Very simple C question (maybe!)

[mdurham]

I've just tried it on Intrepid with Anjuta 2.4.2, whereas before I used Hardy with Anjuta 2.4.1.
On Interpid it compiles both ways, no problems!

A note to stroyan.
The original problem was not to do with sqrt(16.0) or dosomething(), but with line 18:

d = sqrt(d); // when ERROR: "main.c:18: undefined reference to `sqrt'"

Thanks guys, Mike

[stroyan]

I've just tried it on Intrepid with Anjuta 2.4.2, whereas before I used Hardy with Anjuta 2.4.1.
On Interpid it compiles both ways, no problems!

A note to stroyan.
The original problem was not to do with sqrt(16.0) or dosomething(), but with line 18:

The "d = sqrt(d)" in line 18 is just one more iteration of constant evaluation.
If the compiler can reduce the previous assignments to d into a constant,
then it can replace the sqrt and assignment in line 18 with a constant.
If the value of d before line 18 is potentially variable then the compiler
needs to really use a function call to sqrt.

[mdurham]

The "d = sqrt(d)" in line 18 is just one more iteration of constant evaluation.
If the compiler can reduce the previous assignments to d into a constant,
then it can replace the sqrt and assignment in line 18 with a constant.
If the value of d before line 18 is potentially variable then the compiler
needs to really use a function call to sqrt.

I think that explains it perfectly. But it's interesting that it works without additional instructions (ie including the maths lib) on Intrepid.
Cheers, Mike

[sajithn]

When you just use sqrt(16.0) you might not need to link with libm.so (-lm option). This is because sqrt() is a built-in function of gcc (on my machine gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3) and when you call sqrt with a constant, it simply uses the built-in function sqrt.

You can disable all built-in functions with -fno-builtin option of gcc, or specifically in the case of sqrt with the option -fno-builtin-sqrt.
Then it does complain about "undefined reference to `sqrt'". If we use either of these options, we need to link with -lm to resolve the symbol sqrt.

But when you use a variable for invoking sqrt, gcc does not use any built-in function. It tries to call the function sqrt, which is typically defined in libm.so forcing us to use -lm always.

Hope this clarifies the issue.

[nvteighen]

When you just use sqrt(16.0) you might not need to link with libm.so (-lm option). This is because sqrt() is a built-in function of gcc (on my machine gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3) and when you call sqrt with a constant, it simply uses the built-in function sqrt.

You can disable all built-in functions with -fno-builtin option of gcc, or specifically in the case of sqrt with the option -fno-builtin-sqrt.
Then it does complain about "undefined reference to `sqrt'". If we use either of these options, we need to link with -lm to resolve the symbol sqrt.

But when you use a variable for invoking sqrt, gcc does not use any built-in function. It tries to call the function sqrt, which is typically defined in libm.so forcing us to use -lm always.

Hope this clarifies the issue.

Ugh... You're responding to a 1.5 years old thread...

Actually, the thing is that when he compiled with g++, it worked because g++ links your program with libm.so by default. gcc doesn't.