文件输入/输出

if(fopen==NULL){
    ...
    exit(EXIT_FAILURE);
}

//reducto.c 把文件压缩成原来的1/3
# define _CRT_SECURE_NO_WARNINGS
# include<stdio.h>
# include<stdlib.h> //提供exi()原型
# include<string.h> //提供strcpy()、strcat()原型
# define LEN 40
int main(int argc, char * argv[])
{
	FILE * in, *out; //声明两个指向FILE的指针
	int ch;
	char name[LEN]; //存储输出文件名
	int count = 0;

	if (argc < 2)//检查命令行参数
	{
		fprintf(stderr, "Usage: %s filename\n", argv[0]);
		exit(EXIT_FAILURE); //表明程序失败退出
	}
	//设置输入，设置mode为可读
	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");
	if ((out = fopen(name, "w")) == NULL) //设置可写方式打开file
	{
		fprintf(stderr, "Can't create output file.\n");
		exit(3);
	}
	//拷贝数据
	while ( (ch=getc(in))!=EOF) //通过使用EOF来告知程序已经读取到文件结尾，从而结束程序。
	{
		if (count++ % 3 == 0)
			putc(ch, out);
	}
	if (fclose(in) != 0 || fclose(out) != 0)
		fprintf(stderr, "Error in closing files.\n");

	return 0;
}

/*addaword.c --fprintf() fscanf() rewind()*/
# define _CRT_SECURE_NO_WARNINGS
# include<stdio.h>
# include<stdlib.h>
# include<string.h>
# define MAX 41

int main(void)
{
	FILE *fp;
	char words[MAX];
	if ((fp = fopen("wordy", "a+")) == NULL) //更新(读写)模式打开文件，只允许在文件末尾添加内容
	{
		fprintf(stdout, "Can't open \"wordy\" file.\n");
		exit(EXIT_FAILURE);
	}
	puts("Enter words to add to the file; press the #");
	puts("key at the beginning of a line to terminate.");
	while ((fscanf(stdin, "%40s", words) == 1) && (words[0] != '#'))
		fprintf(fp, "%s\n", words);
	puts("File contents:");
    /** 关于 rewind() 函数的介绍说明
     *
     * rewind()函数 在头文件 stdio.h 中
     * 用于将文件指针重新指向文件的开头，同时清除和文件流相关的错误和EOF标记。
     * 相当于调用 fseek()函数
     * 函数原型：void rewind(FILE *stream);
     *
     */
	rewind(fp);//回到文件开始处。
	while (fscanf(fp, "%s", words) == 1)
		puts(words);
	puts("Done!");
	if (fclose(fp) != 0)
		fprintf(stderr, "Error closing file\n");

	return 0;
}

fseek(fp,0L,SEEK_SET); //定位至文件开始处
fseek(fp,10L,SEEK_SET); //定位至文件中的第10个字节
fseek(fp,2L,SEEK_CUR); //从文件当前位置前移2个字节
fseek(fp,0L,SEEK_END); // 定位至文件结尾
fseek(fp,-10L,SEEK_END); // 从文件结尾处回退10个

# include<stdio.h>

int main()
{
	FILE *stream;
	int len;

	stream = fopen("file.txt","r"); // 只读方式打开
	if(stream == NULL)
	{
		perror("打开文件错误");
		return(-1);
	}
	fseek(stream,0,SEEK_END);
	len = ftell(stream); //返回类型为long，把file.txt文件开始处到文件结尾的字节数赋给len
	fclose(stream);

	printf("file.txt的总大小为 = %d 字节\n",len);

	return(0);
}

www.github.com/solerho

[root@centos8 examples]# gcc ftell.c
[root@centos8 examples]# ls
a.out  file.txt  ftell.c
[root@centos8 examples]# ./a.out
file.txt的总大小为 = 23 字节

int fgetpos( FILE *stream, fpos_t *position );

int fgetpos(FILE * restrict stream,fpos_t * restrict pos);

int fsetpos(FILE *stream,const fpost_t *pos);

size_t fwrite(const void * restrict ptr,size_t size,size_t nmemb,FILE * restrict stream);

size_t fread(void * restrict ptr,,size_t size,size_t nmemb,FILE * restrict stream);

/* append.c -- 把文件附加到另一个文件末尾 */
# 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(void)
{
   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);
   }
   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);
   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);
}

char * s_gets(char * st, int n)
{
   char * ret_val;
   char * find;
   ret_val = fgets(st, n, stdin);
   if (ret_val)
   {
     find = strchr(st, '\n');   // 查找换行符
     if (find)          // 如果地址不是NULL，
        *find = '\0';      // 在此处放置一个空字符
     else
        while (getchar() != '\n')
          continue;
   }
   return ret_val;
}

/* randbin.c -- 用二进制I/O进行随机访问 */
# include <stdio.h>
# include <stdlib.h>
# define ARSIZE 1000
int main()
{
   double numbers[ARSIZE];
   double value;
   const char * file = "numbers.dat";
   int i;
   long pos;
   FILE *iofile;
   // 创建一组 double类型的值
   for (i = 0; i < ARSIZE; i++)
     numbers[i] = 100.0 * i + 1.0 / (i + 1);
   // 尝试打开文件
   if ((iofile = fopen(file, "wb")) == NULL)
   {
     fprintf(stderr, "Could not open %s for output.\n", file);
     exit(EXIT_FAILURE);
   }
   // 以二进制格式把数组写入文件
   fwrite(numbers, sizeof(double), ARSIZE, iofile);
   fclose(iofile);
   if ((iofile = fopen(file, "rb")) == NULL)
   {
     fprintf(stderr,
        "Could not open %s for random access.\n", file);
     exit(EXIT_FAILURE);
   }
   // 从文件中读取选定的内容
   printf("Enter an index in the range 0-%d.\n", ARSIZE - 1);
   while (scanf("%d", &i) == 1 && i >= 0 && i < ARSIZE)
   {
     pos = (long) i * sizeof(double);  // 计算偏移量
     fseek(iofile, pos, SEEK_SET);    // 定位到此处
     fread(&value, sizeof(double), 1, iofile); //读取该位置上的数据值
     printf("The value there is %f.\n", value);
     printf("Next index (out of range to quit):\n");
   }
   // 完成
   fclose(iofile);
   puts("Bye!");
   return 0;
}