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--- os_cp:open [2023/04/05 12:34]
jkonczak [Creating, opening and closing files]
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-===== Warm-up =====
-~~Exercise.#~~ Create a program, compile it under the name //prog//.
-~~Exercise.#~~ Print a static text with:
-  - C standard I/O library (''#include <stdio.h>''):
-    - ''printf'',
-    - ''fprintf'',
-  - 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.#~~ Print the first argument (== the one that has number 1) using the ''write'' function.
-\\ <small>The ''strlen'' function from the ''string.h'' library calculates c-__str__ing (== [[https://en.wikipedia.org/wiki/Null-terminated_string|null-terminated string]]) __len__gth.</small>
-~~Exercise.#~~ Print the first argument omitting its first three bytes.
-~~Exercise.#~~ Print all arguments (including the program name) using the ''write'' function.
-~~Exercise.#~~ Copy the first argument to an automatic (== on stack) array of pre-defined size, then display it.
-\\ <small>The ''strncpy'' function from the ''string.h'' library copies data up to first null byte, but no more than //n// characters. </small>
-~~Exercise.#~~ Copy the first argument to a manually allocated (== on heap) array of a correct size, then display it.
-\\ <small>''malloc'' (and ''calloc'') from ''stdlib.h'' allocates memory, and ''free'' releases it. \\ ''memcpy'' (or ''memmove'') from ''string.h'' copies data.</small>
-~~Exercise.#~~ Output the first argument in uppercase.
-\\
-<small>The ''toupper'' function from ''ctype.h'' covers a single character to upper case.</small>
-~~Exercise.#~~ Read some text from standard output (and then display it), using the following functions:
-  - C standard I/O library:
-    - ''scanf'',
-    - ''fscanf'',
-  - 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 =====
-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 [[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/contents.html|specifies]] what should be present in the [[https://en.cppreference.com/w/c/header|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 [[https://en.wikipedia.org/wiki/C_POSIX_library|C POSIX library]] Wikipedia page.
-The ''[[https://en.wikipedia.org/wiki/Unistd.h|unistd.h]]'' library defines a number of basic constants and functions for interfacing the operating system.
-\\
-The ''[[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html|fcntl.h]]'' library defines constants and functions for __f__ile __c__o__nt__ro__l__.
-==== Data types ====
-POSIX [[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_types.h.html|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.
-[[https://en.wikipedia.org/wiki/Strong_and_weak_typing|Type system]] of C/C++
-checks types after resolving ''typedef''s, therefore variables of such 'types'
-won't even generate compiler warnings when used interchangeably.
-==== Return value conventions ====
-**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 [[https://en.cppreference.com/w/c/error/errno|part of C]] standard (since 1989).
-\\
-Possible values of ''errno'' after executing some function are explained in the
-function's documentation.
-\\
-<small>
-All standard values of ''errno'' are documented [[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html|here]].
-</small>
-To get a human-readable error explanation of the number //errnum//, one can use:
-  * ''char * strerror(int errnum)'' defined by [[https://en.cppreference.com/w/c/string/byte/strerror|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 [[https://pubs.opengroup.org/onlinepubs/9699919799/functions/strerror.html|POSIX]] and are thread-safe,
-  * ''void perror(const char *//str//)'' a [[https://en.cppreference.com/w/c/io/perror|C standard]] function that always uses ''errno'' to obtain //errnum// and prints ''//str//: //explanation//'' to standard error (or just ''//explanation//'' if //str// is ''NULL'').
-===== Reading & writing data =====
-To read or write data, POSIX defines: \\
-<html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html"></html>
-''ssize_t **read** (int //fildes//,       void *//buf//, size_t //nbyte//)''
-<html></a></html>
-\\
-<html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html"></html>
-''ssize_t **write**(int //fildes//, const void *//buf//, size_t //nbyte//)''
-<html></a></html>
-''//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 <small>if the file is in the (default)
-blocking mode</small> 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:
-\\
-<html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html"></html>
-''off_t **lseek**(int //fd//, off_t //offset//, int //whence//)'';
-<html></a></html>
-\\
-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.#~~ 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.#~~ 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.
-<small>
-~~Exercise.#~~ 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.
-</small>
-===== Creating, opening and closing files =====
-To create or open a file, POSIX defines: \\
-<html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html"></html>
-''int **open**(const char *//pathname//, int //flags//)'' \\
-''int **open**(const char *//pathname//, int //flags//, mode_t //mode//)''
-<html></a></html>\\
-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 [[https://en.cppreference.com/w/c/language/integer_constant|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:
-<code c>
-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
-</code>
-To close a file, POSIX defines:
-\\
-<html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html"></html>
-''int **close**(int //filedes//)''
-<html></a></html>
-\\
-that closes a file number ''//filedes//''.
-\\
-<small>On Linux, invoking ''close(fd)'' always closes ''fd'', even if ''close'' returns ''-1''.</small>
-~~Exercise.#~~ Open a file with hardcoded filename, read its contents and
-write it to standard output.
-~~Exercise.#~~ Open a file specified as the first argument of your program, read
-its contents and write it to standard output.
-~~Exercise.#~~ 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//'').
-\\
-<small>''memchr'' looks up a character (e.g., ''\n'') in memory.</small>
-~~Exercise.#~~ Implement a program that checks if two files have the same contents. \\
-<small>''memcmp'' compares two memory areas.</small>
-<small>
-~~Exercise.#~~ Implement a program that works as ''paste [//file//]...''. \\
-Hint: the dirty solution reads single character a time.
-</small>
-~~META:
-language = en
-~~

Jan Kończak

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