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The file access method in C language can only be sequential access?

angryTom

Oct 28, 2019 am 09:29 AM

cdocument

c语言的文件存取方式只能是顺序存取？

c语言的文件存取方式包括顺序存取方式和随机存取（需要使用fseek等函数帮助）方式两种。

顺序读取

顺序读取也就是从上往下，一笔一笔读取文件的内容。保存数据时，将数据附加在文件的末尾。这种存取方式常用于文本文件，而被存取的文件则称为顺序文件。

随机存取

随机存取方式多半以二进制文件为主。它会以一个完整的单位来进行数据的读取和写入，通常以结构为单位。

打开文件

函数原型为：_CRTIMP FILE * __cdecl fopen(const char *, const char *);

第一参数为文件名，第二个参数为打开模式。

打开成功，fopen返回一个结构指针地址，否则返回一个NULL。如果没有指定文件路径，则默认为当前工作目录。如：

FILE *fp;
fp = fopen("c:\\temp\\test.txt", "r") //由于反斜杠\是控制字符，所以必须再加一个反斜杠

使用fopen()函数打开的文件会先将文件复制到缓冲区。注意：所下达的读取或写入动作，都是针对缓冲区进行存取而不是磁盘，只有当使用fclose()函数关闭文件时，缓冲区中的数据才会写入磁盘。

文件打开模式

"r"：只能从文件中读数据，该文件必须先存在，否则打开失败
"w"：只能向文件写数据，若指定的文件不存在则创建它，如果存在则先删除它再重建一个新文件
"a"：向文件增加新数据(不删除原有数据)，若文件不存在则打开失败，打开时位置指针移到文件末尾
"r+"：可读/写数据，该文件必须先存在，否则打开失败
"w+"：可读/写数据，用该模式打开新建一个文件，先向该文件写数据，然后可读取该文件中的数据
"a+"：可读/写数据，原来的文件不被删去，位置指针移到文件末尾

打开二进制文件的模式与打开文本文件的含义是一样的，不同的是模式名称里面多一个字母'b’，以表示以二进制形式打开文件。

关闭文件

函数原型为：_CRTIMP int __cdecl fclose(FILE *);

关闭成功返回值0，否则返回非零值。

注：在执行完文件的操作后，要进行“关闭文件”操作。虽然程序在结束前会自动关闭所有的打开文件，但文件打开过多会导致系统运行缓慢，这时就要自行手动关闭不再使用的文件，来提高系统整体的执行效率。

例1. 打开文件并进行判断和关闭文件

FILE *fp;
fp = fopen("c:\\temp\\test.txt", "r");
if(fp == NULL)
    printf("fail to open the file! \n");
else
{
    printf("The file is open! \n");
    fclose(fp);
}

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