Arjun Suresh (talk | contribs) |
Arjun Suresh (talk | contribs) |
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<metadesc>Undefined value in C language, sequence point</metadesc> | <metadesc>Undefined value in C language, sequence point</metadesc> | ||
| − | <syntaxhighlight lang="c" > | + | <syntaxhighlight lang="c" name="undefined_value" > |
#include<stdio.h> | #include<stdio.h> | ||
int main() | int main() | ||
| Line 8: | Line 8: | ||
printf("%d %d %d %d ", y, x++, x, ++x); | printf("%d %d %d %d ", y, x++, x, ++x); | ||
} | } | ||
| − | </syntaxhighlight | + | </syntaxhighlight> |
'''a)''' 22,10,11,13 | '''a)''' 22,10,11,13 | ||
<syntaxhighlight lang="c" name="undefined_value" >
int main() {
GATECSE int x=10, y;
y = (x++) + (++x);
printf("%d %d %d %d ", y, x++, x, ++x);
} </syntaxhighlight>
a) 22,10,11,13
b) 22,11,11,11
c) 12,10,11,13
d) 22 13 13 13
None of the choices are correct as per C standard. This is because the statement
y = (x++) + (++x);
can causes undefined behavior, so does the statement
printf("%d %d %d %d ", y, x++, x, ++x);
C standard says that the side effects of an operation (for b = ++a; modification of a is a side-effect) need to be completed only before the next sequence point. This relaxation is given so that the compiler would be able to generate the most optimal code (which run faster). But as a consequence, programmer shouldn't do a read and a write from a memory location within a sequence point or otherwise the result would be undefined.
In the statement,
y = (x++) + (++x);
There are 2 reads to the memory location of x and 2 writes to the same within a sequence point. So, the result of this statement has no guarantee as per C standard.
Similarly, the statement
printf("%d %d %d %d ", y, x++, x, ++x);
is undefined. In a function call in C, the arguments are pushed onto the stack from right to left (the left most argument would be on top of the stack, so that variable no. of arguments can work in C). But the order in which the argument expressions are processed is not defined. The only thing defined is that all arguments must be processed before the next sequence point- which here is the execution of the called function.
I have seen many people saying C is so stupid because of these undefined behaviors. But that's the power of C. A programmer in C should know about these behaviors and avoid writing undefined codes.
Undefined value means compiler can give any value. i.e.; different compilers or even different versions of the same compiler can give different answers. As per C standard, no programmer should write this code. This is different from compiler dependent, in which case the output is clearly defined by the compiler and programmer can write those code as long as he is aware of the compiler. Example of compiler dependent code is sizeof(int).
<syntaxhighlight lang="c" >
int main() {
int x=10, y;
y = (x++) + (++x);
printf("%d %d %d %d ", y, x++, x, ++x);
} </syntaxhighlight name="undefined_value">
a) 22,10,11,13
b) 22,11,11,11
c) 12,10,11,13
d) 22 13 13 13
None of the choices are correct as per C standard. This is because the statement
y = (x++) + (++x);
can causes undefined behavior, so does the statement
printf("%d %d %d %d ", y, x++, x, ++x);
C standard says that the side effects of an operation (for b = ++a; modification of a is a side-effect) need to be completed only before the next sequence point. This relaxation is given so that the compiler would be able to generate the most optimal code (which run faster). But as a consequence, programmer shouldn't do a read and a write from a memory location within a sequence point or otherwise the result would be undefined.
In the statement,
y = (x++) + (++x);
There are 2 reads to the memory location of x and 2 writes to the same within a sequence point. So, the result of this statement has no guarantee as per C standard.
Similarly, the statement
printf("%d %d %d %d ", y, x++, x, ++x);
is undefined. In a function call in C, the arguments are pushed onto the stack from right to left (the left most argument would be on top of the stack, so that variable no. of arguments can work in C). But the order in which the argument expressions are processed is not defined. The only thing defined is that all arguments must be processed before the next sequence point- which here is the execution of the called function.
I have seen many people saying C is so stupid because of these undefined behaviors. But that's the power of C. A programmer in C should know about these behaviors and avoid writing undefined codes.
Undefined value means compiler can give any value. i.e.; different compilers or even different versions of the same compiler can give different answers. As per C standard, no programmer should write this code. This is different from compiler dependent, in which case the output is clearly defined by the compiler and programmer can write those code as long as he is aware of the compiler. Example of compiler dependent code is sizeof(int).