简介

C/C++中可以使用以fopen、fclose为代表的文件操作函数对文件进行读写。

注：本文在Linux平台进行演示。(Ubuntu 12.04 LTS + gcc version 4.6.3)

fopen/ fclose

在C中，对文件的操作套路都是先用fopen打开文件，做些读写的操作，然后关闭文件。

FILE *fopen(const char *path, const char *mode);

• path 指文件路径及文件名
• mode 打开模式，主要集中在r w a + b t这几种模式组合
• 返回值：失败则返回NULL，成功返回FILE指针

int fclose(FILE *fp);

• fp 为打开的文件指针
• 返回值成功返回0，失败返回EOF（多为-1）

b t 这两种模式，指二进制模式和文本模式，在Linux平台下并没有任何区别。在windows平台下对文本模式的换行符\n会处理为\r\n。

常用模式说明如下

mode	description
r	只读方式打开，文件必须存在否则失败
r+	读写方式打开，文件必须存在否则失败
rb+	读写方式打开二进制文件，文件必须存在否则失败
w	只写方式打开，文件不存在则新建，存在则清空文件内容
w+	读写方式打开，文件不存在则新建，存在则清空文件内容
a	只写方式打开，文件不存在则新建，存在则写入的内容追加到文件尾部
a+	读写方式打开，文件不存在则新建，存在则写入的内容追加到文件尾部

示例

我们通过打开当前目录下的data.txt文件，用argv[1]传递打开模式，测试r r+ rb+ 三种模式

#include <stdio.h>
#include <stdlib.h>

const char* TEST_FILE_PATH = "./data.txt";

int main(int argc, char **argv)
{
    if ((2 != argc) || (NULL == argv[1]))
    {
        printf("Please Run As: ./a.out r \n");
        return -1;
    }

    FILE *fp = fopen(TEST_FILE_PATH, (const char*)argv[1]);
    if(NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }

    fclose(fp);
    fp = NULL;

    return 0;
}

编译 g++ test_mode_r.cpp
当前目录下文件情况

yyl@Machine:/media/sf_share/lc++/io_file$ ls
a.out test_mode_r.cpp

分别用r r+ rb+ 方式打开，若不存在文件则都打开失败

yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out r
FileName[./data.txt] Mode[r] Open Failed
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out r+
FileName[./data.txt] Mode[r+] Open Failed
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out rb+
FileName[./data.txt] Mode[rb+] Open Failed
yyl@Machine:/media/sf_share/lc++/io_file$

用w模式打开，使其自动建立文件，再用r三种模式打开则可以成功

yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out w
yyl@Machine:/media/sf_share/lc++/io_file$ ls
a.out data.txt test_mode_r.cpp
yyl@Machine:/media/sf_share/lc++/io_file$ cat data.txt
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out r
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out r+
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out rb+

对w a模式的理解离不开对文件的读写，这里就先介绍下文件的读写和文件指针位置，然后接着介绍w 模式和 a模式

fread/fwrite

fread和fwrite是经典的读写函数，主要功能是将数据从文件读到buf或者从buf写到文件中，常用于二进制文件的读写当中。

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

• ptr buf首地址
• size 读取的每一个数据的大小
• nmemb 读取多少个数据项(总共读取的大小为 size * nmemb)
• stream 文件指针
• 返回值 成功返回真正读取/写入文件的数据项的个数(可以根据此判断读取或者写入的数据是否有效)

ftell/fseek

ftell和 fseek 用来获取和文件指针的位置，可以把文件内容看做一个字符串，我们用指针对其进行操作，fseek可以移动到字符串的头、尾等位置

int fseek(FILE *stream, long offset, int whence);
long ftell(FILE *stream);

• stream 文件指针
• offset whence，以whence为基准移动offset个字节的指针位置。基准位置有（SEEK_SET/SEEK_CUR/SEEK_END）分别代表文件首位置和文件尾位置
• 返回值，成功返回0，失败返回-1

下面修改测试代码后，我们继续测试w a两种模式打开文件

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

typedef unsigned char BYTE;

const BYTE  MAX_BUF_LEN    = 0xfe;
const char* TEST_FILE_PATH = "./data.txt";

int main(int argc, char **argv)
{
    if ((2 != argc) || (NULL == argv[1]))
    {
        printf("Please Run As: ./a.out r \n");
        return -1;
    }

