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.