File I/O: Basics¶

File Descriptors ¶

Caution

Windows-Programmers: no errors, warnings, and debug output to standard output!!

`open()`	Opens a file (or creates it ⟶ tons of flags)
`read()`	Reads bytes
`write()`	Writes bytes
`close()`	Closes the file

open() creates file descriptors that are associated with path names (files, named pipes, device special files, …). Other “factory” functions …

Network programming:

Miscellaneous:

#include <fcntl.h>

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

Swiss army knife among system calls. Multiple actions, governed by bitwise-or’ed flags:

Access Mode

Creating a File

O_CREAT: create if not exists, else simply open
O_CREAT|O_EXCL: exclusive creation
- error if file exists
- ⟶ to prevent race conditions when two parties try to create a file at the smae time
- security measure

Miscellaneous

#include <unistd.h>

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

#include <unistd.h>

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

Return value: number of bytes written (-1 on error, as always)
Can write less than count (usually with network I/O)
SIGPIPE (process termination) when …
- Writing on a network connection that has been closed by peer
- Writing on a pip where receiver end has exited (hence SIGPIPE)

#include <unistd.h>

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

read() and write() manipulate the offset (position where the next operation begins).

Explicit positioning. whence is one of

Macro	Description
`SEEK_SET`	The file offset is set to offset bytes.
`SEEK_CUR`	The file offset is set to its current location plus offset bytes.
`SEEK_END`	The file offset is set to the size of the file plus offset bytes.

Obscure feature: files with holes

#include <sys/ioctl.h>

int ioctl(int fd, unsigned long request, ...);