System Calls

Overview

System calls provide the interface between user applications and the kernel. They allow applications to request services from the operating system.

Implementation

The system call interface is implemented in kernel/syscalls/:

• syscall.asm: INT 0x80 handler and syscall dispatch
• syscall_table.asm: Syscall table and registration
• syscalls.asm: Basic syscall implementations

Syscall Mechanism

System calls are invoked using the INT 0x80 instruction:

; User code calls syscall:
mov eax, SYS_CALL_NUMBER  ; Syscall number
mov ebx, arg1            ; First argument
mov ecx, arg2            ; Second argument
mov edx, arg3            ; Third argument
int 0x80                 ; Invoke syscall
; Return value in EAX

Syscall Handler

The syscall handler (syscall_handler) is registered in the IDT at interrupt 0x80. It:

1. Receives syscall number in EAX
2. Looks up handler in syscall table
3. Calls appropriate handler function
4. Returns result in EAX

System Call Categories

1. File Operations

Complete file I/O operations:

• open() - Open file or device
• close() - Close file descriptor
• read() - Read from file
• write() - Write to file
• lseek() - Seek in file
• stat() - Get file status
• fstat() - Get file status by descriptor
• ioctl() - I/O control

2. Directory Operations

Directory management operations:

• mkdir() - Create directory
• rmdir() - Remove directory
• opendir() - Open directory
• closedir() - Close directory
• readdir() - Read directory entry
• getdents() - Get directory entries (Linux-compatible)

3. Process Operations

Process and thread management:

• fork() - Create new process
• execve() - Execute program
• wait() - Wait for child process
• waitpid() - Wait for specific child process
• exit() - Exit process
• getpid() - Get process ID
• getppid() - Get parent process ID
• getuid() - Get user ID
• geteuid() - Get effective user ID
• getgid() - Get group ID
• getegid() - Get effective group ID
• setuid() - Set user ID
• setgid() - Set group ID
• seteuid() - Set effective user ID
• setegid() - Set effective group ID

4. Memory Operations

Memory management operations:

• mmap() - Map memory
• munmap() - Unmap memory
• mprotect() - Change memory protection
• brk() - Change data segment size
• mremap() - Remap memory region

5. Signal Operations

Signal handling operations:

• signal() - Set signal handler (simplified)
• sigaction() - Set signal action (POSIX)
• kill() - Send signal to process
• raise() - Send signal to current process
• sigprocmask() - Set signal mask
• sigpending() - Get pending signals
• sigsuspend() - Suspend until signal received

6. Socket Operations

Network socket operations:

• socket() - Create socket
• bind() - Bind socket to address
• listen() - Listen for connections
• accept() - Accept connection
• connect() - Connect to address
• send() - Send data
• recv() - Receive data
• sendto() - Send data to address
• recvfrom() - Receive data from address
• shutdown() - Shutdown socket
• getsockopt() - Get socket option
• setsockopt() - Set socket option

7. IPC Operations

Inter-process communication:

• Shared Memory: shmget, shmat, shmdt, shmctl
• Message Queues: msgget, msgsnd, msgrcv, msgctl
• Semaphores: semget, semop, semctl

8. System Information

System information operations:

• uname() - Get system information
• sysinfo() - Get system statistics

9. Time Operations

Time and clock operations:

• time() - Get current time
• gettimeofday() - Get time with microseconds
• settimeofday() - Set time
• clock_gettime() - Get clock time
• clock_settime() - Set clock time
• nanosleep() - Sleep for nanoseconds

10. Service Management

Kernel-level service management operations:

• service_manager_init() (204) - Initialize service manager
• service_register() (200) - Register a service
• service_start() (201) - Start a service
• service_stop() (202) - Stop a service
• service_status() (203) - Get service status

11. I/O Operations

I/O control operations:

• ioctl() - I/O control
• fcntl() - File control

File System Calls

sys_read

int sys_read(int fd, void* buf, size_t count);

Parameters:

• fd: File descriptor
• buf: Buffer to read into
• count: Number of bytes to read

Returns: Number of bytes read, or -1 on error

sys_write

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

Parameters:

• fd: File descriptor
• buf: Buffer to write from
• count: Number of bytes to write

Returns: Number of bytes written, or -1 on error

sys_open

int sys_open(const char* path, int flags);

Parameters:

• path: File path
• flags: Open flags (O_RDONLY, O_WRONLY, O_RDWR, etc.)

Returns: File descriptor, or -1 on error

sys_close

int sys_close(int fd);

Parameters:

• fd: File descriptor to close

Returns: 0 on success, -1 on error

sys_exit

void sys_exit(int status);

Parameters:

• status: Exit status code

Returns: Never returns (terminates process)

sys_gettime

uint32_t sys_gettime(void);

Returns: System time in milliseconds since boot

sys_sleep

void sys_sleep(uint32_t milliseconds);

Parameters:

• milliseconds: Sleep duration in milliseconds

Returns: None (blocks until sleep completes)

sys_execve

int sys_execve(const char* pathname, char* const argv[], char* const envp[]);

Parameters:

• pathname: Path to executable file
• argv: Array of command-line argument strings (NULL-terminated)
• envp: Array of environment variable strings (NULL-terminated)

Returns: On success, does not return (process is replaced). On error, returns -1.

Description: Loads and executes an ELF executable, replacing the current process image. Uses the ELF loader and filesystem to load the program.

See execve() System Call for complete documentation.

Syscall Mechanism

System calls are invoked using the INT 0x80 instruction (32-bit) or SYSCALL instruction (64-bit):

; 64-bit user code calls syscall:
mov rax, SYS_CALL_NUMBER  ; Syscall number
mov rdi, arg1            ; First argument
mov rsi, arg2            ; Second argument
mov rdx, arg3            ; Third argument
syscall                   ; Invoke syscall
; Return value in RAX

System Call Convention

System calls follow standard conventions:

• Function names prefixed with sys_
• Return values: 0 or positive on success, -1 on error
• Error codes set in errno (future implementation)

Related Documentation

• Service Management System - Kernel-level service management
• execve() System Call - Complete execve() documentation
• ELF Loader - Loading ELF executables
• File System - Reading files from disk
• Process Management - Process creation and management

System Call Numbers

System calls use Linux-compatible numbers for compatibility:

Status