示例：将多个文件中的内容拷贝到目标文件中

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

#define BUFSIZE 4096
#define SLEN 81

void append(FILE *source, FILE *dest);
char * s_gets(char* st, int n);

int main(int argc, char* argv[])
{
	FILE *fa, *fs; //fa指向目标文件，fs指向源文件
	int files = 0; //附加的文件数量
	char file_app[SLEN]; //目标文件名
	char file_src[SLEN]; //源文件名
	int ch;

	puts("Enter name of destination file:");
	s_gets(file_app, SLEN);
	if ((fa = fopen(file_app, "a+")) == NULL)
	{
		fprintf(stderr, "Can't open %s\n", file_app);
		exit(EXIT_FAILURE);
	}
	//为fa重新分配缓冲区 _IOFBF:在缓冲区满时刷新
	if (setvbuf(fa, NULL, _IOFBF, BUFSIZE) != 0)
	{
		fputs("Can't create output buffer\n", stderr);
		exit(EXIT_FAILURE);
	}
	puts("Enter name of first source file (empty line to quit):");
	while (s_gets(file_src, SLEN) && file_src[0] != '\0')
	{
		if (strcmp(file_src, file_app) == 0)
			fputs("Can't append file to itself\n", stderr);
		else if ((fs = fopen(file_src, "r")) == NULL)
			fprintf(stderr, "Can't open %s\n", file_src);
		else
		{
			if (setvbuf(fs, NULL, _IOFBF, BUFSIZE) != 0)
			{
				fputs("Can't create input buffer\n", stderr);
				continue;
			}
			append(fs, fa);
			if (ferror(fs) != 0)
				fprintf(stderr, "Error in reading file %s.\n", file_src);
			if (ferror(fa) != 0)
				fprintf(stderr, "Error in writing file %s.\n", file_app);
			fclose(fs);
			files++;
			printf("File %s appended.\n", file_src);
			puts("Next file (empty line to quit):");
		}
	}
	printf("Done appending. %d files appended.\n", files);
	rewind(fa);
	printf("%s contents:\n", file_app);
	while ((ch = getc(fa)) != EOF)
		putchar(ch);
	puts("Done displaying.");
	fclose(fa);

	system("pause");
	return 0;
}

void append(FILE *source, FILE *dest)
{
	size_t bytes;
	static char temp[BUFSIZE]; //只分配一次
	while ((bytes = fread(temp, sizeof(char), BUFSIZE, source)) > 0)
		fwrite(temp, sizeof(char), bytes, dest);
}

// 自己实现读取函数
char* s_gets(char* st, int n)
{
	char* ret_val;
	int i = 0;
	ret_val = fgets(st, n, stdin);
	if (ret_val) //即，ret_val != NULL
	{
		while (st[i] != '\n' && st[i] != '\0')
			i++;
		if (st[i] == '\n')
			st[i] = '\0';
		else
			while (getchar() != '\n')
				continue;
	}
	return ret_val;
}

运行测试