エラー

C で書いた関数を Nim で使うとき

add.c

int addInts(int a, int b) {
    return a + b;
}

add.nim

{.compile "add.c".}
proc addInts(a, b: cint): cint {.importc.}

when isMainModule:
    echo addInts(3, 7)

みたいにすると

$ nim c -r add.nim
Hint: used config file '/etc/nim.cfg' [Conf]
Hint: system [Processing]Hint: add [Processing]
CC: add
CC: add
CC: stdlib_system
Hint:  [Link]/home/username/Develop/Nim/nimcache/add.o: In function `PreMainInner':
add.c:(.text+0x1b): multiple definition of `PreMainInner'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0x1b): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `addDatInit000':
add.c:(.text+0x26a): multiple definition of `addDatInit000'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0x26a): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `PreMain':
add.c:(.text+0x2c): multiple definition of `PreMain'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0x2c): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `NimMainInner':
add.c:(.text+0x7c): multiple definition of `NimMainInner'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0x7c): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `NimMainModule':
add.c:(.text+0x1b1): multiple definition of `NimMainModule'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0x1b1): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `NimMain':
add.c:(.text+0x88): multiple definition of `NimMain'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0x88): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `main':
add.c:(.text+0xd8): multiple definition of `main'
/home/username/Develop/Nim/nimcache/add.o:add.c:(.text+0xd8): first defined here
/home/username/Develop/Nim/nimcache/add.o: In function `NimMainModule':
add.c:(.text+0x225): undefined reference to `addInts'
/home/username/Develop/Nim/nimcache/add.o: In function `NimMainModule':
add.c:(.text+0x225): undefined reference to `addInts'
collect2: error: ld returned 1 exit status
Error: execution of an external program failed: 'gcc   -o /home/username/Develop/Nim/add  /home/username/Develop/Nim/nimcache/add.o /home/username/Develop/Nim/nimcache/add.o /home/username/Develop/Nim/nimcache/stdlib_system.o    -ldl'

エラーが出る。

解決策

*.nim と *.c の名前は同じにできないらしい。
add.c の名前を logic.c に、
add.nim を calc.nim に変え、

calc.nim

{.compile: "logic.c".}
proc addInts(a, b: cint): cint {.importc.}

when isMainModule:
    echo addInts(3, 7)

のように修正すると

$ nim c -r calc.nim
Hint: used config file '/etc/nim.cfg' [Conf]
Hint: system [Processing]
Hint: calc [Processing]
CC: logic
CC: calc
CC: stdlib_system
Hint:  [Link]
Hint: operation successful (11721 lines compiled; 0.821 sec total; 22.008MiB peakmem; Debug Build) [SuccessX]
Hint: /home/username/Develop/Nim/calc  [Exec]
10

動いた。

参考

Nim Backend Integration - https://nim-lang.org/docs/backends.html
Multiple definition errors #1059 - https://github.com/nim-lang/Nim/issues/1059