将用 read() 读取的数据写入 C 中的 int 数组

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.