    FILE *fp = fopen(TEST_FILE_PATH, (const char*)argv[1]);
    if(NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }
    printf("OpenMode[%s] ftell=%ld \n", argv[1], ftell(fp));

    char byBuf[MAX_BUF_LEN + 1] = {0};

    int iRet = snprintf(byBuf, MAX_BUF_LEN, "ABCDEFG%d",7);

    iRet = fwrite((const void*)byBuf, 1, iRet, fp);
    printf("fwrite iRet = %d, strlen byBuf = %d, byBuf = %s,ftell = %ld\n", iRet, strlen(byBuf), byBuf, ftell(fp));

    (void)memset(byBuf, 0, MAX_BUF_LEN + 1);
    // (void)fseek(fp, 0, SEEK_SET);
    // (void)fflush(fp);        // 注意这里。我重新rb打开文件，w/wb/wb+读不出来我没读出来。
    fclose(fp);
    fp = NULL;

    fp = fopen(TEST_FILE_PATH, "rb");
    if(NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }
    printf("OpenMode[%s] ftell=%ld \n", "rb", ftell(fp));
    
    iRet = fread((void*)byBuf, 1, MAX_BUF_LEN, fp);
    printf("fread  iRet = %d, strlen byBuf = %d, byBuf = %s,ftell = %ld\n", iRet, strlen(byBuf), byBuf, ftell(fp));

    fclose(fp);
    fp = NULL;

    return 0;
}

w w+ wb+模式，不建议使用w w+ wb+模式进行文件读取，在我机器上测试时发现直接用这三种模式没有读出来，也不知道是不是个例。

fread iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out w
OpenMode[w] ftell=0
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
OpenMode[rb] ftell=0
fread iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out wb
OpenMode[wb] ftell=0
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
OpenMode[rb] ftell=0
fread iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out wbwb+
OpenMode[wbwb+] ftell=0
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
OpenMode[rb] ftell=0

fread iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out wb+
OpenMode[wb+] ftell=0
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
OpenMode[rb] ftell=0
fread iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 8
yyl@Machine:/media/sf_share/lc++/io_file$

跟预期基本一致，只是C中并没有对模式进行严格校验，wbwb+ 这种也可以。

下面我们再看一下a a+ ab+ 这三种模式

yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out a
OpenMode[a] ftell=8
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 16
OpenMode[rb] ftell=0
fread iRet = 16, strlen byBuf = 16, byBuf = ABCDEFG7ABCDEFG7,ftell = 16
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out a+
OpenMode[a+] ftell=0
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 24
OpenMode[rb] ftell=0
fread iRet = 24, strlen byBuf = 24, byBuf = ABCDEFG7ABCDEFG7ABCDEFG7,ftell = 24
yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out ab+
OpenMode[ab+] ftell=0
fwrite iRet = 8, strlen byBuf = 8, byBuf = ABCDEFG7,ftell = 32
OpenMode[rb] ftell=0
fread iRet = 32, strlen byBuf = 32, byBuf = ABCDEFG7ABCDEFG7ABCDEFG7ABCDEFG7,ftell = 32
yyl@Machine:/media/sf_share/lc++/io_file$

可以看到三种模式均可以在尾部写文件，a模式打开后，文件指针直接在文件尾部，而a+ ab+文件指针则在文件首部，我没有使用fseek操作，写的数据也自动加到了文件尾部，而且fwrite后会自动修改文件指针。

那么这里如果用a模式打开是否可以通过fseek把文件指针移动到文件首部呢？

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

typedef unsigned char BYTE;

const BYTE  MAX_BUF_LEN    = 0xfe;
const char* TEST_FILE_PATH = "./data.txt";

int main(int argc, char **argv)
{
    if ((2 != argc) || (NULL == argv[1]))
    {
        printf("Please Run As: ./a.out r \n");
        return -1;
    }

    FILE *fp = fopen(TEST_FILE_PATH, (const char*)argv[1]);
    if(NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }
    printf("OpenMode[%s] ftell=%ld \n", argv[1], ftell(fp));

    int iRet = fseek(fp, 0, SEEK_SET);
    printf("fseek SEEK_SET iRet = %d,ftell=%ld \n", iRet, ftell(fp));

    fclose(fp);
    fp = NULL;

    return 0;
}

测试结果

yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out a
OpenMode[a] ftell=32
fseek SEEK_SET iRet = 0,ftell=0
yyl@Machine:/media/sf_share/lc++/io_file$

