Jan Kończak

Exercise 1 Create a program, compile it under the name prog.

Exercise 2 Print a static text with:

1. C standard I/O library (#include <stdio.h>):
   1. printf,
   2. fprintf,
2. POSIX write function (see man 3p write) that is made available by #include <unistd.h>
   For now, use the following syntax: write(STDOUT_FILENO, pointer_to_data, number_of_bytes_to_write);

Exercise 3 Print the first argument (== the one that has number 1) using the write function.
The strlen function from the string.h library calculates c-string (== null-terminated string) length.

Exercise 4 Print the first argument omitting its first three bytes.

Exercise 5 Print all arguments (including the program name) using the write function.

Exercise 6 Copy the first argument to an automatic (== on stack) array of pre-defined size, then display it.
The strncpy function from the string.h library copies data up to first null byte, but no more than n characters.

Exercise 7 Copy the first argument to a manually allocated (== on heap) array of a correct size, then display it.
malloc (and calloc) from stdlib.h allocates memory, and free releases it.
memcpy (or memmove) from string.h copies data.

Exercise 8 Output the first argument in uppercase.
The toupper function from ctype.h covers a single character to upper case.

Exercise 9 Read some text from standard output (and then display it), using the following functions:

1. C standard I/O library:
   1. scanf,
   2. fscanf,
2. POSIX read function from unistd.h. For now, use the following syntax:
   int actual_number_of_read_bytes = read(STDIN_FILENO, pointer_where_to_store_read_data, max_number_of_bytes_to_read);

POSIX – the Portable Operating System Interface standard – apart from standardising shell and utilities, it also defines functions, macros, and external variables to support applications portability at the C-language source level.

POSIX specifies what should be present in the C standard libraries (e.g., stdio.h or stdlib.h), and also specifies user-level API to the operating system (those two do intersect).

Libraries required by POSIX are cleanly summarised at C POSIX library Wikipedia page.

The unistd.h library defines a number of basic constants and functions for interfacing the operating system.
The fcntl.h library defines constants and functions for file control.

POSIX defines certain data types that are usually fancy names for certain integer types.

For instance, ssize_t shall be used to store signed size of arbitrary data, pid_t shall be used to store process identifiers, uid_t shall be used for user identifiers, time_t shall be used for storing numbers of seconds etc.

Programmers shall use these types, even if they know that time_t (as well as ssize_t) is in fact a long int in the POSIX implementation they use.
This contributes to code portability and legibility.

Type system of C/C++ checks types after resolving typedefs, therefore variables of such 'types' won't even generate compiler warnings when used interchangeably.

The majority of POSIX functions upon unsuccessful execution return -1 and set the errno variable accordingly to the failure reason.

To access the errno (error number) variable, one shall #include <errno.h>.
errno is a part of C standard (since 1989).
Possible values of errno after executing some function are explained in the function's documentation.
All standard values of errno are documented here.

To get a human-readable error explanation of the number errnum, one can use:

• char * strerror(int errnum) defined by C standard, might not be thread-safe,
• strerror_r that requires the programmer to provide a buffer for the message, and strerror_l that allows to specify locale(=language); both are defined by POSIX and are thread-safe,
• void perror(const char *str) a C standard function that always uses errno to obtain errnum and prints str: explanation to standard error (or just explanation if str is NULL).

To read or write data, POSIX defines:
ssize_t read (int fildes,       void *buf, size_t nbyte)
ssize_t write(int fildes, const void *buf, size_t nbyte)

fildes is the file descriptor. The operating system maintains for each process an array of open files. A file descriptor is an index in this array. Functions that open files return this number.
For the standard stream (that are assumed to be open upon start) one may use numbers 0, 1 and 2, or, more verbosely, the equivalent fancy constants STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO.

buf is the location in the memory where the data to be written is read from, or where the data read from file should be written to.

nbyte tells how many bytes shall be read/written.
Notice that buf must point to sufficient space.

The functions return number of bytes successfully read/written (unless an error occurred).
Both read and write may return less bytes than they were ordered to read/write.
When the files are ordinary files, this usually means that either (upon read) the file has ended, or (upon write) that the disk is full.

The thread executing read/write blocks if the file is in the (default) blocking mode until the operation completes.
Two concurrent I/O function calls are guaranteed to execute atomically.

Reading/writing advances the position in the file.
In some files (this include ordinary files), one may change the position within the file with:
off_t lseek(int fd, off_t offset, int whence);
In this function, fd selects the file which position should be changed, whence chooses if the new position is given relative to beginning of the file, current position or the end of the file by, respectively, providing SEEK_SET, SEEK_CUR and SEEK_END, and finally offset chooses the offset from the chosen whence.

An attempt to read a file when the position is at (or beyond) the end of file returns 0.

Exercise 10 Read standard input until the end of file, and write the read data to standard output. Test this both by using Ctrl+d to indicate end of file and by redirecting a file to the standard input.

Exercise 11 Read standard input until the end of file, and write the read data to standard output. When you reach end of file, use lseek to set position in the file to its beginning (that is, 0 bytes from SEEK_SET). Test this by redirecting some file as standard input.

Exercise 12 Replace the standard output with a file descriptor of value 4. Test whether the program works if you tell the shell to open file number 4 for your program by doing a 4>file redirection.

To create or open a file, POSIX defines:
int open(const char *pathname, int flags)
int open(const char *pathname, int flags, mode_t mode)
and a creat function that is a shorthand to open(pathname, O_WRONLY|O_CREAT|O_TRUNC, mode).

pathname is a path.

mode is used only if open creates a new file, and it defines its permissions. Either use octal number, or use symbolic constants described in the manual.

flags is a rat's nest.
flags must contain exactly one of the following flags: O_RDONLY, O_WRONLY, or O_RDWR that choose whether file is opened for reading, writing or both.
flags may additionally contain other flags, including the following:

• O_APPEND sets file position to the end of a file before every write
• O_TRUNC (shall be used only in conjunction with O_WRONLY or O_RDWR) truncates the file (sets its size to 0)
• O_CREAT tells open that if the file does not exist, it should be created
• O_EXCL (shall be used only in conjunction with O_CREAT) makes open fail when the file exists
• and at least a dozen of other flags

For example:

int fd1 = open("/tmp/foo", O_RDONLY);
if (fd1 == -1) perror("Opening /tmp/foo for reading failed");
 
int fd2 = open("/tmp/baz", O_WRONLY|O_APPEND);
if (fd2 == -1) perror("Opening /tmp/baz for appending (write-only) failed");
 
int fd3 = open("/tmp/bar", O_RDWR|O_CREAT|O_EXCL, 0600);
if (fd3 == -1) perror("Creating a new file /tmp/bar failed");
// if open succeeds, the file is open for reading and writing and has permissions of 0600

To close a file, POSIX defines:
int close(int filedes)
that closes a file number filedes.
On Linux, invoking close(fd) always closes fd, even if close returns -1.

Exercise 13 Open a file with hardcoded filename, read its contents and write it to standard output.

Exercise 14 Open a file specified as the first argument of your program, read its contents and write it to standard output.

Exercise 15 Open a file specified as the first argument of your program, read its contents and write it to standard output with line numbers (just like cat -n file).
memchr looks up a character (e.g., \n) in memory.

Exercise 16 Implement a program that checks if two files have the same contents.
memcmp compares two memory areas.

Exercise 17 Implement a program that works as paste [file]....
Hint: the dirty solution reads single character a time.