Introduction to POSIX

The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE for maintaining compatibility between operating systems. POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems.

File Operations

POSIX offers a variety of APIs to handle file operations:

Opening a File: open

  int fd = open("example.txt", O_RDONLY);

Reading from a File: read

  char buffer[100];
  ssize_t bytesRead = read(fd, buffer, sizeof(buffer));

Writing to a File: write

  const char *data = "Hello, World!";
  ssize_t bytesWritten = write(fd, data, strlen(data));

Closing a File: close

  close(fd);

Directory Operations

POSIX compliant systems also provide APIs to handle directories:

Creating a Directory: mkdir

  int status = mkdir("/new_directory", S_IRWXU);

Opening a Directory: opendir

  DIR *dir = opendir("/some_directory");

Reading a Directory: readdir

  struct dirent *entry;
  while ((entry = readdir(dir)) != NULL) {
      printf("%s\n", entry->d_name);
  }

Closing a Directory: closedir

  closedir(dir);

Process Control

POSIX includes powerful APIs for process control:

Forking a Process: fork

  pid_t pid = fork();
  if (pid == 0) {
      // Child process
  } else {
      // Parent process
  }

Executing a Program: exec

  execl("/bin/ls", "ls", NULL);

Waiting for a Process to Change State: wait

  int status;
  wait(&status);

Terminating a Process: exit

  exit(0);

Example Application

Combining several POSIX APIs, we can create a simple application to list files and their sizes in a directory:

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

  int main() {
      DIR *dir;
      struct dirent *entry;
      struct stat fileStat;
     
      dir = opendir(".");
      if (!dir) {
          perror("opendir");
          return EXIT_FAILURE;
      }

      while ((entry = readdir(dir)) != NULL) {
          if (stat(entry->d_name, &fileStat) == 0) {
              printf("%s: %ld bytes\n", entry->d_name, fileStat.st_size);
          } else {
              perror("stat");
          }
      }

      closedir(dir);
      return EXIT_SUCCESS;
  }

This example lists all files in the current directory along with their sizes using POSIX file and directory APIs.

By understanding and utilizing POSIX APIs, developers can write more portable and robust system-level software.

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