概要

本文将从文件IO的几个方面来总结操作文件的方式：

打开文件

读文件

写文件

关闭文件

各种用法demo

第一部分：system call

若文件存在则清空，不存在则以0644许可创建(owner可读可写，其他只可读)

int fd;
fd = open(file, O_WRONLY | O_CREAT | O_TRUNC, 0644);
if(-1 == fd){
  //error
}

读指定Bytes

unsigned long word;
ssize_t nr;
nr = read(fd, &word, sizeof(unsigned long));
if(-1 == nr){
  //error
}

读所有bytes

//fstat()获取文件长度(C library)
struct stat st;
if(fstat(fd, &st)){
  //error
}
size_t len = st.st_size;
//read all bytes
ssize_t ret;
while(len != 0 && (ret = read(fd, buf, len)) != 0){
  if(-1 == ret){
    if(EINTR == errno) continue;
    perror("read");
    break;
  }
  len -= ret;
  buf += ret;
}

写一些bytes

unsigned long word = 1720;
size_t count = 0;
ssize_t nr = 0;

count = sizeof(word);
nr = write(fd, &word, count);
if(-1 == nr){
  //error,and check errno
}
else if(nr != count){
  //only write part bytes
}

ssize_t ret = 0, nr = 0;
while(0 != len && (ret = write(fd, buf, len)) != 0){
  if(-1 == ret){
    if(EINTR == errno) continue;
    perror("write");
    break;
  }
  len -= ret;
  buf += ret;
}

第二部分：C语言

通过FILE*打开文件

FILE* stream = nullptr;
stream = fopen(path, "r");
if(!stream){
  //error
}

通过File Descriptor打开文件

FILE* stream = nullptr;
stream = fdopen(fd, "r");
if(!stream){
  //error
}

读写一个字节

//read
int c = 0;
c = fgetc(stream);
if(EOF == c){
  //error
}

//write
if(fputc(c, stream) == EOF){
  //error
}

读写一个字符串

//read
char buf[BUFFER_SIZE];
if(!fgets(buf, BUFFER_SIZE, stream)){
  //error
}
//write
if(fputs(buf, stream) == EOF){
  //error
}

读写binary data

//read
char buf[64];
size_t nr;
nr = fread(buf, sizeof(buf), 1, stream);
if(0 == nr){
  //error
}

//write
struct pirate{
  char name[100];
  unsigned long booty;
  unsigned int beard_len;
}blackbeard = {"JACK", 950, 48};

if(!fwrite(&blackbeard, sizeof(struct pirate), 1, stream)){
  //error
}

第三部分：C++

打开文件

//打开可读文件
ifstream in(path, ios::in);
if(!in){
  //error
}

//打开可写文件
ofstream out(path, ios::out);
if(!out){
  //error
}

读写文件

//读
//用相应数据类型以及>>组合使用
//学号 姓名 课程 成绩
int id, score;
string name, course;
while(in >> id >> name >> course >> score){
  //do
}

//写
//将vector<int>写入文件
vector<int> data{1,2,3,4};
for(int i = 0; i < data.size(); i++){
  out << data[i] << endl;
}

附录

参考资料：
c++方式可以参考<<c++ primer(第五版)>>
c/system call可以参考<<Linux System Programming,第二版>>

理解文件打开关系

参考link/文献/引用

fstat() : https://www.go4expert.com/articles/understanding-linux-fstat-example-t27449/
ifstream:http://www.cplusplus.com/reference/fstream/ifstream/
ofstream:http://www.cplusplus.com/reference/fstream/ofstream/