使用C语言标准I/O库进行文件读写的实践

1) Line-I/O via fgets and fputs

小试牛刀

源文件 main.c

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

#define BUFFER_SIZE 1024
char *lineBuffer = NULL;

void buildLineBuffer() {
    if (lineBuffer == NULL) {
        lineBuffer = (char *)malloc(BUFFER_SIZE);
    }
}

void dropLineBuffer() {
    if (lineBuffer != NULL) {
        free(lineBuffer);
    }
}

void checkRightAfterFopen(FILE *fp) {
    if (fp == NULL) {
        fputs("Failed to open file.
", stderr);
        exit(1);
    }
}

int writeContents2File(const char *contents, const char *fileName) {
    FILE *fp = fopen(fileName, "w");
    checkRightAfterFopen(fp);
    fputs(contents, fp);
    fclose(fp);

    return 0;
}

char *readLineFromFile(const char *fileName) {
    FILE *fp = fopen(fileName, "r");
    checkRightAfterFopen(fp);
    buildLineBuffer();
    fgets(lineBuffer, BUFFER_SIZE, fp);
    fclose(fp);
    return lineBuffer;
}

void showAllContentsInFile(const char *fileName) {
    FILE *fp = fopen(fileName, "r");
    checkRightAfterFopen(fp);
    buildLineBuffer();
    while (fgets(lineBuffer, BUFFER_SIZE, fp) != NULL) {
        printf(lineBuffer);
    }
    fclose(fp);
}

int appendContentsFromConsole2File(const char *fileName) {
    FILE *fp = fopen(fileName, "a");
    checkRightAfterFopen(fp);
    buildLineBuffer();
    while (fgets(lineBuffer, BUFFER_SIZE, stdin) != NULL && fputs(lineBuffer, fp) != EOF);
    fclose(fp);

    return 0;
}

/*
**由目录名和文件名取得文件的全限定名
**---------------------------------
**建议设定用户环境变量"C_DEV_HOME"的值为该目录的父目录全限定名
**若当前系统环境中尚未设置该环境变量，
**则给出提醒并默认采用当前目录作为该目录的父目录全限定名
*/
char *getFullyQualifedFileName(const char *dirName, const char *fileName) {
    /*访问用户环境变量以取得根目录的全限定名*/
    char *homedir = getenv("C_DEV_HOME");
    if (homedir == NULL) {
        printf("Info: User/System Environment Varable "C_DEV_HOME" has NOT been set yet!
");
        homedir = ".";
    }

    /*创建缓存区，用于存放文件的全限定名*/
    char *fullyQualifedFileNameBuffer = (char *)malloc(strlen(homedir) + strlen(dirName) + strlen(fileName) + 3);
    *fullyQualifedFileNameBuffer = ''; // 初始化为空字符串

    /*设定路径分隔符*/
    char *pathSeparator = "/";
#ifdef _WIN32
    pathSeparator = "\";
#endif

    /*调用系统函数来创建目标目录*/
    char
        *cmd_prefix = "mkdir ",
        *fullyQualifedDirName = strcat(strcat(strcat(fullyQualifedFileNameBuffer, homedir), pathSeparator), dirName),
        *mkdir_CommandBuffer = (char *)malloc(strlen(fullyQualifedDirName) + strlen(cmd_prefix) + 1);
    *mkdir_CommandBuffer = '';
    system(strcat(strcat(mkdir_CommandBuffer, cmd_prefix), fullyQualifedDirName));
    free(mkdir_CommandBuffer);

    /*返回 由目标目录的全限定名和文件名组合成文件的全限定名*/
    return strcat(strcat(fullyQualifedDirName, pathSeparator), fileName);
}

int main(void) {
    char
        *filePath = getFullyQualifedFileName("FileIO", "demo"),
        *contents = "Hello, World! Here is C I/O demo.
";
    printf("##FilePath: %s
", filePath);

    /*建立缓存容器*/
    buildLineBuffer();

    /*Write*/
    writeContents2File(contents, filePath);

    /*Read*/
    printf(readLineFromFile(filePath));

    /*Append*/
    printf("You can Append More Contents Here:
");
    appendContentsFromConsole2File(filePath);

    /*显示文件的全部内容*/
    showAllContentsInFile(filePath);

    /*销毁缓存容器*/
    dropLineBuffer();

    return 0;
}

想要在Visual Studio中正常使用C语言的标准I/O库函数，而不使用Visual Studio自己的xxx_s安全函数，需要在 项目 > 属性 > C/C++ > 预处理器 > 预处理器定义 中
添加 _CRT_SECURE_NO_WARNINGS

跑一波

On Windows 10

打开磁盘上的文件瞅瞅

 

On CentOS 7

2) Formatted-I/O via fscanf and fprintf

以下内容待续

3) Binary-I/O via fread and fwrite

二进制I/O可以直接对内存中的数据对象(变量、数组、结构体，等等)进行写出或读入。
下面以数组对象的二进制I/O为例：
源文件 main.c

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

int main(void) {
    /*定义输出数据缓冲区、输入数据缓冲区、循环步进变量*/
    int outBuf[20], inBuf[20], i;

    /*对outBuf填入特定的值*/
    for (i = 0; i < 20; ++i) {
        outBuf[i] = (i + 1)*(i + 2);
    }

    /*创建二进制输出的目标目录*/
    system("mkdir .\BinaryIO-Temp"); // 在Linux系统上编译前请把该行改为：system("mkdir -p ./BinaryIO-Temp");

    /*以二进制写的方式打开要输出到的目标文件*/
    FILE *fp = fopen(".\BinaryIO-Temp\a", "wb+"); // 在Linux系统上编译前，请把这里的路径分隔符"\"都替换为"/"
    if (fp == NULL) {
        printf("Failed when trying to open file.
");
        exit(EXIT_FAILURE);
    }

    /*执行二进制写操作，把outBuf中的数据写入目标文件中*/
    if (fwrite(outBuf, sizeof(int), 20, fp) != 20) {
        printf("Failed when doing binary-writing.
");
        exit(EXIT_FAILURE);
    } else {
        printf("Succeeded in doing binary-writing.
");
    }

    /*将文件读写位置标记重置到文件开头*/
    rewind(fp);

    /*执行二进制读操作，把目标文件中的数据写入inBuf中*/
    if (fread(inBuf, sizeof(int), 20, fp) != 20) {
        printf("Failed when doing binary-reading.
");
        exit(EXIT_FAILURE);
    } else {
        printf("Succeeded in doing binary-reading.
");
    }

    /*结束I/O操作后，关闭文件*/
    fclose(fp);

    /*显示outBuf中的数据*/
    printf("Data of outBuf:
");
    for (i = 0; i < 20; ++i) {
        printf("%d ", outBuf[i]);
    }
    printf("
");

    /*显示inBuf中的数据*/
    printf("Data of inBuf:
");
    for (i = 0; i < 20; ++i) {
        printf("%d ", inBuf[i]);
    }
    printf("
");

    return 0;
}

 跑一下

on Windows

on CentOS