how to write to an fd instead of terminal with execv()

[TaiL]

Hi guys I'm trying to write to a file descriptor with execv() except instead of writing/executing where I want it, it goes straight to terminal. here's what I'm trying :

        FILE* clnt_write;
	clnt_write = fdopen(dup(clnt_sock),"w");
	send_data(clnt_write, ct , newFile, ext);   // sending file !!!

void send_data(FILE *fp, char* ct, char* file_name, char ext[])
	if( strcmp(ext, "cgi") == 0){

		dup2(fp, 1);
		dup2(fp, 2);
		close(fp);

		char dot_slash[] = "./";
		char *dsPtr = dot_slash;
		strcat(dsPtr, file_name);
		puts(dsPtr);
	
		char *arg[2];
		arg[0] = data;	
		arg[1] = NULL;
	
		execv(dsPtr, arg);
	}

I get the following warnings:

lab2.c: In function 'send_data':
lab2.c:246: warning: passing argument 1 of 'dup2' makes integer from pointer without a cast
lab2.c:247: warning: passing argument 1 of 'dup2' makes integer from pointer without a cast

thanks.

[MK27]

dup2 does not work on FILE pointers (the "high level" file interface). It works on file descriptors (the "low level" interface), which are just unique ints. A FILE* has corresponding file descriptor associated with it (for example, stdout is a stream (a FILE*); it's descriptor is 1. stdin is 0, stderr is 2) that you can get with fileno(), but a file descriptor does not necessarily have a file pointer unless explicitly created (eg, with fdopen()). So you might try:

dup2(fileno(fp), 1);

You should look at popen(). You can use popen() like system(), but it returns a FILE* to the processes stdout or stdin. In other words, the output of the process will not go to the terminal if you use

FILE *p = popen("command","r");

Instead, it gets piped to "p". You can then write that to a file descriptor if you want.

I presume that is what you are trying to do, since, nb, exec() is no good for just opening files.

[TaiL]

thanks that helped a lot. I'm just curious to see, how would you write a file pointer to another file pointer using popen()

[TaiL]

nvm I got it. this would work however, popen() seems to send the whole file instead of executing. is that the purpose of popen()?

		fwrite(p, 1, sizeof(p), fp);

[MK27]

If the file is an EXECUTABLE, such as another program, then popen executes that program. In "r" mode the FILE* is the output (stdout) from the program. In "w" mode the FILE* can be used to write to the program's stdin.

If the file is not an executable program, then both popen() and exec() are inappropriate.

Nb this:

		fwrite(p, 1, sizeof(p), fp);

is problematic, altho it will work (since p is a pointer, it will write 4 or 8 chunks of 1 byte each). Ie, the sizeof(p) is not relevant; you probably want to do this in a while loop.

[TaiL]

thanks. I'm still having troubles on how to execute my cgi script with execv() or popen(). I've read that I should be using execve() if i'm trying to execute anything with a #! but have no idea what to pass to the 2nd and 3rd arguments.

If i don't have any arguments for the 2nd and 3rd args of execve() will that be the end of the world for my program?

[MK27]

The fact that it has a shebang (#!) shouldn't matter. As long as it has the execution bit set, ie, you can run it yourself from the command-line. Presumably you have already run the script somehow and so that is the case?

The 2nd arg to execve is the *argv[] of the process, and argv[0] should be the program name itself. If the 1st arg is "./myscript.cgi" then the first element of the 2nd arg should be "myscript.cgi".

As for the third arg, well, if your cgi script uses the environment to get it's query string (rather than stdin), which can be the case, then perhaps you can mimic whatever ENV var apache sets there using the third arg...I dunno.

[TaiL]

lol thanks. I'm beginning to hate this little program.

edit:: oh wow my script is not a compiled script ! meaning i would have to use execl() GUH!!!

can someone dumb down how to use execl() if my cgi script is "this.cgi"

thanks for the much appreciated help