O · Key I/O System Calls |opens the file identified by pathname, returning a file descriptor....

File I/OCS 3113 Fall 2018

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Be sure to install all code prerequisites

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Download TLPI book code

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File Descriptors

| A nonnegative integer that may refer to:regular files, pipes, FIFOs, sockets, terminals or devices.

| Each process has its own assigned set of file descriptors.

| Used by the system to refer to files (not filenames)

| When requested, the lowest-numbered unused file descriptor is assigned

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Standard File Descriptors

| When a shell program is run, these descriptors are copied from the terminal to the running program.

| I/O redirection may modify this assignment.

| IDEs may map output to stderr to a red color

| POSIX names are available in <unistd.h>

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./myprog <input.txt >output.txt 2>error.txt

New process/program to be run

stdin is mapped to input.txt

stdout is mapped to output.txt

stderr is mapped to error.txt

Key I/O System Calls

| opens the file identified by pathname, returning a file descriptor.

| reads at most count bytes from the open file referred to by fd and stores them in buffer.

| writes up to count bytes from buffer to the open file referred to by fd.

| is called after all I/O has been completed, in order to release the file descriptor fdand its associated kernel resources.

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Example: fileio/copy.c

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Universality of I/O

same four system calls—open(), read(), write(), and close()—are used to perform I/O on all types of files.

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Openopens the file identified by pathname, returning a file descriptor.

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Readreads at most count bytes from the open file referred to by fd and stores them in buffer.

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Writewrites up to count bytes from buffer to the open file referred to by fd.

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Closeis called after all I/O has been completed, in order to release the file descriptor fdand its associated kernel resources.

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Always check for errors.

Seeking

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File offset

| Also called read- write offset or pointer

| the kernel records a file offset for each open file.

| The first byte of the file is at offset 0.

| The file offset is set to point to the start of the file when the file is opened and is automatically adjusted by each subsequent call to read() or write()

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Example: fileio/seek_io.c

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lseek + read + write

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du tfile # The number of blocks used

du tfile # The number of blocks used

./seek_io tfile s10000 wefg # write efg starting at byte point 10000