Programming Tricks, Part 1 - lyuanschool/SystemProgramming GitHub Wiki

Who needs an editor? IDE? We can just use cat! You've seen cat being used to read the contents of files but it can also be used to read the standard-input and send it back to standard output.

$ cat
HELLO
HELLO

To finish reading from the input stream close the input stream by pressing CTRL-D

Let's use cat to send standard input to a file. We will use '>' to redirect its output to a file:

$ cat > myprog.c
#include <stdio.h>
int main() {printf("Hi!");return 0;}

(Be careful! Deletes and undos are not allowed...) Press CTRL-D when finished.

A useful trick if you have several text files (e.g. source code) to change is to use regular expressions. perl makes this very easy to edit files in place. Just remember 'perl pie' and search on the web...

An example. Suppose we want to change the sequence "Hi" to "Bye" in all .c files in the current directory. Then we can write a simple substitution pattern that will be executed on each line at time in all files:

$ perl -p -i -e 's/Hi/Bye/' *.c

(Don't panic if you get it wrong, original files are still there; they just have the extension .bak) Obviously there's a lot more you can do with regular expressions than changing Hi to Bye.

To re-run the last command just type !! and press return To re-run the last command that started with g type !g and press return

Tired of running make or gcc and then running the program if it compiled OK? Instead, use && to chain these commands together

$ gcc program.c && ./a.out

You might also try putting a line in your Makefile that will compile, and then run your program.

run : $(program)
        ./$(program)

Then running

$ make run

will make sure any changes you've made are compiled, and run your program in one go. Also good for testing many inputs at once. Although you probably would just rather write a regular shell script for that.

Use the C pre-processor to redefine common keywords e.g.

#define if while

Protip: Put this line inside one of the standard includes e.g. /usr/include/stdio.h

OK, so this is more of a gotcha. Be careful when using macros that look like functions...

#define min(a,b) a<b?a:b

A perfectly reasonable definition of a minimum of a and b. However the pre-processor is just a simple text wrangler so precedence can bite you:

int value = -min(2,3); // Should be -2?

Is expanded to

int value = -2<3 ? 2 :3; // Ooops.. result will be 2

A partial fix is to wrap every argument with () and also the whole expression with ():

#define min(a,b) (  (a) < (b) ?(a):(b) )

However this is still not a function! For example can you see why min(i++,10) might increment i once or twice!?