According to the C Standard, Annex J, J.1 [ISO/IEC 9899:2011], the behavior of a program is unspecified in the circumstances outlined the following table. The descriptions of unspecified behaviors in the "Description" column are direct quotes from the standard. The parenthesized numbers refer to the subclause of the C Standard (C11) that identifies the unspecified behavior. The "Guideline" column in the table identifies the coding practices that address the specific case of unspecified behavior (USB).
The manner and timing of static initialization (5.1.2).
The termination status returned to the hosted environment if the return type of
|3||The values of objects that are neither lock-free atomic objects nor of type |
The behavior of the display device if a printing character is written when the active position is at the final position of a line (5.2.2).
The behavior of the display device if a backspace character is written when the active position is at the initial position of a line 5.2.2).
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 (5.2.2).
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 (5.2.2).
How an extended source character that does not correspond to a universal character name counts toward the significant initial characters in an external identifier (18.104.22.168).
Many aspects of the representations of types (6.2.6).
The value of padding bytes when storing values in structures or unions (22.214.171.124).
The values of bytes that correspond to union members other than the one last stored into (126.96.36.199).
The representation used when storing a value in an object that has more than one object representation for that value (188.8.131.52).
The values of any padding bits in integer representations (184.108.40.206).
Whether certain operators can generate negative zeros and whether a negative zero becomes a normal zero when stored in an object (220.127.116.11).
Whether two string literals result in distinct arrays (6.4.5).
The order in which subexpressions are evaluated and the order in which side effects take place, except as specified for the function-call
The order in which the function designator, arguments, and subexpressions within the arguments are evaluated in a function call (18.104.22.168).
The order of side effects among compound literal initialization list expressions (22.214.171.124).
The order in which the operands of an assignment operator are evaluated (6.5.16).
The alignment of the addressable storage unit allocated to hold a bit-field (126.96.36.199).
Whether a call to an inline function uses the inline definition or the external definition of the function (6.7.4).
Whether or not a size expression is evaluated when it is part of the operand of a
The order in which any side effects occur among the initialization list expressions in an initializer (6.7.9).
The layout of storage for function parameters (6.9.1).
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
The order in which
The state of the floating-point status flags when execution passes from a part of the program translated with
The order in which
The results of the
|31||The numeric result of the |
The result of rounding when the value is out of range (188.8.131.52, 184.108.40.206, F.10.6.5).
|33||The value stored by the |
|34||Whether a comparison macro argument that is represented in a format wider than its semantic type is converted to the semantic type (7.12.14).|
The hexadecimal digit before the decimal point when a non-normalized floating-point number is printed with an
The value of the file position indicator after a successful call to the
The details of the value stored by the
The details of the value returned by the
The order and contiguity of storage allocated by successive calls to the
The amount of storage allocated by a successful call to the
|44||Whether a call to the |
|45||Whether a call to the |
Which of two elements that compare as equal is matched by the
The order of two elements that compare as equal in an array sorted by the
The encoding of the calendar time returned by the
The characters stored by the
|50||Whether an encoding error occurs if a |
The conversion state after an encoding error occurs (220.127.116.11, 18.104.22.168, 22.214.171.124, 126.96.36.199, 188.8.131.52.2, 184.108.40.206.3, 220.127.116.11.1, 18.104.22.168.2).
The resulting value when the "invalid" floating-point exception is raised during IEC 60559 floating to integer conversion (F.4).
Whether conversion of non-integer IEC 60559 floating values to integer raises the "inexact" floating-point exception (F.4).
|54||Whether or when library functions in |
|55||Whether or when library functions in |
The exponent value stored by
The numeric result returned by the
The sign of one part of the complex result of several math functions for certain exceptional values in IEC 60559 compatible implementations (G.6.1.1, G.6.2.2, G.6.2.3, G.6.2.4, G.6.2.5, G.6.2.6, G.6.3.1, G.6.4.2).