files.c

 /* basic operations with files, UNIX-native */

 /* pointer arithmetics:
  *
  * push(pointer, v) INDICATES *(pointer++) = v
  * pop(pointer) INDICATES *(--pointer)
  * (*++v)[0] INDICATES (**++v)
  *           INDICATES first char in v
  *           INDICATES name of string/vector v
  * likewise, *v[0] INDICATES **v
  *       and *v[n] INDICATES **(v + n)
  * returntype (*funct)(args) INDICATES a function funct with arguments args which returns...
  * char **argv INDICATES pointer to char pointer
  * int(*v)[len] INDICATES pointer "v" to a vector of "len" int elements
  * int *v[len] INDICATES vector "v" of "len" pointers to int elements
  * void *funct() INDICATES function "funct" that returns a pointer-to-void
  * void (*funct)() INDICATES pointer to a function "funct" that returns void
  *
  */

 /* useful characters: [] # */

 # include <stdio.h>
 # include <fcntl.h> // option_flags are defined here
 // # include <unistd.h> // execlp is defined here; undeclared because it causes error with unlink

 # define BUFFERSIZE 1024

 int main(int argc, char * argv[])
    {
     /* create file */
     if(creat("abc.txt", 0666) == -1) // creates the new file
        printf("File could not be created\n");


    /* get descriptor and close file */
    int descriptor = open("abc.txt", O_RDWR|O_CREAT); // obtains the descriptor of a file
    if(descriptor == -1) printf("File could not be opened\n");
    else close(descriptor); // if the descriptor is valid, the file is closed


    /* read and write */
    // DESCRIPTOR will be used without being re-instantiated
    char * buffer[BUFFERSIZE];
    descriptor = open("testfile.txt", O_RDONLY);
    read(descriptor, buffer, 10); // reads the first 10 CHARS from the file referenced by descriptor
    buffer[10] = '\0'; // escape; indicates end of line
    printf("Read: %s\n", buffer); // writes on standard output stdout
    close(descriptor);


    /* copies the input file into another file */
    // BUFFER will be used without being re-instantiated
    int i; // ID of the source file; if <= 0, it's invalid
    int file = open(*(argv + 1), O_RDONLY); // opens the source file
    int filecopy = open(*(argv + 2), O_WRONLY|O_CREAT|O_TRUNC, 0666); // creates the copy (in a blank state)
    while((i = read(file, buffer, BUFFERSIZE)) > 0) // as long as there are characters to copy...
    write(filecopy, buffer, i); // ...they are copied into the output file


    /* linking */
    /* TEST RESULT:
       link(A, B) creates a "copy" of A, named B
       unlink(A, B) removes A!
     */
    link(*(argv + 1), "secondalias.txt"); // the two files are linked (they are aliases of each other)
    unlink(*(argv + 1), "secondalias.txt"); // then they are unlinked


    /* duplication */
    // DESCRIPTOR will be used without being re-instantiated
    /* TEST RESULT:
       dup(A) duplicates the file ON STDOUT and does NOT create a hdd copy
     */
    descriptor = open("secondalias.txt", O_RDONLY);
    close(0); // closes the standard input
    dup(descriptor);
    execlp("more", "more", 0); // executes system call "more": shows file content on stdout
    }