# tips-c -- tips on C programming
# $Id$
# Carlos Duarte <cgd@teleweb.pt>, 000126

Tricks
------

. toggling states

	wanted: toggle a=0 a=1 a=0 a=1 for a unknown number of times

	a = 0
	for (...) { a = !a } 
	for (...) { a ^= 1 } 
	for (...) { a++;   } a &= 1
		this one is OK if `a' isn't needed inside the loop

. #defines that act as procedures
	
	wanted: a macro that can be safely included anywhere in code

	#define x() do { ... } while (0)
		works for multiple statments

	#define x() switch (0) default: ... 
		works for one statment only, ex: 

		#define PUTCHAR(c) \
			switch (putchar(c)) \
			case EOF: fprintf(stderr, "putchar failed"), exit(1)

		...
		PUTCHAR(c); 

	#define x() if (0) {} else ... 
		works for one statment also, and might generate 
		warnings if used on a non embraced if

		#define PUTCHAR(c) if (putchar(c)!=EOF) {} else \
			fprintf(stderr, "putchar failed"), exit(1)

		...
		PUTCHAR(c);   		/* ok */
		if (cond) PUTCHAR(c);   /* ok, but as of gcc 2.95, a warning
					   is generated */

. optimal conditional code inclusion

	wanted: compile some code on certain conditions

	extern char fake_var[2];
	...
	switch (sizeof(fake_var)) {
	case sizeof(char[1]): printf("1\n"); break; 
	case sizeof(char[2]): printf("2\n"); break; 
	}

	one can't declare fake_var as needed to select certain
	parts of code, on different C files.

. fetching data from pointers
	
	wanted: get different data types values from a pointer 

	one way, is to declared different data type pointers: 

		char *cp = &any_ptr; 
		printf("char: %c\n", *cp); 

		int *ip = &any_ptr; 
		printf("int: %d\n", *ip); 

	other way, is to use a type cast: 

		printf("char: %c\n", *(char *)&any_ptr); 
		printf("int: %c\n", *(int *)&any_ptr); 

	beware from what can be taken from `any_ptr'. 
	for example: if it isn't word aligned, and we are taking a
	4-byte long, it may cause errors, depending on the architecture
	and implementation

Common Mistakes
---------------

. pointing to the last char (non '\0') on string

	usually:   t = s+strlen(s)-1
	ERROR:     if s is empty, t == &s[-1]
	SOLUTION:  t = *s ? s+strlen(s)-1 : s

	typical case: want to chop the last character

		while (fgets(buf, sizeof buf, stdin)) {
			buf[strlen(buf)-1] = '\0'; /** dangerous **/

			if (*buf) 
			buf[strlen(buf)-1] = '\0'; /** safer **/

			/* but in this case */
			char *t; 
			if (*buf && *(t = buf+strlen(buf)-1) == '\n')
				*t = 0; 
		}

. beware with automatic castings: pre declare, or use implicit casts

	example: 
		#include <stdio.h>
		main() {
			double x = atof("3.14"); 
			printf("%f\n", x); 
		}
		-> 1073741824.000000

	because atof() isn't pre-declared, it is seen as: 

		x = (double)( (int)atof(..) )

	SOLUTION: to pre-declare atof():
			#include <stdlib.h>
		  or:
			double atof(); 

	in general, always be sure the correct type-casts are 
	being used, either with implicit casts, or with pre-declarations