fmt, args, num_args, [=](fmt::format_args format_args) {
auto result = fmt::vformat_to_n(buffer, size, fmt, format_args);
return static_cast<int>(result.size);
});
}

extern "C" int fmt_vprint(FILE* stream, const char* fmt, const fmt_arg* args,
size_t num_args) {
return fmt_c_wrapper(fmt, args, num_args, [=](fmt::format_args format_args) {
fmt::vprint(stream, fmt, format_args);
return 0;
});
}

extern "C" int fmt_vprintln(FILE* stream, const char* fmt, const fmt_arg* args,
size_t num_args) {
return fmt_c_wrapper(fmt, args, num_args, [=](fmt::format_args format_args) {
fmt::vprintln(stream, fmt, format_args);
return 0;
});
}
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