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The codebase I am working with uses pragma comment lib to express the dependencies of code to the library that it needs. Its build is very broken. I made an experiment to try to understand the use of pragma comment lib. It is the simplest library and client of the library that I could produce.

Foo.lib
foo.cpp 
void foo() { printf("hello\n"); }
foo.h : 

#pragma comment(lib, "foo.lib")
void foo();

FooTest.exe
main.cpp : 

#include "foo.h"
void main()
{
    foo();
}

I am using Visual Studio 2005 (for compatibility with a big commercial program that only accepts plugins made with this version). The problem is that FooTest.exe does not recompile when foo.lib is updated. I make a change to foo.cpp, save, build. Then build the fooTest.exe solution. The fooTest.exe solution does not seem to understand that its dependency has been changed.

I can get around this problem by adding the linker dependency to FooTest.exe but that defeats the entire point of pragma comment lib.

I have read many posts about pragma comment lib and figure it is working for lots of people. In my overall codebase and my test it is not working. I must be missing something.

Scott

Scott Johnson
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2 Answers2

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The comment lib pragma inserts a linker directive. The linker sees this and adds that library to its list of inputs. That is all it does. What you want is a project dependency, which you seem to know how to configure.

Visual Studio 2005 pragma comment documentation

Khouri Giordano
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This is a known limitation of MSBuild (the build engine behind VS2010+ versions) and most probably of VCBuild (the engine in VS2005 builds), and really to every reasonable build engine:

Dependency analysis receives as input only the project files - either vcproj, vcxproj, make files or whatever. If you inject the dependency into the source files (with #pragma lib), you are making it invisible to the build system altogether, thereby bypassing its dependency analysis and preventing a needed build when the dependency changes.

It is good practice to use pragma-comment-lib to express dependencies only on very stable components. If you occasionally do need to respond to changes in the component, pragma lib is just not the tool for you.

Ofek Shilon
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