Reading return value from C program

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

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Re: Reading return value from C program by jdporter (Paladin) on Mar 15, 2007 at 17:59 UTC
On most operating systems like Windows and UNIX, one program can call (execute) another. When the called program ends, it returns to the calling program a status code, which is an integer. (Normally, a code of 0 indicates success.) If the called program is written in C, it returns this status code via the return value of the `main()` function. Other languages do it differently. In Perl, you can do it by giving an argument to the `exit` function. When the calling program is written in Perl, and the called program is executed using the `system` function, the return value of that function is (or rather, contains) the status code returned by the called program. So you can do `my $child_code = system( "other_program" );` [download] There are other ways to execute another program (such as the `qx` operator), and they don't return the child's status code quite so conveniently, so in those cases you look for it in the special `$?` variable. E.g.: `my $result_text = qx( other_program ); my $child_code = $?;` [download] A word spoken in Mind will reach its own level, in the objective world, by its own weight	[reply] [d/l] [select]
Re: Reading return value from C program by ikegami (Patriarch) on Mar 15, 2007 at 17:57 UTC
If you launched the program using `system`, the child's exit code is returned by `system`. It is also placed in `$?`. See the example in the docs on how to do error handling and on how to extract the exit code. If the child was launched asynchronously, `waitpid` will place the exit code in `$?`.	[reply] [d/l] [select]
Re: Reading return value from C program by Anno (Deacon) on Mar 15, 2007 at 17:56 UTC
How does your C program return an integer? Does it print it to the screen? Return it as its exit value? Something else? The answer to your question depends entirely on that. As a Perl expression, `!$?` is the logical negation of the value of the system variable `$?`. If you are in fact after the exit value of a program, this (the variable, not its negation) would be the place to look, but read about `$?` in `perldoc perlvar`. Anno	[reply] [d/l] [select]
Re^2: Reading return value from C program by Anonymous Monk on Mar 25, 2007 at 07:06 UTC
I got my answer. We can use "$?>>8" to get the read the return value, however it's returning only positive value.Any one knows how to read the negative values as well	[reply]
Re^3: Reading return value from C program by Anno (Deacon) on Mar 25, 2007 at 12:31 UTC
You can interpret the byte as a signed byte: `my $exit_code = $? >> 8; $exit_code -= 256 if $exit_code > 127;` [download] But that won't tell you whether the program has specified its exit code as a negative number. All you get is the least significant eight bytes of the value. Anno	[reply] [d/l]
Re: Reading return value from C program by agianni (Hermit) on Mar 15, 2007 at 17:57 UTC
You might find this thread insightful. As for !$?, $? is a Perl special variable for child error. So if your script forks a child process and there is an error, this will contain that error. ! is the not operator. I'm guessing you're seeing something like: `if ( !$? ){ # do the stuff you do after a forked process completes sucessfully }` [download] But that would be much easier to read as: `use English qw(no_match_vars); if ( not $CHILD_ERROR ){ # do the stuff you do after a forked process completes sucessfully }` [download] Updated comments to reflect meaning of $?	[reply] [d/l] [select]
Re^2: Reading return value from C program by ikegami (Patriarch) on Mar 15, 2007 at 18:00 UTC
`fork` sets `$!`, not `$?`.	[reply] [d/l] [select]
Re^3: Reading return value from C program by Anno (Deacon) on Mar 15, 2007 at 18:20 UTC
agianni's code wouldn't make sense after a `fork()`, that's true. It would make sense after a `waitpid`. Maybe that was the tacit assumption. It would have been better made explicit. Anno	[reply] [d/l] [select]
Re^4: Reading return value from C program by ikegami (Patriarch) on Mar 15, 2007 at 19:08 UTC
Re^5: Reading return value from C program by agianni (Hermit) on Mar 15, 2007 at 20:40 UTC