System Calls in Operating Systems

Without System Calls: To save a file, you'd need to:

• Calculate exact disk sector locations
• Control spindle motor speed (5400/7200 RPM)
• Position read/write head using stepper motors
• Manage disk cache and buffer
• Handle concurrent access from other programs
• Implement error correction codes
• Deal with bad sectors

With System Calls:

write(file_descriptor, data, size);

One line! The OS kernel handles all the complexity.

Common System Calls:

• load() - Load a program into memory
• execute() - Start program execution
• end() - Normal program termination
• abort() - Abnormal termination (error)
• fork() - Create a child process (copy of current process)
• wait() - Parent process waits for child to complete
• exec() - Replace current process with new program
• exit() - Terminate calling process

Example in C (Linux):

#include <unistd.h>
#include <sys/wait.h>

int main() {
    pid_t pid = fork();  // Create child process
    
    if (pid == 0) {
        // Child process
        execl("/bin/ls", "ls", "-l", NULL);  // Run 'ls -l'
    } else {
        // Parent process
        wait(NULL);  // Wait for child to finish
        printf("Child completed\n");
    }
    return 0;
}

Common System Calls:

• create() - Create a new file
• delete() / unlink() - Delete a file
• open() - Open a file for reading/writing
• close() - Close an open file
• read() - Read data from file
• write() - Write data to file
• lseek() - Move file pointer to specific position
• stat() - Get file information (size, permissions, dates)

Example in C (Linux):

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

int main() {
    int fd = open("data.txt", O_RDONLY);  // Open file for reading
    if (fd < 0) {
        perror("File open failed");
        return 1;
    }
    
    char buffer[100];
    int bytes = read(fd, buffer, sizeof(buffer));  // Read data
    
    close(fd);  // Close file
    return 0;
}

Common System Calls:

• request() / ioctl() - Request device access
• release() - Release device
• read() - Read from device
• write() - Write to device
• ioctl() - Device-specific control operations

Devices Managed:

• Printers
• Disk drives
• Tape drives
• USB devices
• Network cards
• Graphics cards

Example:

int printer = open("/dev/lp0", O_WRONLY);  // Open printer
write(printer, document, size);  // Print document
close(printer);  // Release printer

Common System Calls:

• getpid() - Get current process ID
• getppid() - Get parent process ID
• time() - Get current system time
• date() - Get system date
• gettimeofday() - Get time with microsecond precision
• sysinfo() - Get system information (uptime, memory)
• uname() - Get OS name and version

Example:

#include <time.h>
#include <unistd.h>

int main() {
    pid_t my_pid = getpid();  // Get my process ID
    time_t current_time = time(NULL);  // Get current time
    
    printf("PID: %d\n", my_pid);
    printf("Time: %s", ctime(&current_time));
    return 0;
}

Methods:

Shared Memory:

• shmget() - Create shared memory segment
• shmat() - Attach to shared memory
• shmdt() - Detach from shared memory

Message Passing:

• pipe() - Create pipe for IPC
• msgget() - Create message queue
• msgsnd() / msgrcv() - Send/receive messages

Network Communication:

• socket() - Create network socket
• bind() - Bind socket to address
• listen() - Listen for connections
• accept() - Accept connection
• send() / recv() - Send/receive data

Example (Pipe):

int fd[2];
pipe(fd);  // Create pipe

if (fork() == 0) {
    // Child writes to pipe
    close(fd[0]);
    write(fd[1], "Hello Parent!", 13);
    close(fd[1]);
} else {
    // Parent reads from pipe
    char buffer[20];
    close(fd[1]);
    read(fd[0], buffer, 13);
    printf("Received: %s\n", buffer);
    close(fd[0]);
}

Common System Calls:

• chmod() - Change file permissions
• chown() - Change file owner
• chgrp() - Change file group
• umask() - Set default file permissions
• setuid() - Set user ID
• setgid() - Set group ID

Example:

#include <sys/stat.h>

// Set file permissions to rw-r--r-- (644 in octal)
chmod("document.txt", S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);

// Or using octal notation
chmod("document.txt", 0644);

Popular APIs:

• ● POSIX API - Portable Operating System Interface (Linux, macOS, Unix)
• ● Win32 API - Windows API
• ● Java API - Java Virtual Machine provides OS abstraction

Examples:

int fd = open("file.txt", O_RDONLY);
if (fd < 0) {
    // Error occurred!
}

pid_t pid = fork();
if (pid < 0) {
    // Fork failed!
}

Common Error Codes:

• ENOENT - No such file or directory
• EACCES - Permission denied
• ENOMEM - Out of memory
• EINVAL - Invalid argument
• EBADF - Bad file descriptor
• EEXIST - File already exists

Example with Error Handling:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>

int main() {
    int fd = open("nonexistent.txt", O_RDONLY);
    
    if (fd < 0) {
        printf("Error: %s\n", strerror(errno));
        
        if (errno == ENOENT) {
            printf("File does not exist!\n");
        } else if (errno == EACCES) {
            printf("Permission denied!\n");
        }
        return 1;
    }
    
    close(fd);
    return 0;
}

Output if file doesn't exist:

Error: No such file or directory
File does not exist!