C使用va_list的函数可以多次使用它吗？

在下面的程序中,可变参数函数process_message通过va_list argp参数将其变量参数传递给print_message. argp又传递给vsnprintf,后者计算格式字符串的长度.

但是,如果将argp传递给另一个从print_message中调用的函数,例如vprintf(fmt,argp),它会产生无意义的输出.使用va_list参数的函数是否可以多次使用它？

#include <stdio.h>
#include <stdarg.h>

void process_message(const char *fmt, ...);
void print_message(const char *fmt, va_list argp);

int main(void) {
  process_message("%s:%d\n", "test message", 1);
  return 0;
}

void process_message(const char *fmt, ...) {
  va_list argp;
  va_start(argp, fmt);
  print_message(fmt, argp);
  va_end(argp);
}

void print_message(const char *fmt, va_list argp) {
  /*Calculate and print the length of the format string*/
  int len = vsnprintf(NULL, 0, fmt, argp);
  printf("Length of format string = %d\n", len);
  /*Print the rendered format string - this produces a nonsense output
   *if argp was aleady passed to another function earlier */
  vprintf(fmt, argp);
}

你可以使用 va_copy：

The va_copy() macro copies the (previously initialized) variable argument list src to dest. The behavior is as if va_start() were applied to dest with the same last argument, followed by the same number of va_arg() invocations that was used to reach the current state of src.

你print_message()可能是：

void print_message(const char *fmt, va_list argp) {
  /*Calculate and print the length of the format string*/ 
  va_list argp_copy;
  va_copy(argp_copy, argp);
  int len = vsnprintf(NULL, 0, fmt, argp_copy);
  va_end(argp_copy);

  printf("Length of format string = %d\n", len);
  /*Print the rendered format string - this produces a nonsense output
   *if argp was aleady passed to another function earlier */
  vprintf(fmt, argp);
}

注意：

使用复制的va_list后不要忘记调用va_end.

C11：

Each invocation of the va_start and va_copy macros shall be matched by a corresponding invocation of the va_end macro in the same function.