1. 概述
read
NAME
read - read from a file descriptor
SYNOPSIS
#include <unistd.h>
//从fd中读，读到buf中去，读count个字节
ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspeci‐
fied.
RETURN VALUE
//如果成功，那么返回成功读取到的字节数；如果读到文件尾了，那么返回的是0个字节
On success, the number of bytes read is returned (zero indicates end of file),
//失败，返回-1，
On error, -1 is returned, and errno is set appropriately. write
NAME
write - write to a file descriptor
SYNOPSIS
#include <unistd.h>
//将buf中的内容，写到fd中去，数据的个数为count个字节
ssize_t write(int fd, const void *buf, size_t count);
DESCRIPTION
write() writes up to count bytes from the buffer pointed buf to the file referred to by the file descriptor fd.
RETURN VALUE
On success, the number of bytes written is returned (zero indicates nothing was written). On error, -1 is returned, and errno
is set appropriately.lseek
NAME
lseek - reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
DESCRIPTION
The lseek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset
according to the directive whence as follows:
SEEK_SET
The offset is set to offset bytes.
SEEK_CUR
The offset is set to its current location plus offset bytes.
SEEK_END
The offset is set to the size of the file plus offset bytes.
The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the
file). If data is later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes ('\0')
until data is actually written into the gap.
RETURN VALUE
Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file.
Otherwise, a value of (off_t) -1 is returned and errno is set to indicate the error.