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atomic operation pread,pwirte

#include

ssize_t pread(int fd, void *buf, size_t nbytes, off_t offset);//Return value: Number of bytes read, 0 if it has reached the end of the file,-1 if it is wrong

ssize_t pwrite(int fd,const void *buf ,size_t nbytes, off_t offset);//Return value: If successful, return number of bytes written, if error, return cause-1

pread,pwirte is equivalent to calling lseek and read or write sequentially, but calling pread,pwrite cannot interrupt its positioning and reading and writing operations, and cannot update the file pointer.

od -c file: View the actual contents of file

File sharing:

File Descriptors: Each process has an entry in the process table. Each entry has an open file descriptor table (which is why it is necessary to copy file descriptors). Each file descriptor table entry is associated with: (a) file descriptor flags. (b)Pointer to a file table entry.

File table: The kernel maintains a file table for all open files, each file table entry containing: (a) file status flags (read, write, add, synchronous, non-blocking, etc.). (b)Offset of the current file. (c)Pointer to the v node table entry of the file.

v node: Every open file (or device) has a v node structure. Contains information about the file type and pointers to functions that perform various operations on the file. For most files, the v node also contains the i node (inode) of the file. This information is read from disk into memory when the file is opened, so all information about the file is quickly available. For example, the i node contains the owner of the file, the length of the file, the device on which the file resides, pointers to the actual data blocks used by the file on disk, and so on.

The fcntl function can change the properties of an already opened file.

int fcntl(int filedes, int cmd ,....);

cmd:

F_DUPFD copies an existing file descriptor.

F_GETFD or F_SETFD Get/Set File Descriptor Flags

F_GETFL or F_SETFL Get/set file status flags, i.e. several states of mode in the open function

F_GETOWN or F_SETOWN Get/set asynchronous I/0 permissions

F_GETLK,F_SETLK or F_SETLKW Get/Set Record Lock

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