System UNIX Programming Lecture 2: Essentials
Lecture 2: Essentials UNIX Libraries
Outline
Let's put the sumtwo function in a static library:
$ $ $ $ g++ -c mymath.cpp # generate mymath.o object file ar rv libmymath.a mymath.o # add obj file to lib ranlib libmymath.a # not required under Linux g++ -o second second.cpp -L. -lmymath
Libraries UNIX Philosophy Essential Commands Files and Directories BLP: Chapter 1 UITL: Chapter 2, 3
Reference
-L. option (required) tells g++ to look in the current directory for libraries. -lmypath tells g++ to open the library name libmypath.a gcc automatically looks in standard directories for libraries: /usr/lib, /usr/local/lib
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Lecture 2: Essentials UNIX Libraries
Lecture 2: Essentials UNIX Libraries
The ar utility is used to create, extract, list, and add modules to libraries.
$ ar t /usr/lib/libm.a # List modules in math lib $ ar x libmymath.a sumtwo.o # Extract obj file
The nm utility can be used to list all of the symbols in a library, object file, or executable.
$ nm -C libmymath.a $ nm -C test
Linux supports static and shared libraries. Shared libraries are used by default. If you want to use only static libraries use the -static option with g++. Statically linked executables are much larger, but may be desired if you want to distribute your program in binary form. Code from shared libraries is loaded at runtime and may be shared by many applications. Shared libraries are usually located in /lib or /usr/ lib and have a .so filename extension.
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Lecture 2: Essentials UNIX Libraries
Lecture 2: Essentials UNIX Philosophy
The ldd utility will list the shared libraries required by a program:
$ ldd /bin/ls libtermcap.so.2 => /lib/libtermcap.so.2 libc.so.6 => /lib/tls/libc.so.6 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 $ ldd test libstdc++.so.5 => /usr/lib/libstdc++.so.5 libm.so.6 => /lib/tls/libm.so.6
Here are a few notes on UNIX programming philosophy:
ldconfig is used by the administrator to make shared libraries available to others on the system.
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Write small programs that do one thing well. Read data from standard input (scanf, cin) and write data to standard output (printf, cout). Such programs are known as filters. Very complex applications can be created using filters and pipes. Move core routines into well-documented libraries so that they can be reused.
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Lecture 2: Essentials Essential Commands
COMMAND DESCRIPTION COMMAND
Lecture 2: Essentials Useful Commands
DESCRIPTION
ls cd directory cd pwd mkdir dirname file filename cat textfile less textfile exit or ^D passwd man command info command
Displays directory listing Change the current (working) directory Return to HOME directory Display the current directory Create a directory Display file type Display file contents Page file contents to display (also more) Leave this session Change your password Display man pages on command Display info pages on command
apropos string Search the whatis (manual page) database for string (same as man -k string) whatis command Display brief description from whatis database (same as man -f command) command help Display brief help on command ps aux Display all processes kill Kill a process (requires ownership) du foo Display disk usage by file or dir foo df Display free disk space grep Search for character strings find Find files meeting criteria tar Standard UNIX archive tool
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Lecture 2: Essentials File Information
Lecture 2: Essentials Soft (Symbolic) Links
Use ls -l to display complete file info:
$ ls -l drwxrwxr-x -rw-r--r-lrwxrwxrwx prw-rw-r-13 ar63 ar63 4096 Jul 25 10:43 qt 1 ar63 ar63 6455 Aug 19 14:53 readme.txt 1 ar63 ar63 23 Aug 8 12:59 rt -> a.txt 1 ar63 ar63 0 Aug 27 15:18 mypipe
Use ln -s to create soft or symbolic links. A soft link is a pointer to a real name for a file. These are similar to Windows shortcuts but are supported by both the CLI and the GUI.
$ ln -s oldfile newname The file may be on another filesystem. You can create soft links to directories. Deleting the link has no effect on the file. Deleting the file can leave a soft link to nothing.
