Is the sizeof operator in c a compile time or run time operator..Because the following code works fine with a gcc compiler
int a;
scanf("%d",&a);
printf("%d",sizeof(a));But I found in one of the tutorials thatsizeof is compile time??Can someone please clear the confusion?
sizeof is a compile time operator, except when used with variable length arrays.
In your example, the result of sizeof(a) does not change from one run of the program to another, so I do not see what is your confusion.
Look up a C++ Reference and learn How To Ask Questions The Smart WayOriginally Posted by Bjarne Stroustrup (2000-10-14)
Sizeof () is compile time. That is why the compiler has to see the full definition of a type, before it can determine the 'sizeof' it. Consider the following:
The compiler counts the length of "some string" + \0 and multiply that by the sizeof a char (Which the compiler already knows) and it can now replace any calls to 'sizeof a' with the pre-calculated size.Code:char a[] = "some string"; ... printf("sizeof a %lu\", sizeof a);
But let's say we have this:
This code is not valid because the compiler doesn't yet know what's contained inside a 'struct some_object' and thus can not determine its size.Code:struct some_object object; ... printf("address of object %p sizeof object %lu\n", &p, sizeof object);
It is a compile time operator, and your use of it doesn't prove one way or the other whether it is compile time or run time.
Since the original standard for C didn't permit variable length arrays (run-time), the sizeof(int) in this context shows that it happens at compile time to provide the dimension of the foo array.Code:#include<stdio.h> int main ( int argc, char *argv[] ) { char foo[sizeof(int)]; char bar[argc]; return 0; } $ gcc -std=c89 -Wall -pedantic bar.c bar.c: In function ‘main’: bar.c:5:3: warning: ISO C90 forbids variable length array ‘bar’ [-Wvla] bar.c:5:8: warning: unused variable ‘bar’ [-Wunused-variable] bar.c:4:8: warning: unused variable ‘foo’ [-Wunused-variable]
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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It is determined at compile-time. When the program is compiled, one intermediate step effectively turns the above into:Assuming you compile it on a system that uses 4-byte ints.Code:int a; scanf("%d",&a); printf("%d",4);
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@salem thankyou for your answer I agree to your point..but then what about the following codeCode:int foo(){return 5;}int main(){ printf("%d",sizeof(foo())); return 0;}here the call to foo() is made at runtime so the sizeof is evaluated at runtime? is the return type determined before the program runs?
The return type is determined at compile time... you even wrote it right there: int.
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But you have managed to demonstrate that using sizeof on expressions with side effects can have surprises attached.
Code:$ cat bar.c #include<stdio.h> int foo ( ) { printf("foo called\n"); return 5; } int main(){ printf("%zd\n",sizeof(foo())); return 0; } $ gcc -W -Wall bar.c $ ./a.out 4
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
There are no if's or but's, sizeof always, and can only produce a value determined at compile time.
There are no examples to the contrary!
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Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"
Here is an example to the contrary:
Hence my assertion in post #2 that "sizeof is a compile time operator, except when used with variable length arrays."Code:#include <stdio.h> int main(int argc, char *argv[]) { int numbers[argc + 1]; printf("%lu\n", (unsigned long)sizeof(numbers)); return 0; }
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Hmph. Damn C99 and it's cursed VLA's!
Okay, there are no examples to the contrary in pre-C99 C then
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Advice: Take only as directed - If symptoms persist, please see your debugger
Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"
