WinExec Return 0x21, But what exactly it means? - windows

Question

When call WinExec to run a .exe, I get return value 0x21.
According to MSDN, a return value greater than 31 (0x1F) means function succeeds.
But what does it mean of 0x21, Why it didn't return other value to me?

Answer 1

It is not useful for you to know what it means. That is an implementation detail. Even if you knew what it meant for this version, the meaning might change in the next version. As a programmer, you are concerned only with programming against the interface, not the underlying implementation.
However, if you are really interested, I will take you through the approach I would take to reverse engineer the function. On my system, WinExec is disassembled to this:
764F2C21 > 8BFF MOV EDI,EDI
764F2C23 55 PUSH EBP
764F2C24 8BEC MOV EBP,ESP
764F2C26 81EC 80000000 SUB ESP,80
764F2C2C 53 PUSH EBX
764F2C2D 8B5D 0C MOV EBX,DWORD PTR SS:[EBP+C]
764F2C30 56 PUSH ESI
764F2C31 57 PUSH EDI
764F2C32 33FF XOR EDI,EDI
764F2C34 47 INC EDI
764F2C35 33F6 XOR ESI,ESI
764F2C37 85DB TEST EBX,EBX
764F2C39 79 4F JNS SHORT kernel32.764F2C8A
764F2C3B 8D45 FC LEA EAX,DWORD PTR SS:[EBP-4]
764F2C3E 50 PUSH EAX
764F2C3F 56 PUSH ESI
764F2C40 57 PUSH EDI
764F2C41 8D45 C8 LEA EAX,DWORD PTR SS:[EBP-38]
764F2C44 50 PUSH EAX
764F2C45 C745 FC 20000000 MOV DWORD PTR SS:[EBP-4],20
764F2C4C E8 90BE0200 CALL <JMP.&API-MS-Win-Core-ProcessThread>
764F2C51 85C0 TEST EAX,EAX
764F2C53 0F84 D2000000 JE kernel32.764F2D2B
764F2C59 56 PUSH ESI
764F2C5A 56 PUSH ESI
764F2C5B 6A 04 PUSH 4
764F2C5D 8D45 F8 LEA EAX,DWORD PTR SS:[EBP-8]
764F2C60 50 PUSH EAX
764F2C61 68 01000600 PUSH 60001
764F2C66 56 PUSH ESI
764F2C67 8D45 C8 LEA EAX,DWORD PTR SS:[EBP-38]
764F2C6A 50 PUSH EAX
764F2C6B C745 0C 00000800 MOV DWORD PTR SS:[EBP+C],80000
764F2C72 897D F8 MOV DWORD PTR SS:[EBP-8],EDI
764F2C75 E8 5CBE0200 CALL <JMP.&API-MS-Win-Core-ProcessThread>
764F2C7A 85C0 TEST EAX,EAX
764F2C7C 0F84 95000000 JE kernel32.764F2D17
764F2C82 8D45 C8 LEA EAX,DWORD PTR SS:[EBP-38]
764F2C85 8945 C4 MOV DWORD PTR SS:[EBP-3C],EAX
764F2C88 EB 03 JMP SHORT kernel32.764F2C8D
764F2C8A 8975 0C MOV DWORD PTR SS:[EBP+C],ESI
764F2C8D 6A 44 PUSH 44
764F2C8F 8D45 80 LEA EAX,DWORD PTR SS:[EBP-80]
764F2C92 56 PUSH ESI
764F2C93 50 PUSH EAX
764F2C94 E8 B5E9F7FF CALL <JMP.&ntdll.memset>
764F2C99 83C4 0C ADD ESP,0C
764F2C9C 33C0 XOR EAX,EAX
764F2C9E 3975 0C CMP DWORD PTR SS:[EBP+C],ESI
764F2CA1 897D AC MOV DWORD PTR SS:[EBP-54],EDI
764F2CA4 0F95C0 SETNE AL
764F2CA7 66:895D B0 MOV WORD PTR SS:[EBP-50],BX
764F2CAB 8D0485 44000000 LEA EAX,DWORD PTR DS:[EAX*4+44]
764F2CB2 8945 80 MOV DWORD PTR SS:[EBP-80],EAX
764F2CB5 8D45 E8 LEA EAX,DWORD PTR SS:[EBP-18]
764F2CB8 50 PUSH EAX
764F2CB9 8D45 80 LEA EAX,DWORD PTR SS:[EBP-80]
764F2CBC 50 PUSH EAX
764F2CBD 56 PUSH ESI
764F2CBE 56 PUSH ESI
764F2CBF FF75 0C PUSH DWORD PTR SS:[EBP+C]
764F2CC2 56 PUSH ESI
764F2CC3 56 PUSH ESI
764F2CC4 56 PUSH ESI
764F2CC5 FF75 08 PUSH DWORD PTR SS:[EBP+8]
764F2CC8 56 PUSH ESI
764F2CC9 E8 A4E3F7FF CALL kernel32.CreateProcessA
764F2CCE 85C0 TEST EAX,EAX
764F2CD0 74 27 JE SHORT kernel32.764F2CF9
764F2CD2 A1 3C005476 MOV EAX,DWORD PTR DS:[7654003C]
764F2CD7 3BC6 CMP EAX,ESI
764F2CD9 74 0A JE SHORT kernel32.764F2CE5
764F2CDB 68 30750000 PUSH 7530
764F2CE0 FF75 E8 PUSH DWORD PTR SS:[EBP-18]
764F2CE3 FFD0 CALL EAX
764F2CE5 FF75 E8 PUSH DWORD PTR SS:[EBP-18]
764F2CE8 8B35 A0054776 MOV ESI,DWORD PTR DS:[<&ntdll.NtClose>] ; ntdll.ZwClose
764F2CEE FFD6 CALL ESI
764F2CF0 FF75 EC PUSH DWORD PTR SS:[EBP-14]
764F2CF3 FFD6 CALL ESI
764F2CF5 6A 21 PUSH 21
764F2CF7 EB 1D JMP SHORT kernel32.764F2D16
764F2CF9 E8 C9E4F7FF CALL <JMP.&API-MS-Win-Core-ErrorHandling>
764F2CFE 48 DEC EAX
764F2CFF 48 DEC EAX
764F2D00 74 12 JE SHORT kernel32.764F2D14
764F2D02 48 DEC EAX
764F2D03 74 0B JE SHORT kernel32.764F2D10
764F2D05 2D BE000000 SUB EAX,0BE
764F2D0A 75 0B JNZ SHORT kernel32.764F2D17
764F2D0C 6A 0B PUSH 0B
764F2D0E EB 06 JMP SHORT kernel32.764F2D16
764F2D10 6A 03 PUSH 3
764F2D12 EB 02 JMP SHORT kernel32.764F2D16
764F2D14 6A 02 PUSH 2
764F2D16 5E POP ESI
764F2D17 F745 0C 00000800 TEST DWORD PTR SS:[EBP+C],80000
764F2D1E 74 09 JE SHORT kernel32.764F2D29
764F2D20 8D45 C8 LEA EAX,DWORD PTR SS:[EBP-38]
764F2D23 50 PUSH EAX
