File I/O
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open
close
lseek
read and write – unbuffered I/O
dup and dup2
File Descriptors
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Kernel maintains file descriptors to
reference open files
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Non-negative integer
The shell defines 3 by convention
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stdin 0
stdout 1
stderr 2
Open
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Functions defined in <unistd.h>
open function
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
Open (cont)
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Required Flags
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O_RDONLY
O_WRONLY
O_RDWR
Useful Optional Flags
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O_APPEND
O_CREAT
O_EXCL (used with O_CREAT to cause the open
function to fail if file already exists)
O_TRUNC
Open Example
int fd;
fd = open(“data.txt”, O_RDONLY |
O_CREAT | O_TRUNC, mode);
if(fd < 0) {
perror(“Unable to open file”);
exit(1);
}
creat
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creat function
int creat(const char *pathname, mode_t mode);
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Same as using open with O_WRONLY |
O_CREAT | O_TRUNC, mode)
close
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close function
int close(int fd);
Returns 0 on success or -1 on error
Kernel will close any file descriptors left
open by a process when it exits
lseek
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lseek function
off_t lseek(int fildes, off_t offset, int whence);
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Returns current file offset if sucessful,
otherwise -1
whence
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SEEK_SET – Sets offset from beginning of file
SEEK_CUR – Relative offset from current position
SEEK_END – Sets offset past end of file
Unbuffered I/O - read
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read function
ssize_t read(int fd, void *buf, size_t count);
Reads count bytes from the file
indicated by the file descriptor fd into
the buffer pointed to by buf
Returns -1 on error, 0 on end of file and
number of bytes read otherwise (may
be less that the number requested)
Unbuffered I/O - write
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write function
ssize_t write(int fd, const void *buf, size_t count);
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Attempts to write out count bytes from
buffer buf to file indicated by file
descriptor fd.
Returns -1 on error, number of bytes
written otherwise
Buffer Size and Efficiency
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Program in Fig 3.4 page 69
Table in Fig 3.5 page 70
File Sharing
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Multiple processes can access a file
simultaneously
3 Kernel level data structures are used
to keep track of file information
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Process table
File table
v-node
See Fig 3.6 and 3.7 on pages 72, 73
Atomic Operations
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Appending to a file
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Separate lseek and write operations can
result in race conditions
Open file with O_APPEND to force an
atomic seek to end of file before each write
Creating a file
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Separate check for file existence and
creation can result in race condition
Use both the O_CREAT and O_EXCL flags
dup and dup2
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Both are functions for duplicating a file
descriptor within a process and return -1 on
error
dup
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int dup(int oldfd);
Returns a new fd that points to the same entry in
the file table
dup2
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int dup2(int oldfd, int newfd);
Returns newfd which now points to the same
entry in the file table as oldfd
fcntl
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Allows us to change properties of an
already opened file
Several uses. See page 78 for more info
All return -1 on error, but meaning of
other return types depends on specific
use
ioctl
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int ioctl(int d, int request, ...);
Catchall for I/O functions that don’t fit
well in other headers.