示例：

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define LEN 40

int main(int argc, char* argv[])
{
	FILE* in, * out;
	int ch;
	char name[LEN];
	int count = 0;

	// 检查命令行参数
	if (argc < 2)
	{
		fprintf(stderr, "Usage: %s filename\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	// 设置输入
	// 以读模式打开文件
	if ((in = fopen(argv[1], "r")) == NULL)
	{
		fprintf(stderr, "I couldn't open the file \"%s\"\n", argv[1]);
		exit(EXIT_FAILURE);
	}

	// 设置输出
	strncpy(name, argv[1], LEN - 5); // 拷贝文件名
	name[LEN - 5] = '\0';
	strcat(name, ".red"); //在文件名后添加.red
	// 以写模式打开文件
	if ((out = fopen(name, "w")) == NULL)
	{
		fprintf(stderr, "Can't create output file.\n");
		exit(3);
	}
	// 拷贝数据
	while ((ch = getc(in)) != EOF)
	{
		if (count++ % 3 == 0)
			putc(ch, out); //打印3个字符中的和1个字符
	}

	// 收尾工作
	if (fclose(in) != 0 || fclose(out) != 0)
		fprintf(stderr, "Error in closing files\n");

	system("pause");
	return 0;
}

运行测试