調査手順

[atoi関数が含まれる、*.aファイルを列挙]
>nm -A -C -g -P "C:\Program Files\MinGW\lib\*.a" | find " T " | find "atoi"

[コマンドの実行結果]
C:\Program Files\MinGW\lib\libcrtdll.a[deqns00340.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr70.a[dgnss00598.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr70.a[dgnss00144.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr70d.a[dgpjs00598.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr70d.a[dgpjs00144.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr71.a[dwvis00600.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr71.a[dwvis00146.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr71d.a[dmals00600.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr71d.a[dmals00146.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr80.a[dwzss00628.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr80.a[dwzss00146.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr80d.a[dykrs00628.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr80d.a[dykrs00146.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr90.a[daqts00628.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr90.a[daqts00146.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcr90d.a[dyuts00629.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcr90d.a[dyuts00146.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcrt.a[duyms00592.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcrt.a[duyms00139.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libmsvcrtd.a[dsdqs00592.o]: atoi T 00000000
C:\Program Files\MinGW\lib\libmsvcrtd.a[dsdqs00139.o]: _atoi64 T 00000000
C:\Program Files\MinGW\lib\libntdll.a[dqkgs00944.o]: atoi T 00000000

[最初にヒットしたlibcrtdll.aを展開]
>ar -x libcrtdll.a

[libcrtdll.aに含まれるdeqns00340.oを逆アセンブラ]
>objdump -D deqns00340.o

[コマンドの実行結果]

deqns00340.o: file format pe-i386

Disassembly of section .text:

00000000 <_atoi>:
0: ff 25 00 00 00 00 jmp *0x0
6: 90 nop
7: 90 nop

Disassembly of section .idata$7:

00000000 <.idata$7>:
0: 00 00 add %al,(%eax)
...

Disassembly of section .idata$5:

00000000 <__imp__atoi>:
0: 00 00 add %al,(%eax)
...

Disassembly of section .idata$4:

00000000 <.idata$4>:
0: 00 00 add %al,(%eax)
...

Disassembly of section .idata$6:

00000000 <.idata$6>:
0: 54 push %esp
1: 01 61 74 add %esp,0x74(%ecx)
4: 6f outsl %ds:(%esi),(%dx)
5: 69 .byte 0x69
...