Lecture 03
Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
File Descriptors
• Small, nonnegative number
• Each process has own set of file descriptors
• Used to refer to all types of open files:
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Regular files
Pipes
Sockets
Terminals
Devices
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File Descriptors
• Must programs use three file descriptors (FDs):
1. FD 0: standard input (stdin)
2. FD 1: standard output (stdout)
3. FD 2: standard error (stderr)
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File Descriptors
• FDs 0, 1, and 2 opened by shell before program starts
• Program inherits FDs 0, 1, and 2 from shell
program
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Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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Universality of I/O
• open(), close(), read(), write() -> I/O on ALL file types
• Not just regular files …
• … but also sockets, pipes, terminals, etc.
• Achieved by ensuring that each file system and device driver
implements the same set of I/O system calls
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System Calls for I/O
• Four key system calls:
fd = open( pathname, flags, mode );
numread = read( fd, buffer, count );
numwritten = write( fd, buffer, count );
status = close( fd );
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Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
open()
int open ( const char *pathname, int flags, mode_t mode );
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#include <fcntl.h>
Attempts to open file identified by pathname
If successful, returns lowest file descriptor
On error, returns -1 (and errno set)
flags is a bit mask that specifies access mode
mode specifies file permissions
• Used only if open() creates the file
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open()
int open ( const char *pathname, int flags, mode_t mode );
• flags bit masks: (see table 4-3 and page 75+ in book)
O_RDONLY – opens file for reading only
O_WRONLY – opens file for writing only
O_RDWR – opens file for both reading and writing
O_CREAT – creates file if it doesn’t exist
O_APPEND – write() appended to the file
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open()
int open ( const char *pathname, int flags, mode_t mode );
• If error occurs, check errno for cause
• Common errno values and causes:
• EACCESS – file permissions do not allow access
• ENDENT – file doesn’t exist
• EROFS – attempted to write to a read-only file
• Use perror() to print error messages!
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Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
read()
ssize_t read( int fd, void *buffer, size_t count );
• #include <unistd.h>
• Attempts to read count bytes into buffer from file fd
• Returns (ssize_t is a signed integer):
• number of bytes read
• 0 on EOF
• -1 on error
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read()
ssize_t read( int fd, void *buffer, size_t count );
• count specifies maximum number of bytes to read from fd
• size_t is an unsigned integer
• buffer is memory address where input data will be placed
• void * type makes it very flexible (can be anything)
• System calls DO NOT allocate memory for buffers
• Assumes buffer has already been allocated!
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Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
write()
ssize_t write( int fd, void *buffer, size_t count );
• #include <unistd.h>
• Attempts to write count bytes from buffer to file fd
• Returns (ssize_t is a signed integer):
• number of bytes written (may be less than count)
• -1 on error
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write()
ssize_t write( int fd, void *buffer, size_t count );
• count specifies number of bytes to write to file fd
• size_t is an unsigned integer
• buffer is memory address of data to be written
• void * type makes it very flexible (can be anything)
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Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
close()
int close( int fd );
• #include <unistd.h>
• Closes an open file descriptor
• Returns -1 on failure
• Good practice to close files when done using them
• File descriptors are a limited resource
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close()
int close( int fd );
• Possible reasons why close() might fail:
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Trying to close an unopened file descriptor
Trying to close a file twice
Network File System (NFS) error
Function call interrupted by a signal
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Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
lseek()
int lseek( int fd, off_t offset, int whence );
• #include <unistd.h>
• lseek() changes the file’s offset
• Returns -1 on error
• Offset – number of bytes relative to file’s beginning
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lseek()
int lseek( int fd, off_t offset, int whence );
• fd is the file descriptor to seek
• offset specifies offset value in bytes
• whence is base pointer from which offset will be interpreted
• SEEK_SET – from beginning of file
• SEK_CUR – relative to current file offset
• SEEK_END – relative to end of file
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lseek()
int lseek( int fd, off_t offset, int whence );
• For each open file, kernel keeps track of file offset
• Automatically adjusts for read / write operations
• Offset points to next byte
• Files start with 0 offset
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lseek()
• Examples:
lseek( fd, 0, SEEK_SET );
//start of file
lseek( fd, 0, SEEK_END );
//next byte after EOF
lseek( fd, -10, SEEK_CUR );
//10 bytes prior to current offset
lseek( fd, 10000, SEEK_END );
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//10001 bytes past EOF
File Holes
• Possible to write bytes well past EOF
• E.g., lseek( fd, 10000, SEEK_END );
• File hole- space between previous EOF and newly written
bytes
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July 29, 2016
Outline
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File descriptors
System calls for universal I/O
open()
read()
write()
close()
lseek()
Live Coding
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July 29, 2016
Live Coding
• Recreate UNIX cp command
• UNIX> my_cp source destination
• Recreate UNIX tee command
• writes stdin to both stdout AND file specified by command line argument
• -a command line option for “append”
• UNIX> my_tee output
• UNIX> my_tee –a output2
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