Writing data read with read () to int array in C

Question

I have a file with numbers on each line. I am trying to write the data read from the file to the int array with the read () function. I can read the file and print it to the terminal. How can i get the numbers i read into arr array ?

Here is my code

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

int main() {
    int len, fd,arr[10000],i=0;
    fd = open("1.txt", O_RDONLY);
    if (fd >= 0) {

        while(read(fd, &len, sizeof(int)) > 0){
            write(1, &len, sizeof(int));

        }
    }

    close(fd);

    return 0;
}

Answer 1

On most (all?) implementations, a integer stores the binary representation of a number in the bits and bytes of that integer. In the other hand, a string and something like your text file uses bytes to store the ASCII value of digits, spaces and line feeds.

int (assuming 4 byte with 8 bit each, big endian) may store the value 1234 this way:

Address   0x42  0x43  0x44  0x45
Value     0x00  0x00  0x04  0xD2
Text       NUL   NUL   EOT    Ò

In the other hand, a string can contain the ASCII values of each character that represents a text. The String "1234" could be stored like this:

Address   0x82  0x83  0x84  0x85  0x86
Value     0x31  0x32  0x33  0x34  0x00
Text        1     2     3     4    NUL

When you do a read, you read the characters of the text file. Reading them into a char array is easy, you do not need to do any conversation, only add the NUL-Byte at the end. When you want to get the number, you have to convert them from a string.

This means you have to read the file, you can do this with read() if you want, and store the content in a char array, add a NUL-Byte and then convert the resulting string with a function like strtol() or sscanf().

What you are doing

What you do is reading the ASCII characters into the int len. When you use a debugger before the write() call, you can check the value of len. In my case i used this as an input file:

0
1
2
3
...

When i stop my debugger before write(), i see that len has the value of 170986032 == 0xA310A30. My system is little endian, means the lowest byte is stored at the lowest address (unlike my previous example). Which means 0x30 comes first, then 0x0a, 0x31 and 0x0A. From this we know that we got the following memory layout of len.

Address Offset   0x00  0x01  0x02  0x03
Value            0x30  0x0A  0x31  0x0A
Text                0    LF     1    LF

As you can see, the text is interpreted as a int.

How to get what you want

You want to store the data into a char array. And then parse it. I use some pseudo code to explain it better. This is not C-Code. You should learn to write your own code. This is just to get you an idea what you have to do:

char buffer[<buffersize>] //create a buffer array
r=read(buffer) //read the file into the buffer. You may need to repeat that multiple times to get everything
if r==error //check for errors
  <do some error handling here>
buffer[<one after the last read byte>]='\0' //add the NUL-Byte st the end, so that we have a string.
int values[<number of ints you want>]  //create an array to store the parsed values
for(<one loop for every int>) //make the loop for every int, to parse the int
  values[<index>]=strtol(buffer,&buffer,0) //parse the text to a int
  if error occured:
    <do some error handling here>

How to implement this in C is your task. Keep buffer sizes in mind so you do not end with UB.

Answer 2

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

int main()
{
    int fd;
    char arr[10000];
    int arr2[10000];
    ssize_t count;
    fd = open("1.txt", O_RDONLY);
    if (fd >= 0)
    {
        count = read(fd, arr, 10000); // reading from file and writing to array.
        if (count == -1)
        {
            printf("Error %d\n", errno);
            exit(0);
        }
        arr[count] = '\0';
    }

    for(int i=0; i<count; i++)  //Converting the character arrays to integer array
    {
        if(arr[i] >='0' && arr[i]<='9' )
        arr2[i] = arr[i]-'0';
        else
        {
            arr2[i] = 0; //For characters other than numbers.
        }
        
    }

    for (int i=0; i<count; i++)
    {
        printf("%d ",arr2[i]);
    }

    close(fd);

    return 0;
}

Here, I read the numbers from the file and stored into a character array, then I converted it to an integer array. If there is any character other than number, I am assigining it as zero here in the array.