Why does using popen result in Error: Failed to close command stream under windows?
You actually have to use: _popen and _pclose (yes WITH the silly underscore) under Windows.
See MSDN Entry on it; it has a nice example
Now as for _popening "ps" you know whether or not you have a ps on your system.
Related
i have a C program that opens an mp3 and extract the jpg artwork in the same folder. If i execute this program with no root privileges i get a crash. If i execute it with sudo it works normally.
Now, i need another C programs who launch the previous program when it needs a jpg artwork for the selected mp3.
I tried to call popen("./firstProgram test.mp3" , "r") function or system("/(absolute path)/firstProgram test.mp3") function by calling them even with sudo in the command or not and either with relative or absolute paths. But no version seems to work.
How can i launch the first program from the second one with success?
Thanks!
fork and then use execl
char sudo[]="/usr/bin/sudo";
char pbin[]="/usr/local/bin/puppet";
NOTICE("running puppet: %s %s",sudo,pbin);
errno=0;
execl(sudo,sudo,pbin,(char *)NULL);
/* we should never get as far as this */
obviously I recommend reading man execl for further info
Unix (Linux) systems have contained a C Programming Manual in them since possibly forever. Look in Section 2, "System Calls".
This Wikipedia Page explains the Unix Manual "sections"
It is section 2 of the manual you can read about "System Calls"
Try the command: man 2 setuid
This will give you the manual for the setuid() system call which I think is what you want.
That manual page will also list references to other related system calls that may be what you want.
Remember when compiling C programs and using system calls that do low-level hardware access, to use the -O2, or -O3 option to gcc. There is a mention of it in the manual.
Ultimately the setuid() system call makes a running process started by one user change the UID of that running process to be running as some other user. (For example, you may see the Apache running as "apache", even though it was started by root).
setuid(0) lets you be root.
I 'm reading coreutils source code to learn programming under linux.
I find that in most of the programs like ls.c, cat.c, they invoke the macro function initialize_main() at the first few lines. So I looked into system.h to find the implementation:
/* Redirection and wildcarding when done by the utility itself.
Generally a noop, but used in particular for native VMS. */
#ifndef initialize_main
# define initialize_main(ac, av)
#endif
I don't understand why define such a macro and I don't understand the comment.
The first step in understanding the comment is to know what VMS is. So here's a link for that:
http://en.wikipedia.org/wiki/OpenVMS
The next step is to understand redirection and wildcarding. In Linux and other members of the unix family, a command like
cat foo* > /tmp/foolist
will call the main function of cat with argv containing the matches for foo*. The output file /tmp/foolist will already be open as stdout before main is entered.
VMS doesn't do that. cat will find the unexpanded string "foo*" and the redirection operator > in its argv. So the utility itself (cat) must do the redirection (opening the output file) and wildcarding (replacing "foo*" with "foo1", "foo2", "foo3"). That's what initialize_main will do on VMS. On unix, it'll do nothing ("Generally a noop").
This is left over from times gone by. OpenVMS is an operating system which roughly competed with Unix in the past. There is still a fair amount of OpenVMS running in the world, but HP have dropped support for it and it will be going away in the next 10-15 years.
Anyway, this function is used on OpenVMS to allow stdout and stderr redirection on VMS.
Since cat foo.txt > stuff.txt on Unix, the cat command only sees one argument foo.txt, but on VMS, which knows nothing of the > symbol, the cat command sees 3 arguments.
The code inside initialize_main on VMS, allows the basic unix style commands to support output redirection, such as ls and
OpenVMS later added a command called pipe which allows redirection to work via any command.
You can view the source code for initialize_main on VMS here: Link
I have tried both system() and popen(). When I run the program on cygwin terminal it works perfectly fine but when I try to run it on windows platform by double clicking on .exe file I get this error:
exception::handle: Exception: STATUS_ACCESS_VIOLATION
I get this error on popen() however I do not get any error for system().
I am running simple commands that work on both unix and windows OSs such as:
system("echo foo>foo.txt");
What am I doing wrong here?
I think that's because both system() and popen() pass their arguments to the /bin/sh which you don't have in the path, you could try adding it to the path first. According to this you need the following in your path:
cyggcc_s-1.dll
cygiconv-2.dll
cygintl-8.dll
cygncursesw-10.dll
cygreadline7.dll
cygwin1.dll
ls.exe
sh.exe
I need to make a debug shell inside each c exe(linux enviroment), and my solution is as follows:
Read elf symbols from exe file, build a symbol->address table in
memory;
Run a thread calling readline to accept user input, some thing
like a c function call;
use Lex & yacc to parse the function name and arg list;
Find address of the function in the symbol table;
Call the function with args list;
Every function written can be input as shell command instantly.
I don't think this is a fresh idea, and my question is: Are there any mature codes implemented already?
Thanks for your help!
Sure. If you had working with VxWorks, you'll find WindShell is what you're looking for. I had port a similar shell to Linux. You can download the source from:
https://sourceforge.net/projects/zprj/
Note: don't use the source in commercial products, since they are ported from WindShell. If you do want a shell in commercial fields, then you shall develop one with LEX/YACC.
I'm just starting programming and going through K&R to try and learn C. I've gotten to the section on command line arguments (5.10) but now I'm stumped. Every time I try and open a program I've written with command line arguments I'm told that file X, X being the argument, doesn't exist.
`gcc -o find find.c
open find test
The file /Documents/Learning_C/test does not exist.`
Any suggestions? Thanks
What system are you on? In Unix/Linux you compile & run your executable via:
gcc -o find find.c
./find test
As others have noted, when you prefix your binary with "./", there wont be any naming conflicts. However, if you made find accessible in your $PATH, you might have some conflicts with find and test--standard programs with most *nix distributions... Maybe you could choose more specific names (i.e. ./myFind testArg)
Try giving your output executable a different name.
I suspect your executing the system find command which is looking for a directory called 'test'.
Or try forcing it by executing
./find toto
Edit: Prepending the ./ to the command is important because it tells the shell to execute the find in the current directory as opposed to the first 'find' that exists in your PATH. It is normally recommended that you don't have . (the current directory) in your PATH for security reasons.
HTH
P.S. Forgot to say good one for working through K&R. I just finished doing the same after working in C for thirty years and it was good to get back and refresh the mind!
Instead of making us all individually guess what exactly you're doing wrong, perhaps you should paste the program you're using for the illustration mentioned ?