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According to Section 1 of Annex J of the C Standard [ISO/IEC 9899:2011], the behavior a program is unspecified in the circumstances outlined the table below. The "Practice" column in the table identifies the coding practice(s) that address the specific case of unspecified behavior (USB).

USB

Description

Practice    

1

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usb_1
usb_1

The manner and timing of static initialization.

 

2

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usb_2
usb_2

The termination status returned to the hosted environment if the return type of main is not compatible with int.

 

3
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usb_3
usb_3
The values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t and the state of the floating-point environment when the processing of the abstract machine is interrupted by receipt of a signal. 

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usb_4
usb_4

The behavior of the display device if a printing character is written when the active position is at the final position of a line.

 

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usb_5
usb_5

The behavior of the display device if a backspace character is written when the active position is at the initial position of a line.

 

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usb_6
usb_6

The behavior of the display device if a horizontal tab character is written when the active position is at or past the last defined horizontal tabulation position.

 

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usb_7
usb_7

The behavior of the display device if a vertical tab character is written when the active position is at or past the last defined vertical tabulation position.

 

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usb_8
usb_8

How an extended source character that does not correspond to a universal character name counts toward the significant initial characters in an external identifier.

 

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usb_9
usb_9

Many aspects of the representations of types.

 

10 

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usb_10
usb_10

The value of padding bytes when storing values in structures or unions.

EXP04-C

11

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usb_11
usb_11

The values of bytes that correspond to union members other than the one last stored into.

 

12

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usb_12
usb_12

The representation used when storing a value in an object that has more than one object representation for that value.

 

13

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usb_13
usb_13

The values of any padding bits in integer representations.

 

14

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usb_14
usb_14

Whether certain operators can generate negative zeros and whether a negative zero becomes a normal zero when stored in an object.

 

15

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usb_15
usb_15

Whether two string literals result in distinct arrays.

 

16

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usb_16
usb_16

The order in which subexpressions are evaluated and the order in which side effects take place, except as specified for the function-call (), &&, ||, ?:, and comma operators.

EXP30-C

17

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usb_17
usb_17

The order in which the function designator, arguments, and subexpressions within the arguments are evaluated in a function call.

 

18

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usb_18
usb_18

The order of side effects among compound literal initialization list expressions.

 

19

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usb_19
usb_19

The order in which the operands of an assignment operator are evaluated.

 

20 

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usb_20
usb_20

The alignment of the addressable storage unit allocated to hold a bit-field.

 

21

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usb_21
usb_21

Whether a call to an inline function uses the inline definition or the external definition of the function.

 

22

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usb_22
usb_22

Whether or not a size expression is evaluated when it is part of the operand of a sizeof operator and changing the value of the size expression would not affect the result of the operator.

EXP06-C

23

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usb_23
usb_23

The order in which any side effects occur among the initialization list expressions in an initializer.

 

24

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usb_24
usb_24

The layout of storage for function parameters.

 

25

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usb_25
usb_25

When a fully expanded macro replacement list contains a function-like macro name as its last preprocessing token and the next preprocessing token from the source file is a (, and the fully expanded replacement of that macro ends with the name of the first macro and the next preprocessing token from the source file is again a (, whether that is considered a nested replacement.

 

26

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usb_26
usb_26

The order in which # and ## operations are evaluated during macro substitution.

 

27

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usb_27
usb_27

The state of the floating-point status flags when execution passes from a part of the program translated with FENV_ACCESS "off" to a part translated with FENV_ACCESS "on".

 

28

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usb_28
usb_28

The order in which feraiseexcept raises floating-point exceptions, except as stated in F.8.6.

 

29

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usb_29
usb_29

Whether math_errhandling is a macro or an identifier with external linkage.

DCL37-C

30

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usb_30
usb_30

The results of the frexp functions when the specified value is not a floating-point number.

 

31
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usb_31
usb_31
The numeric result of the ilogb functions when the correct value is outside the range of the return type. 

32

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usb_32
usb_32

The result of rounding when the value is out of range.

 

33
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usb_33
usb_33
The value stored by the remquo functions in the object pointed to by quo when y is zero. 
34
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usb_34
usb_34
Whether a comparison macro argument that is represented in a format wider than its semantic type is converted to the semantic type. 

35 

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usb_35
usb_35

Whether setjmp is a macro or an identifier with external linkage.

DCL37-C

36

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usb_36
usb_36

Whether va_copy and va_end are macros or identifiers with external linkage.

DCL37-C

37

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usb_37
usb_37

The hexadecimal digit before the decimal point when a non-normalized floating-point number is printed with an a or A conversion specifier.

 

38

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usb_38
usb_38

The value of the file position indicator after a successful call to the ungetc function for a text stream, or the ungetwc function for any stream, until all pushed-back characters are read or discarded.

 

39

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usb_39
usb_39

The details of the value stored by the fgetpos function.

 

40 

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usb_40
usb_40

The details of the value returned by the ftell function for a text stream.

 

41 

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usb_41
usb_41

Whether the strtod, strtof, strtold, wcstod, wcstof, and wcstold functions convert a minus-signed sequence to a negative number directly or by negating the value resulting from converting the corresponding unsigned sequence.

 

42 

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usb_42
usb_42

The order and contiguity of storage allocated by successive calls to the calloc, malloc, and realloc functions.

 

43 

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usb_43
usb_43

The amount of storage allocated by a successful call to the calloc, malloc, and realloc function when 0 bytes was requested.

MEM04-C

44
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usb_44
usb_44
Whether a call to the atexit function that does not happen before the exit function is called will succeed. 
45
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usb_45
usb_45
Whether a call to the at_quick_exit function that does not happen before the quick_exit function is called will succeed. 

46 

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usb_46
usb_46

Which of two elements that compare as equal is matched by the bsearch function.

 

47 

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usb_42
usb_42

The order of two elements that compare as equal in an array sorted by the qsort function.

 

48

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usb_48
usb_48

The encoding of the calendar time returned by the time function.

MSC05-C

49

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usb_49
usb_49

The characters stored by the strftime or wcsftime function if any of the time values being converted is outside the normal range.

 

50
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usb_50
usb_50
Whether an encoding error occurs if a wchar_t value that does not correspond to a member of the extended character set appears in the format string for a function in 7.29.2 or 7.29.5 and the specified semantics do not require that value to be processed by wcrtomb. 

51 

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usb_51
usb_51

The conversion state after an encoding error occurs.

 

52 

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usb_52
usb_52

The resulting value when the "invalid" floating-point exception is raised during IEC 60559 floating to integer conversion.

 

53 

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usb_53
usb_53

Whether conversion of non-integer IEC 60559 floating values to integer raises the "inexact" floating-point exception.

 

54
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usb_54
usb_54
Whether or when library functions in <math.h> raise the "inexact" floating-point exception in an IEC 60559 conformant implementation. 
55
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usb_55
usb_55
Whether or when library functions in <math.h> raise an undeserved "underflow" floating-point exception in an IEC 60559 conformant implementation. 

56 

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usb_56
usb_56

The exponent value stored by frexp for a NaN or infinity.

 

57 

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usb_57
usb_57

The numeric result returned by the lrint, llrint, lround, and llround functions if the rounded value is outside the range of the return type.

 

58 

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usb_58
usb_58

The sign of one part of the complex result of several math functions for certain exceptional values in IEC 60559 compatible implementations.

 

...