importc

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{.interpreted-text role="c"}:

{.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 $$.