POSIX Files API

POSIX functions for working with files include

• open
  • returns a file descriptor
    • used to identify files
    • passed to other functions
• read
  • copies data from file to process’s Address Space
• write
  • copies data from process’s Address Space into a file
• lseek
  • specifies where to point at in the file
  • non-sequential reading and writing
• close
  • invalidates a file descriptor
    OS Kernel tracks which file descriptors are valid for which process

stdio library in C works on top of these functions

• fopen, fscanf, …

Manipulating File Descriptors

dup2

int dup2(int oldfd, int newfd);

• Makes newfd an exact copy of oldfd

• Closes what was originally in newfd if it was a valid FD
• Both will share same offset
  • lseek affects both

fd = open(infile, O_RDONLY);
dup2(fd,0);

fcntl (file control)

int fcntl(int fd, F_SETFD, int val);

• Sets the close-on-exec flag

  • if val=1, execvp will close the file FD
  • if val=0, execvp will leave open the file FD
  • if execvp was unsuccessful, FD is left open

Pipes

int pipe(int fds[2]);

• Returns two file descriptors in fds[0] and fds[1]

• Output of fds[1] becomes input of fds[0]
  • fds[1] | fds[0]

1 void doexec (void) {
2   while (outcmd) {
3   int pipefds[2]; pipe(pipefds);
4   switch (fork()) {
5     case -1: // error
6       perror("fork"); exit(1);
7     case 0: // child writes to the pipe
8       dup2(pipefds[1], 1); //stdout is pipe input
9       close(pipefds[0]); close(pipefds[1]);
10     outcmd = NULL;
11     break;
12   default: // parent reads from the pipe
13     dup2(pipefds[0], 0); // stdin is pipe output
14     close(pipefds[0]); close(pipefds[1]);
15     parse_input(&av, &outcmd, outcmd);
16     break;
17   }
18 }