Can't spawn "cmd.exe":

emalossi has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Can't spawn "cmd.exe": by almut (Canon) on Mar 18, 2009 at 04:35 UTC
If the program does run and produce normal output, the message might be due to an incorrect interpretation of its status/return code¹. Maybe some variation of the issue discussed in What error is "No Error", where the problem seems to have been a negative return code of the program in question. What return code do you get when you run the same command interactively from the command line? ___ ¹ this is the respective snippet from Perl's `win32.c` where the warning (presumably) originates from: `if (status < 0) { if (ckWARN(WARN_EXEC)) Perl_warner(aTHX_ packWARN(WARN_EXEC), "Can't spawn \"%s\": %s +", argv[0], strerror(errno)); status = 255 * 256; }` [download]	[reply] [d/l] [select]
Re^2: Can't spawn "cmd.exe": by emalossi (Novice) on Mar 18, 2009 at 14:26 UTC
almut, I think you've hit the nail. I ran this manually from the CLI and then did echo %ERRORLEVEL% and the value returned was -1. Is this the proper way to check this? I wonder if you could remind me, in windows what would be the "normal" return code, 0 or 1? That's a very strange error message to return in the event of a negative return code. Not just meaningless but confusing as well. Is the reason why it is seen when run from perl but not when run directly in the CLI because cmd doesn't pass warnings, but perl does? Thanks !!	[reply]
Re^3: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 15:32 UTC
in windows what would be the "normal" return code, 0 or 1? 0 for success, like unix. then did echo %ERRORLEVEL% and the value returned was -1. That's weird. Error codes are unsigned in Win32. `IF ERRORLEVEL` even relies on that. `@echo off program.exe if errorlevel 3 goto ERROR if errorlevel 1 goto WARNING rem 0 <= errorlevel < 1 echo Success goto END :WARNING rem 1 <= errorlevel < 3 echo Warning goto END :ERROR rem 3 <= errorlevel echo Error :END` [download] That's a very strange error message to return in the event of a negative return code. No, the message is correct for a negative code being returned from the function that implements `system`. The problem is that the function is returning negative when the child was successfully launched. In Win32, exit codes are unsigned 32 bit values. Perl treats it as a signed 32-bit value. `DWORD status; ... GetExitCodeProcess(ProcessInformation.hProcess, &status); ret = (int)status;` [download] The second line is a bug. Sufficiently high exit codes will be converted to negative. Negatives are used to signal a problem launching the process.	[reply] [d/l] [select]
Re^4: Can't spawn "cmd.exe": by BrowserUk (Patriarch) on Mar 19, 2009 at 14:16 UTC
Re^3: Can't spawn "cmd.exe": by runrig (Abbot) on Mar 18, 2009 at 15:23 UTC
"Normal" return code seems to be zero. E.g., do a "dir" and then echo %ERRORLEVEL%. Now do "dir file_that_doesnt_exist" and echo %ERRORLEVEL% again. Now do " `perl -e exit(-1)`".	[reply] [d/l]
Re^3: Can't spawn "cmd.exe": by almut (Canon) on Mar 18, 2009 at 15:51 UTC
echo %ERRORLEVEL% ... Is this the proper way to check this? Yes, I think so, but I'm no Windows expert... As to the negative return code, IIRC, I was able to reproduce the issue last time with a self-compiled perl-5.8.8 on a Win XP (32-bit) vmware image. Unfortunately, I don't have access to that image at the moment, and any attempts to reproduce the problem on the Windows box I currently have access to (XP x64 SP2, with ActiveState's Perl 5.8.8) have failed so far... Maybe I'll continue to play with this later. In case I should dig up something of interest, I'll report back.	[reply]
Re: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 04:44 UTC
In this case I'm not even asking it to start cmd.exe, but yet it's telling me that it can't spawn it. A command that requires the shell (such as one contains quotes) causes Perl to invoke the shell. This and related details is documented. "Can't spawn "cmd": Bad file descriptor at temp2.plx line 11." Are you checking `$!` to see if an error occurred? That would be incorrect. `.plx`? Do you get the error without whatever packager you are using?	[reply] [d/l] [select]
Re^2: Can't spawn "cmd.exe": by emalossi (Novice) on Mar 18, 2009 at 14:30 UTC
Hi, I'm not sure what you mean by packager. temp2.plx is just a perl script. As I mentioned above, I don't get the error when that program is run directly in the cmd CLI.	[reply]
Re: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 15:21 UTC
Independent of the problem you are reporting, your usage of `system` (as follows) is wrong. `my $test = "/C \"C:\\...\\EXCEL.EXE\" C:\\test.xls"; system ("cmd",$test);` [download] You can either pass the whole command as one argument: `my $test = "/C \"C:\\...\\EXCEL.EXE\" C:\\test.xls"; system ("cmd $test");` [download] or pass the arguments separately (note the lack of quotes): `my @test= ("/C", "C:\\...\\EXCEL.EXE", "C:\\test.xls"); system ("cmd", @test);` [download] You are currently relying on a bug.	[reply] [d/l] [select]
Re^2: Can't spawn "cmd.exe": by BrowserUk (Patriarch) on Mar 18, 2009 at 16:54 UTC
Could you expand a little on the nature of the bug? The current behaviour seems harmless; maybe even useful. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply]
Re^3: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 17:07 UTC
`system` documents that the value of `$test` in `system ("cmd",$test);` will be passed as a single argument. If `system` behaved as documented, it would execute `cmd "/C excel.exe text.xls"` [download] `cmd` does not accept that.	[reply] [d/l] [select]
