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How can I use read () and write () without including "unistd.h"?

I used syscalls read () and write () in my WITHOUT program, including the unistd.h header file in the program. But still, the program works and gives the expected results.

After starting the program, I thought that I would read the man page for read () and write ().

The man 2 page for read () and write () in the SYNOPSIS section mentions that I need to include the unistd.h header file to use read () or write ().

SYNOPSIS
   #include <unistd.h>

   ssize_t read(int fd, void *buf, size_t count);


SYNOPSIS
   #include <unistd.h>

   ssize_t write(int fd, const void *buf, size_t count);

So, I am surprised how my program worked, although I did not enable unistd.h?

Below is my program. This is a program for copying the contents of the source file to the target file using the read () and write () system calls.

#include<stdio.h>
#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<stdlib.h>

int main()
{
    /* Declaring the buffer. */
    /* Data read by read() will be stored in this buffer. */
    /* Later when write() is used, write() will take the contents of this buffer and write to the file.*/

    char buffer[512];

    /* Decalring strings to store the source file and target file names. */

    char source[128], target[128];

    /* Declaring integer variables in which integer returned by open() will be stored. */
    /* Note that this program will open a source file, and a target file. So, 2 integers will be needed. */

    int inhandle, outhandle;

    /* Declaring integer variable which will specify how much bytes to read or write.*/

    int bytes;

    /* Taking source filename from keyboard.*/

    printf("\nSource File name: ");
    scanf("%s",source);

    /* Open the source file using open().*/

    inhandle = open(source, O_RDONLY);

    /* If there is error while opening source file.*/

    if (inhandle == -1)
    {
            perror("Error opening source file.\n");
            exit(1);
    }

    /* Taking target filename from keyboard.*/

    printf("\nTarget File name: ");
    scanf("%s",target);

    /* Open the target file using open().*/

    outhandle = open(target, O_CREAT | O_WRONLY, 0660);

    /* If there is error while opening target file.*/

    if (outhandle == -1)
    {
            perror("Error opening target file.\n");
            close(inhandle);
            exit(2);
    }

     /* Below code does following:
       1. First reads (at most) 512 bytes from source file
       2. Then copies them to buffer
       3. If bytes read is greater than 0, write the content stored in buffer to target file.
    */

    while((bytes = read(inhandle, buffer, 512)) > 0)
    {
                    write(outhandle, buffer, bytes);
    }

    /* Close both source and target files. */
    close(inhandle);
    close(outhandle);

    return 0;
}
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