1. C/C++ Linux System Programming

• • Session 16

• User-space System Programming

• session 6

2. Outline

Filesystem concepts

File I/O Ops

3. Filesystem

Traditionally: An abstraction for storage device access

Why?

• Common sensible organization

• Encapsulate OS HW interaction, e.g. performance considerations

4. VFS

Wider-range abstraction:

• special FS, different types of disk FS, network FS

• Common user interface

• Multiple FS's

• Common handling

5. Mounts

Superblocks filesystem control block

Mount point

Syscalls

• int mount(const char *source, const char *target, const char *filesystemtype, unsigned long mountflags, const void *data);

• int umount(const char *target);

6. FS Objects and Metadata

Inode file control block

• A unique ID

• Access/Owner info

• Memory maps

• Block device info

Dirent file as a directory entry (not physical)

File file data and hook to meta (not physical)

7. Journaling

Problem:

• operations on metadata are non-atomic, can be interrupted by power loss

Physical vs logical journals

Metadata-only journals

8. Disk Cache

Buffers

Page cache

Writeback pdflush

Read-ahead

9. File Descriptors

Descriptors index into process file table

int open(const char *pathname, int flags);

int open(const char *pathname, int flags, mode_t mode);

int creat(const char *pathname, mode_t mode);

• Open with O_CREAT (disk files only)

int close(int fd); /* notice status !! */

10. File I/O modes

int fcntl(int fd, int cmd, long arg); // F_SETFL

Nonblocking: If not ready, EAGAIN - O_NONBLOCK

Synchronized: Wait until data is on HW - O_SYNC

• int fsync(int fd);

Asynchronous: Signal when ready - O_ASYNC

• SIGIO handler

• fcntl: F_GETSIG / F_SETSIG, F_SETOWN/F_GETOWN (process getting signal)

Direct: Directly from user buffer - O_DIRECT

11. More File control

int unlink(const char *pathname);

int truncate(const char *path, off_t length);

• int ftruncate(int fd, off_t length);

• O_TRUNC on open

12. Descriptor I/O

ssize_t read(int fd, void *buf, size_t count);

ssize_t write(int fd, const void *buf, size_t count);

off_t lseek(int fd, off_t offset, int whence);

• SEEK_SET, SEEK_CUR, SEEK_END

EOF

13. IO Vectors

ssize_t readv(int fd, const struct iovec *iov, int iovcnt);

ssize_t writev(int fd, const struct iovec *iov, int iovcnt);

struct iovec { void*iov_base;/* Starting address */ size_t iov_len;/* Number of bytes to transfer */ }; 14. int echo_main(int argc, char **argv) { struct iovec io[argc]; struct iovec *cur_io = io; char *arg; char *p; ... while (1) { int c; cur_io->iov_base = p = arg; ... while ((c = *arg++)) { if (c == eflag) { /* Check for escape seq. */ if (*arg == 'c') { /* 'c' means cancel newline and ignore all subsequent chars. */ cur_io->iov_len = p - (char*)cur_io->iov_base; cur_io++; goto ret; } ... c = bb_process_escape_sequence( (void*) &arg); } *p++ = c; } arg = *++argv; if (arg) *p++ = ' '; cur_io->iov_len = p - (char*)cur_io->iov_base; cur_io++; if (!arg) break; } ret: return writev(1, io, (cur_io - io)) >= 0; } 15. Memory Mapped file

void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);

int munmap(void *start, size_t length);

Important flags:

• No anonymous, MAP_SHARED, MAP_FIXED, MAP_POPULATE ( | MAP_NONBLOCK)

int msync(void *start, size_t length, int flags); // MS_SYNC or MS_ASYNC

void *mremap(void *old_address, size_t old_size, size_t new_size, int flags);

16. Locking

Mandatory Locking (BSD)

• ~S_IXGRP | SGID ( + mount flag MS_MANDLOCK)

• Racy (mmap)

