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The C Standard identifies five distinct situations in which undefined behavior (UB)may arise as a result of invoking a function using a declaration that is incompatible with its definition or by supplying incorrect types or numbers of arguments:
| UB | Description |
A pointer is used to call a function whose type is not compatible with the referenced type (6.3.2.3). | |
For a call to a function without a function prototype in scope, the number of arguments does not equal the number of parameters (6.5.2.2). | |
For a call to a function without a function prototype in scope where the function is defined with a function prototype, either the prototype ends with an ellipsis or the types of the arguments after promotion are not compatible with the types of the parameters (6.5.2.2). | |
| For a call to a function without a function prototype in scope where the function is not defined with a function prototype, the types of the arguments after promotion are not compatible with those of the parameters after promotion (with certain exceptions) (6.5.2.2). | |
A function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function (6.5.2.2). |
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The header <tgmath.h> provides type-generic macros for math functions. Although most functions from the <math.h> header have a complex counterpart in <complex.h>, several functions do not. Calling any of the following type-generic functions with complex values results in is undefined behavior.
Functions That Should Not Be Called with Complex Values
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If the clog2() function is not available for an implementation as an extension, the programmeng programmer can take the base-2 logarithm of a complex number, using log() instead of log2(), because log() can be used on complex arguments, as shown in this compliant solution:
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fd = open(ms, O_CREAT | O_EXCL | O_WRONLY | O_TRUNC); |
Note that, technically, it is also incorrect to pass a third argument to open() when not creating a new file (that is, with the O_CREAT flag not set).
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