Page 102 - C-Language

Note: VLAs are optional as of C11. If your implementation supports C11 and defines the macro
__STDC_NO_VLA__ to 1, you are stuck with the pre-C99 methods.

Using character constants instead of string literals, and vice versa

In C, character constants and string literals are different things.

A character surrounded by single quotes like 'a' is a character constant. A character constant is
an integer whose value is the character code that stands for the character. How to interpret
character constants with multiple characters like 'abc' is implementation-defined.

Zero or more characters surrounded by double quotes like "abc" is a string literal. A string literal is
an unmodifiable array whose elements are type char. The string in the double quotes plus
terminating null-character are the contents, so "abc" has 4 elements ({'a', 'b', 'c', '\0'})

In this example, a character constant is used where a string literal should be used. This character
constant will be converted to a pointer in an implementation-defined manner and there is little
chance for the converted pointer to be valid, so this example will invoke undefined behavior.

#include <stdio.h>

int main(void) {
const char *hello = 'hello, world'; /* bad */
puts(hello);
return 0;
}

In this example, a string literal is used where a character constant should be used. The pointer
converted from the string literal will be converted to an integer in an implementation-defined
manner, and it will be converted to char in an implementation-defined manner. (How to convert an
integer to a signed type which cannot represent the value to convert is implementation-defined,
and whether char is signed is also implementation-defined.) The output will be some meaningless
thing.

#include <stdio.h>

int main(void) {
char c = "a"; /* bad */
printf("%c\n", c);
return 0;
}

In almost all cases, the compiler will complain about these mix-ups. If it doesn't, you need to use
more compiler warning options, or it is recommended that you use a better compiler.

Ignoring return values of library functions

Almost every function in C standard library returns something on success, and something else on
error. For example, malloc will return a pointer to the memory block allocated by the function on
success, and, if the function failed to allocate the requested block of memory, a null pointer. So

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