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| -rw-r--r-- | doc/info/chap/language-facilities.texinfo | 109 |
1 files changed, 109 insertions, 0 deletions
diff --git a/doc/info/chap/language-facilities.texinfo b/doc/info/chap/language-facilities.texinfo index f593d5b..04b95cf 100644 --- a/doc/info/chap/language-facilities.texinfo +++ b/doc/info/chap/language-facilities.texinfo @@ -10,6 +10,8 @@ * Variable alignment:: Aligment of variables and data types. * Member offsets:: Getting the offset of members of a structure. * Variadic functions:: Support for variadic functions. +* Assertions:: Checking for impossible errors. +* NULL:: The constant @code{NULL}. @end menu @@ -320,3 +322,110 @@ arguments. @command{slibc} does not implement the non-standard macro @code{va_count}. + + +@node Assertions +@section Assertions + +@hfindex assert.h +@cpindex Assertion +@fnindex assert +@lvindex NDEBUG +The @sc{ANSI}@tie{}C standard defines the header +file @file{<assert.h>} which defines the function +macro @code{assert} that is used to verify in +runtime that a condition is true. This is often +considered a C language facility. + +@table @code +@item void assert(bool expression) +If and only if the expression evaluates to a zero +value, the program prints what failed --- the +expression, the filename, the line in the source +code, and if C99 is used, the function where the +assertion took place --- and aborts the process. +If the expression evaluates non-zero value, +nothing happens. + +The expression must not have said effects. +@sc{POSIX} specifies that the expression +must not be evaluated if @code{NDEBUG} is +defined. If @code{NDEBUG} is defined, +@code{assert} has no effect, but is defined. +@end table + +If @command{slibc} extensions or GNU extensions +are enabled, the function macro @code{assert_perror} +is defined. + +@table @code +@item void assert_perror(int errnum) +@fnindex assert_perror +This macro is identical to @code{assert}, except +the argument should be an error code, and a +description of it is printed before the process +exits. (Nothing happens if @code{errnum} is zero.) +@end table + +@fnindex static_assert +@cpindex Static assertion +@fnindex _Static_assert +The @sc{ISO}@tie{}C11 standard adds the keyword +@code{_Static_assert}, and the macro @code{static_assert} +that expands to said keyword if @file{<assert.h>} +is included. @code{static_assert} is similar to +@code{assert}, however it has a second parameter: +the message to print if the assertion failed, +and the assertion is evaluated at compile-time. + +Assertions should only be used to make sure that +impossible conditions are not reached. + + + +@node NULL +@section @code{NULL} + +@lvindex NULL +@code{NULL} is an implementation-defined +lvalue macro that is a sentinel value of a +non-existing pointer. In @sc{ISO}@tie{}C99 +it was declared that it should have a zero-value. +This enabled implicit @code{NULL}-checking. +@sc{POSIX} however declares further that it +shall have the type @code{void*}. @code{NULL} +is hence declared as @code{((void*)0)}. + +@sc{POSIX}'s requirement on @code{NULL} is +important for variadic functions. Because it +ensures that it it has the same width as +any pointer type, @code{NULL} does not need +to be casted when used in variadic functions. + +@hfindex stddef.h +@code{NULL} is defined by many header files, +however its canonical location is @file{<stddef.h>}. + +Deferring @code{NULL} causes undefined behaviour. +The usual behaviour (implemented in the operating +system kernel) is to abort the process because of +a segmentation fault or access violation. However, +programs that do not run under an operating system +kernel will access the memory address 0 (assuming +@code{NULL} is zero) which is a valid address. + +@cpindex @code{NULL} considerationed harmful +@cpindex @code{NULL} considerationed harmful considerationed harmful +Note on considerations of @code{NULL} being harmful: +It is important to remember than @code{sizeof(NULL)} +does not equal @code{sizeof(int)} on all machines. +Therefore, in variadic arguments, it is important +not to substitute @code{x} for @code{x != NULL}. This +would cause horrible bugs. If you insist on not +using @code{NULL}, correct substitutions would be +@code{!!x} or @code{x != 0}. + +Note that @code{NULL} is genuinely harmful in C++, +but excessive use of C++, and especially it +features, is harmful too. + |