Variadic functions - cppreference.com
Variadic functions
Variadic functions are functions (e.g. printf) which take a variable number of arguments.
The declaration of a variadic function uses an ellipsis as the last parameter, e.g.
int printf(const char* format, ...);. See variadic arguments for additional detail on the syntax and automatic argument conversions.
Accessing the variadic arguments from the function body uses the following library facilities:
Types
va_list: holds the information needed by va_start, va_arg, va_end, and va_copy (typedef)
Macros
Defined in header <stdarg.h>
va_start:
enables access to variadic function arguments
(function macro)
va_arg:
accesses the next variadic function argument
(function macro)
va_copy:
(C99)
makes a copy of the variadic function arguments
(function macro)
va_end:
ends traversal of the variadic function arguments
(function macro)
Example
Run online compiler: Coliru
Print values of different types.
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| #include <stdarg.h> #include <stdio.h> void simple_printf(const char* fmt, ...) { va_list args; for (va_start(args, fmt); *fmt != '\0'; ++fmt) { switch(*fmt) { case 'd': { int i = va_arg(args, int); printf("%d\n", i); break; } case 'c': { // A 'char' variable will be promoted to 'int' // A character literal in C is already 'int' by itself int c = va_arg(args, int); printf("%c\n", c); break; } case 'f': { double d = va_arg(args, double); printf("%f\n", d); break; } default: puts("Unknown formatter!"); goto END; } } END: va_end(args); } int main(void) { simple_printf("dcff", 3, 'a', 1.969, 42.5); } |
Output:
3
a
1.969000
42.50000
Variadic arguments - cppreference.com
Variadic functions are functions that may be called with different number of arguments.
Only prototyped function declarations may be variadic. This is indicated by the parameter of the form ... which must appear last in the parameter list and must follow at least one named parameter(until C23). The ellipsis parameter and the proceeding parameter must be delimited by ,.
// Prototyped declaration
int printx(const char* fmt, ...); // function declared this way
printx("hello world"); // may be called with one
printx("a=%d b=%d", a, b); // or more arguments
int printz(...); // OK since C23 and in C++
// Error until C23: ... must follow at least one named parameter
// int printy(..., const char* fmt); // Error: ... must be the last
// int printa(const char* fmt...); // Error in C: ',' is required; OK in C++
At the function call, each argument that is a part of the variable argument list undergoes special implicit conversions known as default argument promotions.
Within the body of a function that uses variadic arguments, the values of these arguments may be accessed using the <stdarg.h> library facilities.
Notes
Although old-style (prototype-less) function declarations allow the subsequent function calls to use any number of arguments, they are not allowed to be variadic (as of C89). The definition of such function must specify a fixed number of parameters and cannot use the stdarg.h macros.
// old-style declaration, removed in C23
int printx(); // function declared this way
printx("hello world"); // may be called with one
printx("a=%d b=%d", a, b); // or more arguments
// the behavior of at least one of these calls is undefined, depending on
// the number of parameters the function is defined to take
Example
#include <stdio.h>
#include <time.h>
#include <stdarg.h>
void tlog(const char* fmt,...)
{
char msg50;
strftime(msg, sizeof msg, "%T", localtime(&(time_t){time(NULL)}));
printf("%s ", msg);
va_list args;
va_start(args, fmt);
vprintf(fmt, args);
va_end(args);
}
int main(void)
{
tlog("logging %d %d %d...\n", 1, 2, 3);
}
Output:
10:21:38 logging 1 2 3...
Defined in header <stdio.h>
(1)
int vprintf( const char* format, va_list vlist );
(until C99)
int vprintf( const char* restrict format, va_list vlist );
(since C99)
(2)
int vfprintf( FILE* stream, const char* format, va_list vlist );
(until C99)
int vfprintf( FILE* restrict stream, const char* restrict format, va_list vlist );
(since C99)
(3)
int vsprintf( char* buffer, const char* format, va_list vlist );
(until C99)
int vsprintf( char* restrict buffer, const char* restrict format, va_list vlist );
(since C99)
(4) (since C99)
int vsnprintf( char* restrict buffer, size_t bufsz, const char* restrict format, va_list vlist );
(5) (since C11)
int vprintf_s( const char* restrict format, va_list vlist );
(6) (since C11)
int vfprintf_s( FILE* restrict stream, const char* restrict format, va_list vlist );
(7) (since C11)
int vsprintf_s( char* restrict buffer, rsize_t bufsz, const char* restrict format, va_list vlist );
(8) (since C11)
int vsnprintf_s( char* restrict buffer, rsize_t bufsz, const char* restrict format, va_list vlist );
Loads the data from the locations, defined by vlist, converts them to character string equivalents and writes the results to a variety of sinks.
-
Writes the results to stdout.
-
Writes the results to a file stream stream.
-
Writes the results to a character string buffer.
-
Writes the results to a character string buffer. At most bufsz - 1 characters are written. The resulting character string will be terminated with a null character, unless bufsz is zero. If bufsz is zero, nothing is written and buffer may be a null pointer, however the return value (number of bytes that would be written not including the null terminator) is still calculated and returned.
Parameters
stream - output file stream to write to
buffer - pointer to a character string to write to
bufsz - up to bufsz - 1 characters may be written, plus the null terminator
format - pointer to a null-terminated character string specifying how to interpret the data
vlist - variable argument list containing the data to print.
The format string consists of ordinary byte characters (except %), which are copied unchanged into the output stream, and conversion specifications.
Return value
1-3) The number of characters written if successful or negative value if an error occurred.
- The number of characters written if successful or negative value if an error occurred. If the resulting string gets truncated due to buf_size limit, function returns the total number of chara
Example
#include <stdarg.h>
#include <stdio.h>
#include <time.h>
void debug_log(const char* fmt, ...)
{
struct timespec ts;
timespec_get(&ts, TIME_UTC);
char time_buf100;
size_t rc = strftime(time_buf, sizeof time_buf, "%D %T", gmtime(&ts.tv_sec));
snprintf(time_buf + rc, sizeof time_buf - rc, ".%06ld UTC", ts.tv_nsec / 1000);
va_list args1;
va_start(args1, fmt);
va_list args2;
va_copy(args2, args1);
char buf1+vsnprintf(NULL, 0, fmt, args1);
va_end(args1);
vsnprintf(buf, sizeof buf, fmt, args2);
va_end(args2);
printf("%s debug: %s\n", time_buf, buf);
}
int main(void)
{
debug_log("Logging, %d, %d, %d", 1, 2, 3);
}
Possible output:
02/20/15 21:58:09.072683 UTC debug: Logging, 1, 2, 3