syscalls are public functions that allow user land operations to access system-level services (such as reading a sector) which otherwise is locked in kernel mode because they require special privileges.
These functions are called completely isolated to another function: 1) private stack frame 2) private memory, etc.
open, close, read, write
kernel mode
kernel mode allows superuser function access such as reading sectors, etc. which would be dangerous if public.
file
open
int open(const char *pathname, int flags, mode_t mode);
Flags are a bitwise OR operations: you have to open with O_RDONLY (read only), O_WRONLY (write only), or O_RDWR (both read and write). This returns \(-1\) if the reading fails.
Other flags:
O_TRUNC(truncate file)O_CREAT(creating a file if not exist), which will require amode_t modeparameter to set the permissionO_EXCL(file must not exist)
close
int close(int fd);
ssize_t is a type that is a size_t which accepts -1.
read
get a block of a file
ssize_t read(int fd, void *buf, size_t count);
Returns the number of bytes actually read (for instance, if count is too large, it will only return the number of bytes read). \(0\) if EOF, \(-1\) on error.
- read my nat read all the bytes
- the OS keeps track of where you are reading from
write
writes a block of a file
ssize_t write(int fd, void *buf, size_t count);
Returns the number of bytes actually read (for instance, if count is too large, it will only return the number of bytes read). \(0\) if EOF, \(-1\) on error.
file descriptor
After we open a file, file descriptors, which are ints, which track where the reading head is in the file; so you can have multiple descriptors each with a different location
file descriptor is used to model access to a variety of resources:
- network connections
- printers/services
and special file descriptors:
- 0:
STDIN_FILENO— input from the terminal - 1:
STDOUT_FILENO— output to the terminal - 2:
STDERR_FILENO— error to the terminal