• primarily for i/o (read/write) on various types
• also memory allocation and process exit

• process management   (e.g. fork(), exec(), _exit(), ...)
• file management   (e.g. open(), read(), fstat(), ...)
• device management   (e.g. ioctl(), ...)
• information maintenance   (e.g. settimeofday(), getuid(), ...)
• communication   (e.g. pipe(), connect(), send(), ...)

User programs invoke sys calls through an API (POSIX + Linux)

Type man syscalls to browse through a list of them