Re^4: Can't spawn "cmd.exe": by BrowserUk (Patriarch) on Mar 18, 2009 at 18:36 UTC
Re^5: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 19:52 UTC
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Re: Can't spawn "cmd.exe": by Sandy (Curate) on Mar 18, 2009 at 14:50 UTC
I'm not sure if this is relevant, but check the following node: Re: system command erroneously states 'can't spawn <executable>' from jimbojones The code (from 5.8.9) win32.c that might be causing the problem int Perl_do_aspawn(pTHX_ SV really, SV mark, SV sp) { char argv; char str; int status; int flag = P_WAIT; int index = 0; if (sp <= mark) return -1; get_shell(); Newx(argv, (sp - mark) + w32_perlshell_items + 2, char); if (SvNIOKp((mark+1)) && !SvPOKp((mark+1))) { ++mark; flag = SvIVx(mark); } while (++mark <= sp) { if (mark && (str = SvPV_nolen(mark))) argv[index++] = str; else argv[index++] = ""; } argv[index++] = 0; status = win32_spawnvp(flag, (const char)(really ? SvPV_nolen(really) : argv[0]), (const char const)argv); if (status < 0 && (errno == ENOEXEC \|\| errno == ENOENT)) { / possible shell-builtin, invoke with shell / int sh_items; sh_items = w32_perlshell_items; while (--index >= 0) argv[index+sh_items] = argv[index]; while (--sh_items >= 0) argv[sh_items] = w32_perlshell_vec[sh_items]; status = win32_spawnvp(flag, (const char)(really ? SvPV_nolen(really) : argv[0] +), (const char* const)argv); } if (flag == P_NOWAIT) { PL_statusvalue = -1; / >16bits hint for pp_system() / } else { if (status < 0) { if (ckWARN(WARN_EXEC)) Perl_warner(aTHX_ packWARN(WARN_EXEC), "Can't spawn \"%s\": %s +", argv[0], strerror(errno)); status = 255 256; } else status = 256; PL_statusvalue = status; } Safefree(argv); return (status); } [download] Notice that if the executable returns a negative error code, the assumption is that the spawn* didn't work.	[reply] [d/l]
Re: Can't spawn "cmd.exe": by BrowserUk (Patriarch) on Mar 18, 2009 at 11:57 UTC
Thinking that this might be some weird sort of 32/64 bit issue, ... I can't reproduce your errors using AS1004 Perl 64-bit running under Vista 64-bit. `#! /usr/bin/perl -slw use strict; print "About to try fail 1"; my $testfile = "test.exe"; my $test = "/C ${testfile} localhost root pwd"; system ("cmd", $test); print "About to try fail 2"; $test = "${testfile} localhost root pwd"; system ($test);` [download] In the following `test.exe` just displays its args: `c:\test\test\test>test.pl About to try fail 1 0 : test.exe 1 : localhost 2 : root 3 : pwd About to try fail 2 0 : test.exe 1 : localhost 2 : root 3 : pwd` [download] In the first case Perl starts cmd.exe, which runs test.exe. In the second, perl runs test.exe directly. No error messages are produced by either. What version/build of Perl are you using? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l] [select]
Re^2: Can't spawn "cmd.exe": by emalossi (Novice) on Mar 18, 2009 at 14:33 UTC
Browser, It's 5.8.8. I think almut is on to something in his reply concerning negative return codes. Can you verify this by returning -1 from your test executable?	[reply]
Re^3: Can't spawn "cmd.exe": by BrowserUk (Patriarch) on Mar 18, 2009 at 16:45 UTC
Indeed I can: `C:\test\test\test>test.pl About to try fail 1 0 : test.exe 1 : localhost 2 : root 3 : pwd Can't spawn "cmd": No error at C:\test\test\test\test.pl line 7. About to try fail 2 0 : test.exe 1 : localhost 2 : root 3 : pwd Can't spawn "test.exe localhost root pwd": No error at C:\test\test\te +st\test.pl line 11.` [download] Same result whether the return value is -1 or -100_000. The problem is with Perl's handling of Windows return codes. Trying to force fit a 32-bit signed int into an unsigned char doesn't go :) As for what constitutes a "valid windows return code"--a signed integer covers it. Long ago, there was a convention that positive return codes were 'informational', negative 'failures', with zero being unadorned success. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]
Re^4: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 17:21 UTC
Re^5: Can't spawn "cmd.exe": by BrowserUk (Patriarch) on Mar 18, 2009 at 23:11 UTC
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Re: Can't spawn "cmd.exe": by ikegami (Patriarch) on Mar 18, 2009 at 04:45 UTC
In this case I'm not even asking it to start cmd.exe, but yet it's telling me that it can't spawn it. A command that requires the shell (such as one contains quotes) causes Perl to invoke the shell. This and related details is documented. "Can't spawn "cmd": Bad file descriptor at temp2.plx line 11." Are you checking `$!` to see if an error occurred? That would be incorrect. `.plx`? Do you get the error without whatever packager you are using?	[reply] [d/l] [select]
Re: Can't spawn "cmd.exe": by Anonymous Monk on Mar 18, 2009 at 04:00 UTC
You don't show any error checking, where are you seeing these errors?	[reply]
Re: Can't spawn "cmd.exe": by Anonymous Monk on Jul 17, 2012 at 04:10 UTC
For me returning -1 from the called script causes the problem. Returning other values i.e -2 solves the problem.	[reply]
Re^2: Can't spawn "cmd.exe": by Anonymous Monk on Sep 19, 2013 at 06:54 UTC
I have a script where -1 is being returned from a lot of places which can not be changed. I have used "system" to call that script. Can anyone suggest how to get it working as I am getting the same can't spawn warning which is breaking my other logic. Thanks in advance	[reply]
Re^3: Can't spawn "cmd.exe": by Anonymous Monk on Sep 19, 2013 at 07:22 UTC
no, not without details (code)	[reply]