[SOLVED] Why do I not always have to explicitly include a system library for it to compile?

carbonbasedlifeform · 10-25-2012, 07:53 AM

Newb question. I am having to switch between two computers (Centos 5.8 and Centos 6.2) and I have noticed that on the CentOS 5 system I do not need to explicitly include the string library while on the CentOS 6 system I do, such as the following:

Code:

#include <iostream>

int main() {

        std::cout << strlen("test") << std::endl;
        return 0;
}

This will compile fine on the CentOS 5 system but on the CentOS 6 system i receive the following:

Code:

stringtest.cpp: In function ‘int main()’:
stringtest.cpp:5: error: ‘strlen’ was not declared in this scope

Could this be a compiler or gcc version difference? Thanks for any tips!

dwhitney67 · 10-25-2012, 08:26 AM

Quote:

Originally Posted by carbonbasedlifeform

Newb question. I am having to switch between two computers (Centos 5.8 and Centos 6.2) and I have noticed that on the CentOS 5 system I do not need to explicitly include the string library while on the CentOS 6 system I do, such as the following:

Code:

#include <iostream>

int main() {

        std::cout << strlen("test") << std::endl;
        return 0;
}

This will compile fine on the CentOS 5 system but on the CentOS 6 system i receive the following:

Code:

stringtest.cpp: In function ‘int main()’:
stringtest.cpp:5: error: ‘strlen’ was not declared in this scope

Could this be a compiler or gcc version difference? Thanks for any tips!

It is probably due to the implementation of iostream. In older versions, it probably included <cstring> or some other header which did, whereas in current versions it does not.

We had a similar problem in my office recently; someone made code changes and only tested the compilation on RHEL5. But on RHEL6, it would not compile.

My suggestion is to write better code.

theNbomr · 10-25-2012, 08:43 AM

Just for the sake of accuracy, C/C++ header files are not libraries. They are source code. Libraries are bound to your application at link time, by a linker (and again at runtime, in the case of shared object libraries). Header files are commonly associated with libraries, as they can be used to describe to the compiler how the library content can be accessed. The libraries are already separately compiled, so the compiler can only know about their content by including source code statements that describe the content.
--- rod.

carbonbasedlifeform · 10-25-2012, 10:57 AM

I found the problem. It is an issue porting from gcc 4.1 to 4.3 and higher.

http://gcc.gnu.org/gcc-4.3/porting_to.html

"As detailed here (Header dependency streamlining), many of the standard C++ library include files have been edited to only include the smallest possible number of additional files. As such, many C++ programs that used std::memcpy without including <cstring>, or used std::auto_ptr without including <memory> will no longer compile.

Usually, this error is of the form:

error: 'strcmp' was not declared in this scope"

NevemTeve · 10-25-2012, 01:51 PM

Note: In real life problems like this arise all the time. There is no point in muling over them, you simply solve them as they come.

carbonbasedlifeform · 10-25-2012, 02:29 PM

Quote:

Originally Posted by NevemTeve

Note: In real life problems like this arise all the time. There is no point in muling over them, you simply solve them as they come.

Note: In real life people also discuss issues as a way to find resolutions.