Last change
on this file since 58c5821 was
7972603,
checked in by Michael Brooks <mlbrooks@…>, 11 months ago
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Missing files from last thesis push
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-
Property mode set to
100644
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File size:
1.8 KB
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Rev | Line | |
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[7972603] | 1 | #include <stdio.h> |
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| 2 | #include <assert.h> |
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| 3 | #include <stdlib.h> |
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| 4 | |
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| 5 | #ifdef ERRS |
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| 6 | #define ERR(...) __VA_ARGS__ |
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| 7 | #else |
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| 8 | #define ERR(...) |
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| 9 | #endif |
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| 10 | |
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| 11 | int main() { |
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| 12 | |
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| 13 | /* |
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| 14 | These attempts to assign @y@ to @x@ and vice-versa are obviously ill-typed. |
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| 15 | */ |
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| 16 | float * x; // x points at a floating-point number |
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| 17 | void (*y)(void); // y points at a function |
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| 18 | ERR( |
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| 19 | x = y; // wrong |
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| 20 | y = x; // wrong |
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| 21 | ) |
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| 22 | |
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| 23 | /* |
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| 24 | The first gets |
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| 25 | warning: assignment to `float *' from incompatible pointer type `void (*)(void)' |
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| 26 | and the second gets the opposite. |
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| 27 | |
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| 28 | Similarly, |
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| 29 | */ |
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| 30 | |
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| 31 | float pi = 3.14; |
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| 32 | void f( void (*g)(void) ) { |
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| 33 | g(); |
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| 34 | } |
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| 35 | ERR( |
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| 36 | f( & pi ); // wrong |
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| 37 | ) |
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| 38 | /* |
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| 39 | gets |
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| 40 | warning: passing argument 1 of `f' from incompatible pointer type |
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| 41 | with a segmentation fault at runtime. |
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| 42 | |
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| 43 | That @f@'s attempt to call @g@ fails is not due to 3.14 being a particularly unlucky choice of value to put in the variable @pi@. |
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| 44 | Rather, it is because obtaining a program that includes this essential fragment, yet exhibits a behaviour other than "doomed to crash," is a matter for an obfuscated coding competition. |
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| 45 | |
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| 46 | A "tractable syntactic method for proving the absence of certain program behaviours |
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| 47 | by classifying phrases according to the kinds of values they compute"*1 |
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| 48 | rejected the program. The behaviour (whose absence is unprovable) is neither minor nor unlikely. |
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| 49 | The rejection shows that the program is ill-typed. |
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| 50 | |
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| 51 | Yet, the rejection presents as a GCC warning. |
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| 52 | |
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| 53 | In the discussion following, ``ill-typed'' means giving a nonzero @gcc -Werror@ exit condition with a message that discusses typing. |
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| 54 | |
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| 55 | *1 TAPL-pg1 definition of a type system |
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| 56 | */ |
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| 57 | |
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| 58 | } |
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