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Shown: file type, permissions, # of hard links, owner, group, size, creation time, and name. File type is indicated by first character
- Regular file d Directory l Soft link p Named pipe c Char device b Block device 9
Lecture 2: Essentials Hard Links
Lecture 2: Essentials File Links
name1 DATA name2 File with two hard links name1 DATA name2 File with one hard link and one soft link 11 12
Use ln to create links. Creating a hard link creates another real name for a file.
$ ln newname oldfile Data is not deleted until all hard links are deleted Data must be on the same filesystem as the link System call to delete a file is actually unlink Only the administrator can create hard links to directories (to prevent filesystem corruption due to directory loops by unwitting users)
Lecture 2: Essentials Special File Names
Lecture 2: Essentials Device Files
A filename whose first character is a period will not show up in normal directory listing.
Device files provide access to hardware. There are two types: character and block
Use 'ls -a' to list hidden files This has NOTHING to do with security.
Every directory automatically includes . and which .. are links to itself and its parent directory.
$ cp /etc/passwd . $ cd ../../../datadir
They reside in the /dev directory. They can be created with mknod by the administrator. Permissions determine access to corresponding hardware. Character devices provide sequential access (serial ports) while block devices are used for random access (drives)
See the MAKEDEV man page and the devices.txt file in the kernel source archive for a list of devices. Also man pages for hd, fd, random, etc.
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Lecture 2: Essentials A Few Standard Device Files
/dev/hda /dev/hdb /dev/hdc /dev/hdd /dev/cdrom /dev/sda /dev/sdc0 /dev/st0 /dev/hda1 /dev/hda2 /dev/hda5 /dev/fd0 /dev/fd0H1440 Master IDE drive on first controller Slave IDE drive on first controller Master IDE drive on second controller (CDROM drive???) Slave IDE drive on second controller (ZIP drive???) Link to actual CDROM device First SCSI drive SCSI CD drive SCSI tape drive First primary partition on hda drive (C: drive in Windows) Second primary partition on hda drive First logical partition First floppy drive (autodetect format) First floppy drive (1.44 MB format)
Lecture 2: Essentials A Few Standard Device Files
/dev/tty /dev/tty1 /dev/pts/1 /dev/fd/0 /dev/stdin /dev/ttyS0 /dev/lp0 /dev/audio /dev/null /dev/zero /dev/random /dev/mem /dev/psaux Current terminal device First console device First pseudo-terminal (xterm) device File descriptor 0 or standard input ( 1 is stdout, 2 is stderr) Alias for /dev/fd/0 (also /dev/stdout, /dev/stderr) First serial port (COM1) First parallel port Sound card i/o Null device (bit bucket) Zero device Random number generator Physical memory PS/2 mouse port
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Lecture 2: Essentials Cool Device Tricks
$ cat /dev/fd0 > floppy.img # Create floppy image $ cat floppy.img > /dev/fd0 # Duplicate a floppy $ dd if=/dev/cdrom of=cdrom.iso # Create ISO image of CD $ cat /dev/audio > sound.au # Record sound from microphone $ cat sound.au > /dev/audio # Play sound $ program 2> /dev/null # Redirect program error messages to bit bucket $ program /dev/stdin # Pass stdin as filename argument $ od -A n -N 8 -t u2 < /dev/random # Generate 4 16-bit unsigned random #s
Lecture 2: Essentials Cool Device Tricks
// Print 10 random 32-bit integers #include <fstream> #include <iostream> using namespace std; int main(void) { int randnum; ifstream randstr(/dev/random); for(int count=0; count<10; count++) { randstr.read((char *)&randnum,sizeof(randnum)); cout << count << : << randnum << endl; } randstr.close(); return 0; }
You can create, format, and mount disk images!
$ dd if=/dev/zero of=f.img bs=1k count=1440 # Create 1.44MB zero file $ mkfs -t msdos f.img # Create a file system $ mount -o loop f.img a # Mount the file system
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Lecture 2: Essentials File Permissions
Lecture 2: Essentials Directory Permissions
Regular file permissions are specified for user (owner), group and other (anyone else).
Permissions apply to user, group and other.
Read implies permission to get a directory listing. Write indicates permission to create and delete files in the directory. (Write permission on the file is n...