• Learning Outcomes
• Supervisor mode
• User mode
• Processor privilege level • System call
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• File descriptor
• File permissions (mode)
• Use the system calls open, close, read, write.
• Chapter 10: 10.1-10.4
CPSC 313 1
Processor Privilege levels
• Modern hardware has multiple privilege levels.
• Different software can run with different privileges.
• Processor hardware typically provides (at least) two different modes of operation:
• User mode: how all “regular programs” run.
• Kernel mode or supervisor mode: how the OS runs.
• Most processors have only two modes; x86 has four; some older machines had 8!
• The mode in which a piece of software is running determines:
• What instructions may be executed.
• What memory locations may be accessed (enforced through translation). • Whether the program can directly interface with devices.
CPSC 313 2
System Calls
• When a program wants to do something that it cannot do in user mode (e.g., interact with a disk), it needs to ask the kernel to perform the activity on its behalf.
• System calls are the mechanism that accomplishes this.
• They are a way to for a program to explicitly request the operating system to
do something.
• System calls appear in section 2 of the man pages.
CPSC 313 3
The POSIX File System API
• The file system APIs are implemented as library functions (in libc) that invoke system calls.
• open: takes a pathname (the name of a file) and returns a file descriptor (fd), which you will use to access that file.
• close: takes an fd and frees up all the OS and library resource that have been allocated to it.
• read: reads data from the file referenced by fd into a user-specified buffer.
• write: takes data from a buffer and writes it to the file referenced by fd.
CPSC 313 4
A Copy Program in Pseudo code
copy(infile, outfile)
fd_in = open infile for reading fd_out = open outfile for writing
while (there is data left to read) read data from fd_in
write data to fd_out
close fd_in close fd_out
CPSC 313 5
• int open(const char *path, int oflag, int mode)
• Parameters:
• path: the path name of the file you wish to open
• oflag: flag values that tell you things such as:
• How you want to access the file: O_RDONLY, O_WRONLY, O_RDWR
• What to do if the file does/does not exist (O_CREATE, O_EXCL
• How the file should behave with respect to (OS) caching (O_SYNC, O_DIRECT).
• mode: if the file does not exist and you create it, what should its access mode be set to (e.g., who is allowed to read/write it). You can ignore this for now; we’ll come back to it.
• Return value: a file descriptor
• non-negative integer on success; -1 on error).
• open(“myfile”, O_CREAT, 0600)
CPSC 313 6
A Brief Digression on File System Modes
total 1592
40 drwx——
280 drwxr-xr-x 40 drwx—— 192 -rw——- 40 drwx—— 64 -rw-r–r–
Permissionsor access modes
5 mseltzer
448 Oct 27 19:28 ./ 23819 Oct 28 17:23 ../
167 Sep 21 10:53 313/ 36600 Mar 29 2020 foo
26 Apr 19 2018 Mail/ 725 Sep 14 17:08 xxx.c
8 mseltzer
1 mseltzer
2 mseltzer
1 mseltzer
Read Write eXecute
_ ___ ___ ___
regular file directory
owner group world
We typically specify permissions using 3 octal digits corresponding to the permissions for each category of user.
• int close(int fd) • Parameters:
• fd: the file descriptor to be closed
• Return value: 0 on success; -1 on error
• int ret = close(fd);
CPSC 313 8
Note: There was some voiceover in the video that might be confusing: A return value of 0 is the only return that always indicates end of file. A short read may indicate end of file but does not necessarily do so. Therefore, after a short read, you should try to read again.
• ssize_t read(int fd, void *buf, size_t nbyte)
• Parameters:
• fd: file descriptor for the file from which to read
• buf: a buffer into which the bytes read are placed • nbyte: the number of bytes to read
• Return value: a ssize_t (a signed long) • On success: the number of bytes read
• On end-of-file: 0
• On failure: -1
• char buf[4096];
• ssize_t bytes_read = read(fd, buf, 4096);
CPSC 313 9
• ssize_t write(int fd, const void *buf, size_t nbyte)
• Parameters:
• fd: file descriptor for the file to which to write • buf: a buffer containing the data to write
• nbyte: the number of bytes to write
• Return value: a ssize_t (a signed long) • On success: the number of bytes written
• On failure: -1
• ssize_t bytes_written = write(fd, buf, 4096);
CPSC 313 10
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