Library Functions Questions and Answers

Answer: A

Correct answer: 50

The preprocessor works on the text in order and takes no notice of braces, so the second #define is in force by the time the printf line is expanded - and the program prints 50. Redefining a macro to a different value without an intervening #undef is not strictly conforming, so expect a warning.

Answer: D

Correct answer: All of the above

The preprocessor does all three: file inclusion through #include, macro expansion through #define, and conditional compilation through #if and its relatives. All of it happens on the text before the compiler proper sees anything.

Answer: C

Correct answer: has # as the first character

A directive is recognised by the #, which must be the first non-blank character on its own line. It may be indented, but nothing else may precede it on that line, and it must not be placed after other code. Nor need it come before the first executable statement - directives may appear anywhere in the file.

Answer: D

Correct answer: NULL

NULL is one of the few names defined in several headers at once, <time.h> among them - so a program gets it from whichever it happens to include. fabs belongs to <math.h>, iscntrl to <ctype.h> and the string functions to <string.h>.

Answer: D

Correct answer: assert

<assert.h> defines exactly one thing: the assert macro, which tests a condition and, if it is false, prints the expression, the file and the line and calls abort(). Defining NDEBUG before the include turns every assertion into nothing, which is how they are removed from a release build.

Answer: A

Correct answer: it returns a string representing the local time

ctime() takes a time_t and returns a printable string in local time, of the form Wed Jun 30 21:49:08 2021\n. It is asctime(localtime(t)) in one call. The string is in a static buffer, so it is overwritten by the next call.

Answer: A

Correct answer: tanh(double x)

A function call or declaration needs the argument in parentheses: tanh(double x). tanh is the hyperbolic tangent, declared in <math.h> as double tanh(double).

Answer: A

Correct answer: true

mbstowcs() converts a multibyte string to a wide-character string and returns the number of wide characters written, not counting the terminating null - or (size_t)-1 if it meets an invalid multibyte sequence.

Answer: B

Correct answer: False

strlen() counts only the characters before the terminating null, so strlen("abc") is 3. The array holding that string nevertheless needs 4 bytes, which is why sizeof and strlen give different answers.

Answer: A

Correct answer: first byte of multibyte character

mblen(s, n) examines the multibyte character beginning at s - the pointer must therefore be at its first byte, since the length can only be determined from there. It returns how many bytes that character occupies, so that a program can step through a multibyte string.