764F2D24 E8 A2BD0200 CALL <JMP.&API-MS-Win-Core-ProcessThread>
764F2D29 8BC6 MOV EAX,ESI
764F2D2B 5F POP EDI
764F2D2C 5E POP ESI
764F2D2D 5B POP EBX
764F2D2E C9 LEAVE
764F2D2F C2 0800 RETN 8
The call convention used under Win32 is stdcall which mandates return values be held in EAX. In the case of WinExec, there is only one exit from the function (0x764F2D2F). Tracing back from there, EAX is set by (at least when the return is 0x21):
764F2D29 8BC6 MOV EAX,ESI
Tracing back further, ESI itself is set from POP ESI which pops the top of the stack into ESI. The value of this is dependent on what was previously pushed on the stack. In the case of 0x21, this happens at:
764F2CF5 6A 21 PUSH 21
Immediately afterwards, a JMP is made to the POP ESI. How we got to the PUSH 21 is interesting only from after the CreateProcess call.
764F2CC9 E8 A4E3F7FF CALL kernel32.CreateProcessA
764F2CCE 85C0 TEST EAX,EAX
764F2CD0 74 27 JE SHORT kernel32.764F2CF9
764F2CD2 A1 3C005476 MOV EAX,DWORD PTR DS:[7654003C]
764F2CD7 3BC6 CMP EAX,ESI
764F2CD9 74 0A JE SHORT kernel32.764F2CE5
764F2CDB 68 30750000 PUSH 7530
764F2CE0 FF75 E8 PUSH DWORD PTR SS:[EBP-18]
764F2CE3 FFD0 CALL EAX
764F2CE5 FF75 E8 PUSH DWORD PTR SS:[EBP-18]
764F2CE8 8B35 A0054776 MOV ESI,DWORD PTR DS:[<&ntdll.NtClose>] ; ntdll.ZwClose
764F2CEE FFD6 CALL ESI
764F2CF0 FF75 EC PUSH DWORD PTR SS:[EBP-14]
764F2CF3 FFD6 CALL ESI
764F2CF5 6A 21 PUSH 21
To see how the path will take you to the PUSH 21, observe different branches. The first occurs as:
764F2CD0 74 27 JE SHORT kernel32.764F2CF9
This is saying if CreateProcess returned 0, call Win-Core-ErrorHandling. The return values are then set differently (0x2, 0x3 and 0xB are all possible return values if CreateProcess failed).
The next branch is a lot less obvious to reverse engineer:
764F2CD9 74 0A JE SHORT kernel32.764F2CE5
What it does is read a memory address which probably contains a function pointer (we know this because the result of the read is called later on). This JE simply indicates whether or not to make this call at all. Regardless of whether the call is made, the next step is to call ZwClose (twice). Finally 0x21 is returned.
So one simple way of looking at it is that when CreateProcess succeeds, 0x21 is returned, otherwise 0x2, 0x3 or 0xB are returned. This is not to say these are the only return values. For example, 0x0 can also be returned from the branch at 0x764F2C53 (in this case, ESI is not used in the same way at all). There are a few more possible return values but I will leave those for you to look into yourself.
What I've shown you is how to do a very shallow analysis of WinExec specifically for the 0x21 return. If you want to find out more, you need to poke around more in-depth and try to understand from a higher level what is going on. You'll be able to find out a lot more just by breakpointing the function and stepping through it (this way you can observe data values).
One other way is to look at the Wine source, where someone has already done all the hard work for you:
UINT WINAPI WinExec( LPCSTR lpCmdLine, UINT nCmdShow )
{
PROCESS_INFORMATION info;
STARTUPINFOA startup;
char *cmdline;
UINT ret;
memset( &startup, 0, sizeof(startup) );
startup.cb = sizeof(startup);
startup.dwFlags = STARTF_USESHOWWINDOW;
startup.wShowWindow = nCmdShow;
/* cmdline needs to be writable for CreateProcess */
if (!(cmdline = HeapAlloc( GetProcessHeap(), 0, strlen(lpCmdLine)+1 ))) return 0;
strcpy( cmdline, lpCmdLine );
if (CreateProcessA( NULL, cmdline, NULL, NULL, FALSE,
0, NULL, NULL, &startup, &info ))
{
/* Give 30 seconds to the app to come up */
if (wait_input_idle( info.hProcess, 30000 ) == WAIT_FAILED)
WARN("WaitForInputIdle failed: Error %d\n", GetLastError() );
ret = 33;
/* Close off the handles */
CloseHandle( info.hThread );
CloseHandle( info.hProcess );
}
else if ((ret = GetLastError()) >= 32)
{
FIXME("Strange error set by CreateProcess: %d\n", ret );
ret = 11;
}
HeapFree( GetProcessHeap(), 0, cmdline );
return ret;
}
33d is 0x21 so this actually just confirms the fruits of our earlier analysis.
In regards to the reason 0x21 is returned, my guess is that perhaps there exists more internal documentation which makes it more useful in some way.

