System Call
===========

Examples
--------

fork

- Call the fork syscall
- Parent waits until child exits
- Child sleeps 2 seconds, afterwards, it exits

.. code-block:: c

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <error.h>
  #include <sys/wait.h>
  
  int main(void)
  {
    pid_t pid;
  
    pid = fork();
    if (pid < 0) {
      perror("fork failed");
      exit(1);
    } else if (pid == 0) {
      printf("I am child, sleep 2 seconds\n");
      sleep(2);
      printf("I am child, 2 seconds have passed, now I exit\n");
    } else {
      printf("I am parent, I wait until child exits\n");
      wait(NULL);
      printf("I am parent, OK, now child has exited, I also exit\n");
    }
  
    return 0;
  }

fork + execvp

- Call the fork syscall
- Parent waits until child exits
- Child calls the execvp syscall to execute the *sleep* UNIX command.

.. code-block:: c

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <error.h>
  #include <sys/wait.h>
  
  int main(void)
  {
    pid_t pid;
  
    pid = fork();
    if (pid < 0) {
      perror("fork failed");
      exit(1);
    } else if (pid == 0) {
      char *argv[3];
      argv[0] = "sleep";
      argv[1] = "2";
      argv[2] = NULL;
      printf("I am child, sleep 2 seconds\n");
      if (execvp(argv[0], argv) < 0) {
        perror("exec failed");
        exit(1);
      }
    } else {
      printf("I am parent, I wait until child exits\n");
      wait(NULL);
      printf("I am parent, OK, now child has exited, I also exit\n");
    }
  
    return 0;
  }

fork + chdir + execvp

- Call the fork syscall
- Parent waits until child exits
- Child moves to a directory */home*.
- Child calls the execvp syscall to execute the *pwd* UNIX command to show the current directory.

.. code-block:: c

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <error.h>
  #include <sys/wait.h>
  
  int main(void)
  {
    pid_t pid;
  
    pid = fork();
    if (pid < 0) {
      perror("fork failed");
      exit(1);
    } else if (pid == 0) {
      char *argv[3];
      printf("I am child, I move to /home\n");
      if (chdir("/home") < 0) {
        perror("chdir failed");
        exit(1);
      }
      argv[0] = "pwd";
      argv[1] = NULL;
      printf("I am child, I execute pwd\n");
      if (execvp(argv[0], argv) < 0) {
        perror("exec failed");
        exit(1);
      }
    } else {
      printf("I am parent, I wait until child exits\n");
      wait(NULL);
      printf("I am parent, OK, now child has exited, I also exit\n");
    }
  
    return 0;
  }

open + read + close

- Open a (special) file "/proc/cpuinfo"
- Read the content and print out
- Close the file

.. code-block:: c

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <string.h>
  #include <fcntl.h>
  #include <error.h>
  
  int main(void)
  {
    int fd;
    ssize_t len = 0;
    char buf[256];
  
    fd = open("/proc/cpuinfo", O_RDONLY);
    if (fd < 0) {
      perror("open failed");
      exit(1);
    }
  
    do {
      memset(buf, 0, sizeof(buf));
      len = read(fd, buf, sizeof(buf)-1);
      if (len < 0) {
        perror("read failed");
        exit(1);
      }
      buf[len] = '\0';
      printf("%s", buf);
    } while (len > 0);
  
    close(fd);
  
    return 0;
  }

open + write + close

- Open a file named "./syscall_write_example.txt"
- Write a string "hello" to the file
- Close the file

.. code-block:: c

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <string.h>
  #include <fcntl.h>
  #include <error.h>
  
  int main(void)
  {
    int fd;
    ssize_t len = 0;
  
    fd = open("./syscall_write_example.txt", O_WRONLY | O_CREAT, 0644);
    if (fd < 0) {
      perror("open failed");
      exit(1);
    }
  
    len = write(fd, "hello\n", 6);
    if (len < 0) {
      perror("write failed");
      exit(1);
    }
  
    close(fd);
  
    return 0;
  }

Differences between system calls and library functions
------------------------------------------------------

We can use library functions for reading and writeing files instead of system calls.

The examples of library functions are *fputc* (write a character) and *fgetc* (read a character).
They are implemented as part of the standard library using system calls. In other words,
such library functions are somewhat wrappers of system calls.

.. figure:: ../images/tutorial3/libcall.png

To confirm the issue, let's use the following program that reads data from procfs.

.. code-block:: c

  #include <stdio.h>
  
  int main(void)
  {
    FILE *fp;
    fp = fopen("/proc/cpuinfo", "r");
    while (1) {
      char c = fgetc(fp);
      if (c == EOF)
        break;
      printf("%c", c);
    }
    fclose(fp);
    return 0;
  }

Please compile the program above and run it with strace which is a command shows which system calls are called while the program is running.::

  $ gcc PROGRAM.c
  $ strace ./a.out

If you do not have the strace command on your VM, please install it using the following command.::

  $ sudo apt install strace

Supposedly, you will find a lot of write and read system calls are called, but you won't see fgetc. This is because fgetc just calls the read system call internally.