When the following little C program is compiled with gcc it does not get a compile error even though the function atof is undefined. If I change it to xatof then it goes get an "undefined reference" error. (I know that atof is a library function, and I know that it resides in stdlib.h. But since I did not include stdlib.h I would think that it would give me an error). What am I missing here???

main()
{
double f;
char s[] = "1.0";
f = atof(s);
}

The compiler doesn't know about libraries linked later, so the only thing it could issue is a warning. Implicit declaration is allowed in C. But, try compiling with "-Wall", i'm quite sure you will get your implicit declaration warning

I have learned to never use a name from the common C libraries.
There are too many paths that the compiler can get to them.
Some of the headers call each other so deep, that I cannot track down
which names might be safe to use.

I suspect the GNU compiler knows about the name even it is not defined or declared. It has many non-standard enhancements. Try to find it in the "info GCC", it might be mentioned.
Test this again with some other stdlib names. I suspect it will work with some and not others. If the compiler is getting to the stdlib header by some sneak path then all the names will behave the same.

well, right, but the OP just misses an error message. the explanation for this is that the linker obviously finds a symbol "atof" somewhere in the standard libs that are linked by default, so it doesn't complain. And, as strange as this may sound, "implicit declaration" (by JUST using a function without declaring it first) is allowed in ISO C.

But, compilers can (and should) issue warnings about implicit declarations because they are a common source of errors ....

Using gcc -Wall does give me an error message, and I thank Zirias for that info. I will use that from now on. I might add, parenthetically, that whatever atof it did come up with does not work. If it had worked, I would have never even realized the error. (The error being not to use #include <stdlib.h>)

That's good practice. I use "-std=c89 -pedantic-errors -Wall -Werror" on any ISO C code i plan to release to the public Sometimes this complains about things I actually wanted that way, but there's always a way around

Implicit declaration makes the compiler assume the function returns an int. So it called the correct atof (which actually returns a double) then it converted the returned value from int to double. When the returned value wasn't actually an int, converting it from int to double gives you garbage.

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Hey, thanks for this interesting fact! As I generally avoid implicit declarations, I wasn't aware of that.

Yeah, I thought it might be something like that. Live and learn. (Die and forget it all).