C C++ POSIX 的一些 IO 操作
一些 C C++ POSIX 的 IO 操作总结
- 文件-内存之间
- 内存-内存之间
- POSIX 有无缓冲的 IO 操作
对文件的操作,读文件至内存,从内存写至文件
// 读文件至内存buf中
void Fread()
{
char buf[BUFSIZE];
size_t flen = 0;
FILE *fp = fopen("./foo", "r+");
flen = fread(buf, sizeof(char), BUFSIZE, fp); // fp -> buf
printf("read len: %lu\n", flen);
printf("Fread text:\n%s\n\n", buf);
fclose(fp);
}
// 将内存 buf 中的内容写入打开的文件 fp 中
void Fwrite()
{
char buf[BUFSIZE] = BUFTEXT;
size_t flen = 0;
FILE *fp = fopen("./foo", "w+");
flen = fwrite(buf, sizeof(char), strlen(buf), fp); // buf -> fp
printf("Fwrite write len: %lu\n\n", flen);
fclose(fp);
}
// 将fp 换为 stdin 输入至 buf 中
void Fscanf()
{
char buf[BUFSIZE];
size_t flen = 0;
FILE *fp = fopen("./foo", "r+");
while(fscanf(fp, "%s", buf) != EOF) // fp -> buf, terminated by blank char
printf("Fscanf text:\n%s\n", buf);
printf("\n");
fclose(fp);
}
// C++ 文件流,输入至内存中,由于流遇到空格等空白符停止所以需循环将流中的内容全部输入至
// 内存(string)中,直至返回 EOF
void Ifstream()
{
ifstream ifs("./foo");
string buf;
while (ifs >> buf) // ifs -> buf
cout <<"Ifstream: " << buf << endl;
// char *str = new char[100];
// ifs.read(str, 10);
// whlile (getline(ifs, buf))
// cout << "Ifstream: " << buf << endl;
ifs.close();
cout << endl;
}
// fp 换为 stdout, buf 中的内容写入至 fp 中
void Fprintf()
{
char buf[BUFSIZE] = "ioi ioi";
int flen = 0;
FILE *fp = fopen("./foo", "w+");
flen = fprintf(fp, "%s", buf); // buf -> fp
printf("Fprintf len: %d\n", flen);
fclose(fp);
}
void Ofstream()
{
ofstream ofs("./foo", ios::app | ios::out);
string buf = "quick,,time go on";
// ofs << buf;
ofs.write(buf.c_str(), buf.size());
ofs.close();
}
字符流的操作,内存至内存
// 文件流 stdio 换为 str, 格式化输入
void Sscanf()
{
char str[BUFSIZE] = BUFTEXT;
char buf[BUFSIZE];
sscanf(str, "%s", buf); // str -> buf
printf("Sscanf text:\n%s\n\n", buf);
}
// istringstream 对象 iss 绑定 buf,作为输入流输入至 word(string)
void Istringstream()
{
string buf = "hello world";
string word;
istringstream iss(buf); // buf -> iss
while (iss >> word) // iss -> word, repeatedly
cout << "Istringstream: " << word << endl;
}
// 与 sscanf() 类似, 格式化输出
void Sprintf()
{
char str[BUFSIZE] = BUFTEXT;
char buf[BUFSIZE];
sprintf(buf, "%s", str); // str -> buf
printf("Sprintf text:\n%s\n\n", buf);
}
// 与 istringstream 类似
void Ostringstream()
{
string buf = BUFTEXT;
string word;
ostringstream oss;
oss << buf; // buf -> oss
word = oss.str(); // oss -> word
cout << "Ostringstream: " << word << endl;
}
Linux 有无缓冲的 IO操作 (参考 UNP)
// 无缓冲, 普通的 read() write() 并不难保证完成所需要传送的字符数
ssize_t rio_readn(int fd, void *usrbuf, size_t n)
{
size_t nleft = n;
ssize_t nread;
char *bufp = usrbuf;
while (nleft > 0) {
if ((nread = read(fd, bufp, nleft)) < 0) {
if (errno == EINTR) // interrupted by sig handler return
nread = 0; // call read() again
else
return -1; // errno set by read()
} else if (nread == 0)
break; // EOF
nleft -= nread;
bufp += nread;
}
return (n - nleft);
}
// not buff read
ssize_t rio_writen(int fd, void *usrbuf, size_t n)
{
size_t nleft = n;
ssize_t nwritten;
char *bufp = usrbuf;
while(nleft > 0) {
if ((nwritten = write(fd, bufp, nleft)) <= 0) {
if (errno == EINTR)
nwritten = n;
else
return -1;
}
nleft -= nwritten;
bufp += nwritten;
}
return n;
}
// IO 缓冲区结构体
typedef struct {
int rio_fd;
int rio_cnt;
char *rio_bufptr;
char rio_buf[RIO_BUFSIZE];
} rio_t;
// 与 read() 语义相同
// 如果缓冲区为空会通过调用 read() 填满它
static ssize_t rio_read(rio_t *rp, char *usrbuf, size_t n)
{
int cnt;
while (rp->rio_cnt <= 0) { // refill if buf is empty
rp->rio_cnt = read(rp->rio_fd, rp->rio_buf, sizeof(rp->rio_buf));
if (rp->rio_cnt < 0) {
if (errno != EINTR)
return -1;
} else if (rp->rio_cnt == 0) { // EOF
return 0;
} else {
rp->rio_bufptr = rp->rio_buf; // reset buffer ptr
}
}
// copy min(n, rp->cnt) bytes from internal buf to user buf
cnt = n;
if (rp->rio_cnt < n)
cnt = rp->rio_cnt;
memmove(usrbuf, rp->rio_bufptr, cnt);
rp->rio_bufptr += cnt;
rp->rio_cnt -= cnt;
return cnt;
}
一点小结
- 本质是操作对象的变化
// C
printf() -- stdout scanf() -- stdin
fprintf() -- FILE* fscanf() -- FILE*
sprintf() -- char* sscanf() -- str
// C++ 与上类似, 都具有缓冲区
ostream istream
ofstream ifstream
ostringstream istringstream
- 带有缓冲区的 rio_read() 有时效率很高,因为不必一直在内核-用户态之间切换