Wrong conversion of String to C String?

[bvdmitri]

I noticed this line of code in GraphViz.jl package, but later on a String object being used here (note the format="julia:cairo"). It doesn't seem right? The function expects C string, not Julia string? Could it be the reason for JuliaLang/julia#56150?

related PR JuliaLang/julia#51275

[bvdmitri]

I couldn't patch it and test it with the change due to #44

[bvdmitri]

Also see this comment JuliaLang/julia#56150 (comment)

[jhlq]

I found the line
ccall((:gvRenderContext,libgvc),Cint,(Ptr{Cvoid},Ptr{Cvoid},Ptr{UInt8},Ptr{Cvoid}),context.handle,g.handle,format,C_NULL)
returns -1 and prints the error Error: renderer for julia:cairo is unavailable.
By changing the lines

const julia_cairo_name = Vector{UInt8}("julia:cairo")
const julia_cairo_libname = Vector{UInt8}("julia:cairo")

to

const julia_cairo_name = Base.unsafe_convert(Cstring,"julia:cairo")
const julia_cairo_libname = Base.unsafe_convert(Cstring,"julia:cairo")

it returns 0 with no error.