Importc pragma

The importc pragma provides a means to import a proc or a variable from C. The optional argument is a string containing the C identifier. If the argument is missing, the C name is the Nim identifier exactly as spelled:

proc printf(formatstr: cstring) {.header: "<stdio.h>", importc: "printf", varargs.}

When importc is applied to a let statement it can omit its value which will then be expected to come from C. This can be used to import a C const:

{.emit: "const int cconst = 42;".}
let cconst {.importc, nodecl.}: cint
assert cconst == 42

Note that this pragma has been abused in the past to also work in the js backend for js objects and functions. : Other backends do provide the same feature under the same name. Also, when the target language is not set to C, other pragmas are available:

• importcpp
• importobjc
• importjs

proc p(s: cstring) {.importc: "prefix$1".}

In the example the external name of p is set to prefixp. Only $1 is available and a literal dollar sign must be written as $$.