Advisory Locking

• Both sides play nice

17. Advisory Locking

int flock(int fd, int operation); // LOCK_SH, LOCK_EX, LOCK_UN

int lockf(int fd, int cmd, off_t len); // F_LOCK, T_LOCK, F_ULOCK, F_TEST

fcntl: F_GETLK, F_SETLK, F_SETLKW

• High level of control (with offset, down to a single byte)

struct flock { ... short l_type;/* Type of lock: F_RDLCK, F_WRLCK, F_UNLCK */ short l_whence;/* How to interpret l_start: SEEK_SET, SEEK_CUR, SEEK_END */ off_t l_start;/* Starting offset for lock */ off_t l_len;/* Number of bytes to lock */ pid_t l_pid;/* PID of process blocking our lock (F_GETLK only) */ ... }; 18. #ifdef F_SETLK #ifndef SEEK_SET #define SEEK_SET 0 #endif struct flock lock_data; lock_data.l_type = F_WRLCK; lock_data.l_whence = SEEK_SET; lock_data.l_start = lock_data.l_len = 0; if (fcntl(pidFd, F_SETLK, &lock_data) == -1) { if (errno == EAGAIN) return oldpid; else return -1; } #else #ifdef LOCK_EX if (flock (pidFd, LOCK_EX|LOCK_NB) == -1) { if (errno == EWOULDBLOCK) return oldpid; else return -1; } #else if (lockf (pidFd, F_TLOCK, 0) == -1) { if (errno == EACCES) return oldpid; else return -1; } #endif #endif } 19. Buffered I/O

Streams:Buffer I/O and write to kernel at once

• Better alignment

• Less system calls

• Yet another cache!!

• FILE *

• Formatting

FILE *fopen(const char *path, const char *mode);

FILE *fdopen(int fd, const char *mode);

int fclose(FILE *fp);

int fileno(FILE *stream);

20. I/O

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

Formatted

• int fprintf(FILE *stream, const char *format, ...);

• int fscanf(FILE *stream, const char *format, ...);

Char

• int fputc(int c, FILE *stream);

• int fgetc(FILE *stream);-- int ungetc(int c, FILE *stream);

String

• int fputs(const char *s, FILE *stream);

• char *fgets(char *s, int size, FILE *stream);

21. Behind the Scenes

Inherently thread-safe

To do your own locking (of the stream, not the file)

• void flockfile(FILE *filehandle);

• int ftrylockfile(FILE *filehandle);

• void funlockfile(FILE *filehandle);

• xxx_unlocked versions (e.g. fread_unlocked)

Flushing the stream (not the page cache)

• int fflush(FILE *stream);

22. Errors

int feof(FILE *stream);

int ferror(FILE *stream);

void clearerr(FILE *stream);

Descriptor ops can not distinguish EOF vs error

23. Positioning

int fseek(FILE *stream, long offset, int whence);

long ftell(FILE *stream);

int fgetpos(FILE *stream, fpos_t *pos);

int fsetpos(FILE *stream, fpos_t *pos);

24. Metadata

int fstat(int fd, struct stat *buf);

int stat(const char *path, struct stat *buf);

• lstat : BSD only

• Exec on all nodes in path

struct stat { dev_tst_dev;/* ID of device containing file */ ino_tst_ino;/* inode number */ mode_tst_mode;/* protection */ nlink_tst_nlink;/* number of hard links */ uid_tst_uid;/* user ID of owner */ gid_tst_gid;/* group ID of owner */ dev_tst_rdev;/* device ID (if special file) */ off_tst_size;/* total size, in bytes */ blksize_t st_blksize; /* blocksize for filesystem I/O */ blkcnt_tst_blocks;/* number of blocks allocated */ time_tst_atime;/* time of last access */ time_tst_mtime;/* time of last modification */ time_tst_ctime;/* time of last status change */ }; 25. Directory Streams

A directory is a file whose entries are other inodes

DIR *opendir(const char *name);

int closedir(DIR *dir);

struct dirent *readdir(DIR *dir);

struct dirent { ino_td_ino;/* inode number */ off_td_off;/* offset to the next dirent */ unsigned short d_reclen;/* length of this record */ unsigned chard_type;/* type of file */ chard_name[256]; /* filename */ }; 26. static pid_list *scan_proc_pids(inode_list *ilist) { DIR *d; struct dirent *de; pid_t pid; pid_list *plist; xchdir("/proc"); d = opendir("/proc"); if (!d) return NULL; plist = NULL; while ((de = readdir(d)) != NULL) { pid = (pid_t)bb_strtou(de->d_name, NULL, 10); if (errno) continue; if (chdir(de->d_name) < 0) continue; plist = scan_link("cwd", pid, ilist, plist); plist = scan_link("exe", pid, ilist, plist); plist = scan_link("root", pid, ilist, plist); .... } closedir(d); return plist; } static pid_list *scan_link(const char *lname, pid_t pid, inode_list *ilist, pid_list *plist) { ino_t inode; dev_t dev; if (!file_to_dev_inode(lname, &dev, &inode)) return plist; if (search_dev_inode(ilist, dev, inode)) plist = add_pid(plist, pid); return plist; } static int file_to_dev_inode(const char *filename, dev_t *dev, ino_t *inode) { struct stat f_stat; if (stat(filename, &f_stat)) return 0; *inode = f_stat.st_ino; *dev = f_stat.st_dev; return 1; } static int search_dev_inode(inode_list *ilist, dev_t dev, ino_t inode) { while (ilist) { if (ilist->dev == dev) { if (option_mask32 & OPT_MOUNT) return 1; if (ilist->inode == inode) return 1; } ilist = ilist->next; } return 0; } 27. I/O Multiplexing

int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);

int pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, const struct timespec *timeout, const sigset_t *sigmask);

