system calls read and write in C

Question

I am wondering how to use the system calls read() and write() in C. I am trying to read in the contents of a pre existing, file within a directory, into a buffer (array) so I can step through the array and determine what type of file was read. I have looked at quite a few different posts on the matter and have not been able to figure out where I am going wrong. I am trying to print out my buffer array at the bottom to make sure it holds the correct contents of a file before stepping though it to determine the file type, but the buffer holds nothing. Any help would be greatly appreciated.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <time.h>

int main(int argc, char *argv[])
{
    char *currentDir = NULL;
    DIR *myDir = NULL;
    struct dirent *myFile = NULL;
    struct stat myStat;

    const void *buf [1024];

    int count;
    int currentFile;

    if (strcmp(argv[1], "ls") == 0 && argc < 3)
    {
        currentDir = getenv("PWD");
        myDir = opendir(currentDir);

        while ((myFile = readdir(myDir)) != NULL)
        {
            if (myFile->d_name[0] != '.')
            {
                puts(myFile->d_name);
                //printf("%s\n", myFile->d_name);
            }
        }
        closedir(myDir);
    }

        if (strcmp(argv[1], "ls") == 0 && strcmp(argv[2], "-t") == 0)
        {
            currentDir = getenv("PWD");
            myDir = opendir(currentDir);

            while ((myFile = readdir(myDir)) != NULL)
            {
                if (myFile->d_name[0] != '.')
                {
                    printf("%s\n", myFile->d_name);
                    stat (myFile->d_name, &myStat);
                    printf("Last Accessed:\t%s\n", ctime(&myStat.st_atime));
                    printf("Last Modified:\t%s\n", ctime(&myStat.st_mtime));
                    printf("Last Changed:\t%s\n", ctime(&myStat.st_ctime));
                }
            }
            closedir(myDir);
        }

            if (strcmp(argv[1], "ls") == 0 && strcmp(argv[2], "-f") == 0)
            {
                currentDir = getenv("PWD");
                myDir = opendir(currentDir);

                while ((myFile = readdir(myDir)) != NULL)
                {
                    //while (count = read(0, buf, 100) > 0)
                    //{

                    //}
                    //write (1, buf, 100);
                    //printf ("Buffer Holds:\n %s\n", buf);
                    if (myFile->d_name[0] != '.')
                    {
                        while (count = read(myFile->d_name, buf, 100) > 0)
                            write (1, buf, count);

                        printf ("Buffer Holds:\n %s\n", buf);
                    }
                }
            }
    return 0;
}

Answer 1

I was able to figure out what was going wrong, I did have to change the buf array to an array of chars, but I had some misconceptions on how read was working. I though that read() was reading bytes from the file and storing it into a temp array, so I thought I needed to use write() to write the information from a temp array into the array that I specified. In actuality, read() read the specified file and stored its contents directly into my char buf [1024] array, so the call to write() was actually overwriting all the information read() had read from the specified file, and stored into the char buf [1024] array.

Thank you all for the reply's, I have only posted on here 1 other time, so I am still trying to figure out how to explain the issues I am encountering with less ambiguity.

Answer 2

You need some more parens here:

while (count = read(myFile->d_name, buf, 100) > 0)

try:

while ((count = read(myFile->d_name, buf, 100)) > 0)

Also, recommend using sizeof:

while ((count = read(myFile->d_name, buf, sizeof(buf))) > 0)

But you've declared buf as an array of pointers:

const void *buf [1024];

which doesn't seem likely to be what you actually want. Are there really pointer values stored in the file? I think you probably meant for buf to be an array of chars:

char buf[1024];