Answer 2

Other than that this means success, the meaning of the return value is not defined. Perhaps it was chosen such that legacy applications will work well with this particular value. One thing is certain: there are more important things to worry about!
http://msdn.microsoft.com/en-us/library/windows/desktop/ms687393(v=vs.85).aspx

Answer 3

EDIT: This answer is wrong because the OP's result is not an error code. I mistakenly thought it was said that it was an error code. I still think the practical info below can be useful, plus that it can be useful to see what a wrong assumption can lead to, so I let this answer stand.
If you have installed Visual Studio (full or express edition), then you have a tool called errlook, which uses the FormatMessage API function to tell you what an error code or HRESULT value means.
In this case,
The process cannot access the file because another process has locked a portion of the file.
You can do much of the same manually by looking in the <winerror.h> file. For example, type an #include of that in a C++ source file in Visual Studio, then right click and ask it to open the header. Where you find that
//
// MessageId: ERROR_LOCK_VIOLATION
//
// MessageText:
//
// The process cannot access the file because another process has locked a portion of the file.
//
#define ERROR_LOCK_VIOLATION 33L
By the way, WinExec is just an old compatibility function. Preferably use ShellExecute or CreateProcess. The ShellExecute function is able to play more nicely with Windows Vista and 7 User Access Control, and it is simpler to use, so it is generally preferable.