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

int ppoll(struct pollfd *fds, nfds_vt nfds, const struct timespec *timeout, const sigset_t *sigmask);

• POLLIN/POLLOUT/POLLPRI/POLLERR

void FD_CLR(int fd, fd_set *set); intFD_ISSET(int fd, fd_set *set); void FD_SET(int fd, fd_set *set); void FD_ZERO(fd_set *set); struct pollfd { intfd;/* file descriptor */ short events;/* requested events */ short revents;/* returned events */ }; 28. Epoll

Decouple interest set registration from poll

• +: O(1) on the wait

• +: Edge trigger

• - : system call for adding onto the set

int epoll_create(int size); //desc, need close

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);

int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);

typedef union epoll_data { void*ptr; intfd; uint32_t u32; uint64_t u64; } epoll_data_t; struct epoll_event { uint32_tevents;/* Epoll events */ epoll_data_t data;/* User data variable */ }; 29. 30. IOCTL

Device / special file control

int ioctl(int d, int request, ...);

Request is specific to device being controlled, and may have a payload (ioctl_list)

31. Filesystem events

int inotify_init(void); // desc, need close

int inotify_add_watch(int fd, const char *pathname, uint32_t mask); // watch desc

int inotify_rm_watch(int fd, uint32_t wd);

FIONREAD ioctl

fcntl: F_NOTIFY

struct inotify_event { int wd;/* watch descriptor */ uint32_t mask;/* mask of events */ uint32_t cookie; /* unique cookie */ uint32_t len;/* size of 'name' field */ char name[];/* null-terminated name */ }; 32. int inotifyd_main(int argc UNUSED_PARAM, char **argv) { unsigned mask = IN_ALL_EVENTS; // assume we want all events struct pollfd pfd; char **watched = ++argv; // watched name list const char *args[] = { *argv, NULL, NULL, NULL, NULL }; // open inotify pfd.fd = inotify_init(); if (pfd.fd < 0) bb_perror_msg_and_die("no kernel support"); // setup watched while (*++argv) { char *path = *argv; char *masks = strchr(path, ':'); int wd; // watch descriptor // if mask is specified -> if (masks) { *masks = ''; // split path and mask // convert mask names to mask bitset mask = 0; while (*++masks) { int i = strchr(mask_names, *masks) - mask_names; if (i >= 0) { mask |= (1 0) { ssize_t len; void *buf; struct inotify_event *ie; // read out all pending events xioctl(pfd.fd, FIONREAD, &len); #define eventbuf bb_common_bufsiz1 ie = buf = (len 0) { int i; char events[12]; char *s = events; unsigned m = ie->mask; for (i = 0; i < 12; ++i, m >>= 1) { if (m & 1) { *s++ = mask_names[i]; } } *s = ''; args[1] = events; args[2] = watched[ie->wd]; args[3] = ie->len ? ie->name : NULL; xspawn((char **)args); // next event i = sizeof(struct inotify_event) + ie->len; len -= i; ie = (void*)((char*)ie + i); } if (eventbuf != buf) free(buf); } return EXIT_SUCCESS; } 33. Asynchronous I/O

Only on O_DIRECT

struct aiocb { int aio_filedes;/* file descriptor * int aio_lio_opcode;/* operation to perform */ int aio_reqprio;/* request priority offset * volatile void *aio_buf;/* pointer to buffer */ size_t aio_nbytes;/* length of operation */ struct sigevent aio_sigevent; /* signal number and value */ /* internal, private members follow... */ }; int aio_read (struct aiocb *aiocbp); int aio_write (struct aiocb *aiocbp); int aio_error (const struct aiocb *aiocbp); int aio_return (struct aiocb *aiocbp); int aio_cancel (int fd, struct aiocb *aiocbp); int aio_fsync (int op, struct aiocb *aiocbp); int aio_suspend (const struct aiocb * const cblist[], int n, const struct timespec *timeout);