从结果中可以看出a模式下也可以通过fseek移动指针。

通过fopen/fclose、fread/fwrite、ftell/fseek，我们可以完成几乎所有的文件操作，下面再介绍下文件其他的IO函数。

fgetc/fputc

int fgetc(FILE *stream);

• stream 文件指针
• 返回值: 成功返回所得字符，失败返回EOF(-1)

int fputc(int c, FILE *stream);

• c 要写入文件的字符
• stream 文件指针
• 返回值: 成功返回写入的字符，失败返回EOF(-1)

同样写和读分开打开

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

typedef unsigned char BYTE;

const BYTE  MAX_BUF_LEN    = 0xfe;
const char* TEST_FILE_PATH = "./data.txt";

int main(int argc, char **argv)
{
    if ((2 != argc) || (NULL == argv[1]))
    {
        printf("Please Run As: ./a.out r \n");
        return -1;
    }

    FILE *fp = fopen(TEST_FILE_PATH, "rb");
    if (NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }

    char ch = 0;
    printf("fgetc:\n");
    while ((ch = fgetc(fp)) != EOF)
    {
        (void)putchar(ch);
    }
    printf("\n");
    fclose(fp);
    fp = NULL;

    fp = fopen(TEST_FILE_PATH, (const char*)argv[1]);
    if (NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }

    (void)fseek(fp, 0, SEEK_SET);
    const char *buf = "Fuck!";
    for (const char *p = buf; *p != '\0'; p++)
    {
        if (*p != fputc(*p, fp))
        {
            printf("fpuc failed ch[%c] \n", *p);
        }
    }
    fclose(fp);
    fp = NULL;

    fp = fopen(TEST_FILE_PATH, "rb");
    if (NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }

    ch = 0;
    printf("fgetc:\n");
    while ((ch = fgetc(fp)) != EOF)
    {
        (void)putchar(ch);
    }
    printf("\n");

    fclose(fp);
    fp = NULL;

    return 0;
}

yyl@Machine:/media/sf_share/lc++/io_file$ cat data.txt
Fuck!yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out w+
fgetc:
Fuck!
fgetc:
Fuck!
yyl@Machine:/media/sf_share/lc++/io_file$

fgets/fputs

char *fgets(char *s, int size, FILE *stream);

• s 字符串buf
• size 读取的大小，最多取size-1个字符，自动添加'\0'，遇到\n EOF会提前停止
• stream 文件指针

int fputs(const char *s, FILE *stream);

• fputs() writes the string s to stream, without its terminating null byte ('\0').

fscanf/fprintf

文件字符串输出格式化IO,类似scanf和printf。注意其返回值。

int fscanf(FILE *stream, const char *format, ...);

• 返回值: 成功则返回读到参数个数，有可能只成功部分参数，注意校验
  int fprintf(FILE *stream, const char *format, ...);
• 返回值: 成功返回输出的字符个数

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

typedef unsigned char BYTE;

const BYTE  MAX_BUF_LEN    = 0xfe;
const char* TEST_FILE_PATH = "./data.txt";

int main(int argc, char **argv)
{

    FILE *fp = fopen(TEST_FILE_PATH, "wb+");
    if (NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }

    int val_a = 23;
    int val_b = 18;
    int iRet = fprintf(fp, "%dDGDG%d", val_a, val_b);
    printf("fprintf iRet = %d\n", iRet);

    fclose(fp);
    fp = fopen(TEST_FILE_PATH, "rb+");
    if (NULL == fp)
    {
        printf("FileName[%s] Mode[%s] Open Failed \n", TEST_FILE_PATH, argv[1]);
        return -1;
    }

    val_a = 0;
    val_b = 0;
    iRet = fscanf(fp, "%dDGDG%d", &val_a, &val_b);
    printf("iRet = %d, a = %d, b = %d \n", iRet, val_a, val_b);

    return 0;
}

yyl@Machine:/media/sf_share/lc++/io_file$ ./a.out
fprintf iRet = 8
iRet = 2, a = 23, b = 18