Files in C

What is a file?

• Resource for storing data
• Typically stored on persistent media (HDD, SSD, etc)
• Grouped together and organized as a file system

Organization

• Linux organization described in FHS
• Tree structure descending from root node /

Virtual File Systems

• Used to read and write data from the kernel
• /dev, /sys, /proc, /run
• e.g. cat /proc/loadavg

Using files

• Kernel provides system calls (open, read, write, close)
• System call headers are available in the kernel source
• Files are accessed as streams of bytes
• Bytes are read and written starting from the beginning of the file by default

File API

1. open file to get a file descriptor for the open file
2. read or write using the file descriptor
3. close to tell the kernel we are done with the file

Basic file reading

#include <unistd.h>
#include <fcntl.h>

void main() {
  char buf[2048];
  int file = open("/proc/loadavg", O_RDONLY);

  int len = read(file, buf, 2048);
  write(1, buf, len);

  close(file);
}

cat implementation

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

void main(int argc, char ** argv) {
  if (argc != 2) exit(1);

  char buf[1024];
  int file = open(argv[1], O_RDONLY);

  int bytes_read;

  while (bytes_read = read(file, buf, 1024)) {
    write(1, buf, bytes_read);
  }

  close(file);
}

File permissions

• On Unix systems, files can be set as readable, writable, and executable
• These can be defined for the file owner, file owner group, and everyone

Permission encoding

• 3 bits
  • Read (4)
  • Write (2)
  • Execute (1)
• 3 numbers used to store user, group, and world respectively

Example

• User read and write, group read, world read:
  • 644
• User read, write, exec, group read and exec, world nothing
  • 750

Modifying Permissions

• chmod - change file mode bits

chmod 775 myfile.c

• chown - change file owner and group

chown user:group myfile.c

• chgrp - change group ownership

chgrp group myfile.c

cp implementation

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

void main(int argc, char ** argv) {
  if (argc != 3) exit(1);
  
  int in = open(argv[1], O_RDONLY);
  int out = open(argv[2], O_WRONLY|O_CREAT, 0644);

  char buf[1024];
  int bytes_read;

  while (bytes_read = read(in, buf, 1024)) {
    write(out, buf, bytes_read);
  }

  close(in);
  close(out);
}