use emit_error in ccall when library-not-found

[timholy]

It seems there are some bumps in writing code that issues ccalls conditionally on the availability of a C library, see https://groups.google.com/forum/?fromgroups=#!topic/julia-users/O2lBbCaJ99w

The solution worked out there seem likely to be fragile with respect to improvements in inlining. The following seems safe:

const libfrobozz = find_library("libfrobozz")
const have_frobozz = !isempty(libfrobozz)

function somefunction()
    if have_frobozz
        eval(:(ccall((:somefunction, $libfrobozz), rettype, argtypes, args...))
    else
        # do something else
    end
end

But that also seems a bit undesirable. Is there a better way?

[vtjnash]

ccall already handles the case where :somefunction is not found. it just needs to better handle the case where libfrobozz could not be found

[yuyichao]

Is this still an issue? All of the following behavior seems reasonable to me?

julia> const libfrobozz = Libdl.find_library("libfrobozz")
""

julia> const have_frobozz = !isempty(libfrobozz)
false

julia> function somefunction()
           if have_frobozz
               eval(:(ccall((:somefunction, $libfrobozz), Void, ())))
           end
       end
somefunction (generic function with 1 method)

julia> function somefunction2()
           if have_frobozz
               ccall((:somefunction, libfrobozz), Void, ())
           end
       end
somefunction2 (generic function with 1 method)

julia> function somefunction3()
           ccall((:somefunction, libfrobozz), Void, ())
       end
somefunction3 (generic function with 1 method)

julia> somefunction()

julia> somefunction2()

julia> somefunction3()
ERROR: ccall: could not find function somefunction in library 
 in somefunction3() at ./REPL[5]:2
 in eval(::Module, ::Any) at ./boot.jl:228

Unless the issue is to make ccall emit an error for empty library string?

[gbaraldi]

This regressed into segfaulting in optimizeDLSyms @pchintalapudi
updated reproducert

using Libdl

const libfrobozz = Libdl.find_library("libfrobozz")
const have_frobozz = !isempty(libfrobozz)

function somefunction()
    if have_frobozz
        eval(:(ccall((:somefunction, $libfrobozz), Cvoid, ())))
    end
end

function somefunction2()
    if have_frobozz
        ccall((:somefunction, libfrobozz), Cvoid, ())
    end
end

function somefunction3()
    ccall((:somefunction, libfrobozz), Cvoid, ())
end

somefunction()

somefunction2()

somefunction3()