syscalls.md

Linux systemcalls

Please do not use this as a reference. These are just my personal notes.

File access

Low-level

Standard streams

fd = 0     - stdin
fd = 1     - stdout
fd = 2     - stderr

Bitwise OR of constants

Flags are bitwise OR'd to combine, e.g.

#ifndef O_TRUNC
#define O_TRUNC		00001000	/* not fcntl */
#endif
#ifndef O_APPEND
#define O_APPEND	00002000
#endif

fd = open("filename.txt", O_RDWR| O_TRUNC | O_APPEND)

Unbuffered IO

• open()
• close()
• read()
• write()

open

int open(const char *pathname, int flags, mode_t mode);

flags:
O_RDONLY   - read only
O_WRONLY   - write only
O_RDWR     - read and write

optional:
O_APPEND   - append to file
O_CREAT    - create file
O_TRUNC    - truncate to zero length if exists

close

Returns the numbers of bytes actually read.

int close(int fd);

close(fd)

read

Returns the numbers of bytes actually read.

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

read(fd, buffer, count)

write

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

write(fd, buffer, count)

Copy example

#include <fcntl.h>
#include <stdlib.h>
#define BSIZE 16384

void main()
{
  int fin, fout;
  char buf[BSIZE];
  int count;
  char inputfile[] = "inputfile";
  char outputfile[] = "outputfile";

  if ((fin = open(inputfile, O_RDONLY)) < 0) {
    perror(inputfile);
    exit(1);
  }

  if ((fout = open(outputfile, O_WRONLY | O_CREAT, 0644)) < 0) {
    perror(outputfile);
    exit(1);
  }

  while ((count = read(fin, buf, BSIZE)) > 0)
    write(fout, buf, count);

  close(fin);
  close(fout);
}