Execvp help

**hambergler** · 06-02-2010

Ok I changed it a bit, but now I'm having trouble getting the file name into the system call... here's what I did:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main (int argc, char *argv[])
{
	int pid, fd;
	char command;
	
	/*attempt to get the file name stored in *argv to append to the sort command*/
	command = strcat("Sort ",argv[1]);

	/*create new process or quit */
	if ( (pid = fork() ) == -1 ) {
		perror("fork"); 
		exit(1);
	}
	/*child does the work */
	if (pid == 0) {
		close(1);
		fd = creat("userlist", 0644);
		system(command);
		perror("System");
		exit(1);
	}
	/*parent waits then reports */
	if(pid != 0) {
		wait(NULL);
		printf("Done running sort.  Results in userlist\n");
	}

}

**MK27** · 06-02-2010

Originally Posted by hambergler

Ok I changed it a bit, but now I'm having trouble getting the file name into the system call... here's what I did:

[code]

char command;

/*attempt to get the file name stored in *argv to append to the sort command*/
command = strcat("Sort ",argv[1]);

"char command" is one character. Use "char command[1024]" or something.

You don't have to use fork() with system. You need it with exec() because exec causes the current process to be replaced (notice, no code after an exec() call gets executed). This is not an issue with system().

So if this is for an assignment that's supposed to teach you about fork and exec, then choose a different task (if not, just use system with no fork).

WRT argv[1], the argv array is broken on whitespace unless you use quotes. So eg, if your executable is called a.out:

./a.out do > this
argv[1] will be "do", argv[2] will be ">", argv[3] will be "this".
./a.out "do > this"
argv[1] will be "do > this".

**hambergler** · 06-02-2010

This is for a project. I'm learning to close a "line" like stdout or stdin and open another file to connect to the closed line. Thus the fork and the previous execlp. I'm just having trouble with the syntax of the exec family.

**MK27** · 06-02-2010

Originally Posted by hambergler

This is for a project. I'm learning to close a "line" like stdout or stdin and open another file to connect to the closed line. Thus the fork and the previous execlp. I'm just having trouble with the syntax of the exec family.

Okay -- well, you have hit a limitation of exec/execlp. Someone may come along with evidence to the contrary, but I am pretty sure this kind of command:

sort somefile > someotherfile

or anything else involving > redirection is not possible. You can't even use pipes (|) with exec. You can really only issue one single command, and provide values for the arguments it accepts. "> somewhere" and "| whatever" are not arguments to the command. They're part of the shell's own I/O functionality. You may be able to pull it off by opening a file and using dup() and pipe() -- but honestly, I would not bother. This is not what is expected of you. You just need to issue a simple command with optional arguments.

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