Workshop D Operating Systems Programming – 300698 1 Introduction In this workshop you will investigate file I/O and file copy operations. 2 Specification Following on from the demonstration program...

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Workshop D Operating Systems Programming – 300698 1 Introduction In this workshop you will investigate file I/O and file copy operations. 2 Specification Following on from the demonstration program cp1.c presented in the lecture, we will make a series of modifi- cations to the program. Firstly, what happens when thecp1.c program is asked to copy a file onto itself, i.e. cp1 input input? Is this what you expect? Modify the program to do something more sensible! As a hint, two files are the same if they are on the same device and have the same i-node number (which stat() can give you), simply comparing the names is not enough. Secondly, a real copy program will assign the same file permissions to the destination as were on the source, modify your answer to the last part to do this. Thirdly, real copy programs allow the second argument to be a directory, modify the answer to the last part to include this functionality. You should allocate the space for the new name dynamically. 1 3 Sample Code 3.1 cp1.c #include #include #include #define BUFFERSIZE 4096 #define COPYMODE 0644 void oops(char *, char *); main(int ac, char *av[]) { int in_fd, out_fd, n_chars; char buf[BUFFERSIZE]; if ( ac != 3 ){ fprintf( stderr, "usage: %s source destination\n", *av); exit(1); } if ( (in_fd=open(av[1], O_RDONLY)) == -1 ) oops("Cannot open ", av[1]); if ( (out_fd=creat( av[2], COPYMODE)) == -1 ) oops( "Cannot creat", av[2]); while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 ) if ( write( out_fd, buf, n_chars ) != n_chars ) oops("Write error to ", av[2]); if ( n_chars == -1 ) oops("Read error from ", av[1]); if ( close(in_fd) == -1 || close(out_fd) == -1 ) oops("Error closing files",""); } void oops(char *s1, char *s2) { fprintf(stderr,"Error: %s ", s1); perror(s2); exit(1); } 2 4 Supplementary Materials The material on the following pages is an extract of the linux system documentation and may prove useful in implementing this Workshop. These manual pages are taken from the Linuxman-pages Project available at: http://www.kernel.org/doc/man-pages/. 3 http://www.kernel.org/doc/man-pages/ OPEN(2) Linux Programmer’s Manual OPEN(2) NAME open, creat − open and possibly create a file or device SYNOPSIS #include #include #include 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); DESCRIPTION Given a pathname for a file, open() returns a file descriptor, a small, non-negative integer for use in subse- quent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process. The new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled). The file offset is set to the beginning of the file (see lseek(2)). A call to open() creates a new open file description, an entry in the system-wide table of open files. This entry records the file offset and the file status flags (modifiable via the fcntl() F_SETFL operation). A file descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. The new open file description is initially not shared with any other process, but sharing may arise via fork(2). The parameter flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-only, write-only, or read/write, respectively. In addition, zero or more file creation flags and file status flags can be bitwise-or’d in flags. The file cre- ation flags are O_CREAT, O_EXCL, O_NOCTTY, and O_TRUNC. The file status flags are all of the remaining flags listed below. The distinction between these two groups of flags is that the file status flags can be retrieved and (in some cases) modified using fcntl(2). The full list of file creation flags and file sta- tus flags is as follows: O_APPEND The file is opened in append mode. Before each write(), the file offset is positioned at the end of the file, as if with lseek(). O_APPEND may lead to corrupted files on NFS file systems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can’t be done without a race condition. O_ASYNC Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when input or output becomes possible on this file descriptor. This feature is only avail- able for terminals, pseudo-terminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2) for further details. O_CREAT If the file does not exist it will be created. The owner (user ID) of the file is set to the effective user ID of the process. The group ownership (group ID) is set either to the effective group ID of the process or to the group ID of the parent directory (depending on filesystem type and mount options, and the mode of the parent directory, see, e.g., the mount options bsdgroups and sysv- groups of the ext2 filesystem, as described in mount(8)). O_DIRECT Try to minimize cache effects of the I/O to and from this file. In general this will degrade perfor- mance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user space buffers. The I/O is synchronous, i.e., at the completion of a read(2) or write(2), data is guaranteed to have been transferred. Under Linux 2.4 transfer sizes, Linux 2.6.12 2005-06-22 1 OPEN(2) Linux Programmer’s Manual OPEN(2) and the alignment of user buffer and file offset must all be multiples of the logical block size of the file system. Under Linux 2.6 alignment must fit the block size of the device. A semantically similar (but deprecated) interface for block devices is described in raw(8). O_DIRECTORY If pathname is not a directory, cause the open to fail. This flag is Linux-specific, and was added in kernel version 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or tape device, but should not be used outside of the implementation of opendir. O_EXCL When used with O_CREAT, if the file already exists it is an error and the open() will fail. In this context, a symbolic link exists, regardless of where it points to. O_EXCL is broken on NFS file systems; programs which rely on it for performing locking tasks will contain a race condition. The solution for performing atomic file locking using a lockfile is to create a unique file on the same file system (e.g., incorporating hostname and pid), use link(2) to make a link to the lockfile. If link() returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful. O_LARGEFILE (LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened. O_NOATIME (Since Linux 2.6.8) Do not update the file last access time (st_atime in the inode) when the file is read(2). This flag is intended for use by indexing or backup programs, where its use can signifi- cantly reduce the amount of disk activity. This flag may not be effective on all filesystems. One example is NFS, where the server maintains the access time. O_NOCTTY If pathname refers to a terminal device — see tty(4) — it will not become the process’s control- ling terminal even if the process does not have one. O_NOFOLLOW If pathname is a symbolic link, then the open fails. This is a FreeBSD extension, which was added to Linux in version 2.1.126. Symbolic links in earlier components of the pathname will still be followed. O_NONBLOCK or O_NDELAY When possible, the file is opened in non-blocking mode. Neither the open() nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait. For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NON- BLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2). O_SYNC The file is opened for synchronous I/O. Any write()s on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. But see RESTRICTIONS below. O_TRUNC If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified. Some of these optional flags can be altered using fcntl() after the file has been opened. The argument mode specifies the permissions to use in case a new file is created. It is modified by the pro- cess’s umask in the usual way: the permissions of the created file are (mode & ˜umask). Note that this mode only applies to future accesses of the newly created file; the open() call that creates a read-only file Linux 2.6.12 2005-06-22 2 OPEN(2) Linux Programmer’s Manual OPEN(2) may well return a read/write file descriptor. The following symbolic constants are provided for mode: S_IRWXU 00700 user (file owner) has read, write and execute permission S_IRUSR 00400 user has read permission S_IWUSR 00200 user has write permission S_IXUSR 00100 user has execute permission S_IRWXG 00070 group has read, write and execute permission S_IRGRP 00040 group has read permission S_IWGRP 00020 group has write permission S_IXGRP 00010 group has execute permission S_IRWXO 00007 others have read, write and execute permission S_IROTH 00004 others have read permission S_IWOTH 00002 others have write permission S_IXOTH 00001 others have execute permission mode must be specified when O_CREAT is in the flags, and is ignored otherwise. creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC. RETURN VALUE open() and creat() return the new file descriptor, or −1 if an error occurred (in which case, errno is set appropriately). NOTES Note that open() can open device special files, but creat() cannot create them; use mknod(2) instead. On NFS file systems with UID mapping enabled, open() may return a file descriptor but e.g. read(2) requests are denied with EACCES. This is because the client performs open() by checking the permis- sions, but UID mapping is performed by the server upon read and write requests. If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of