Page 71 - C-Language

All integers or pointers can be used in an expression that is interpreted as "truth value".

int main(int argc, char* argv[]) {
if (argc % 4) {
puts("arguments number is not divisible by 4");
} else {
puts("argument number is divisible by 4");
}
...

The expression argc % 4 is evaluated and leads to one of the values 0, 1, 2 or 3. The first, 0 is the
only value that is "false" and brings execution into the else part. All other values are "true" and go
into the if part.

double* A = malloc(n*sizeof *A);
if (!A) {
perror("allocation problems");
exit(EXIT_FAILURE);
}

Here the pointer A is evaluated and if it is a null pointer, an error is detected and the program exits.

Many people prefer to write something as A == NULL, instead, but if you have such pointer
comparisons as part of other complicated expressions, things become quickly difficult to read.

char const* s = ....; /* some pointer that we receive */
if (s != NULL && s[0] != '\0' && isalpha(s[0])) {
printf("this starts well, %c is alphabetic\n", s[0]);
}

For this to check, you'd have to scan a complicated code in the expression and be sure about
operator preference.

char const* s = ....; /* some pointer that we receive */
if (s && s[0] && isalpha(s[0])) {
printf("this starts well, %c is alphabetic\n", s[0]);
}

is relatively easy to capture: if the pointer is valid we check if the first character is non-zero and
then check if it is a letter.

Defining a bool type using typedef

Considering that most debuggers are not aware of #define macros, but can check enum constants,
it may be desirable to do something like this:

#if __STDC_VERSION__ < 199900L
typedef enum { false, true } bool;
/* Modern C code might expect these to be macros. */
# ifndef bool
# define bool bool
# endif

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