sa1.c

#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

#include <sys/types.h>
#include <sys/stat.h>

int main(int argc, char *argv[])
{
    int ch;
    char *filename;
    FILE *pf;

    if (argc > 1)
    {
        filename = argv[1];
    }
    else
    {
        printf("Usage: %s <file-name-to-read>\n", argv[0]);
        exit(EXIT_SUCCESS);
    }

    if (chmod(filename, S_IRUSR | S_IWUSR) < 0)
    {
        perror("chmod()");
        exit(EXIT_FAILURE);
    }

    if (access(filename, F_OK) < 0)
    {
        perror("File does not exist");
        exit(EXIT_FAILURE);
    }

    if ((pf = fopen(filename, "r")) == NULL)
    {
        perror("fopen()");
        exit(EXIT_FAILURE);
    }

    fprintf(stderr, "File %s is opened.\n", filename);

    if (chmod(filename, S_IWUSR) < 0)
    {
        perror("chmod()");
        exit(EXIT_FAILURE);
    }

    if (access(filename, R_OK) < 0)
    {
        fprintf(stderr, "File can't be read: %s.\n", strerror(errno));
        errno = 0;
    }
    else
    {
        fprintf(stderr, "File is still readable!\n");
        exit(EXIT_FAILURE);
    }

    while (! feof(pf))
    {
        if ((ch = fgetc(pf)) >= 0)
        {
            putchar(ch);
        }
    }

    fclose(pf);

    fprintf(stderr, "File is successfuly readed!\n");

    if (chmod(filename, S_IRUSR | S_IWUSR) < 0)
    {
        perror("chmod()");
        exit(EXIT_FAILURE);
    }

    return EXIT_SUCCESS;
}