Thread: Problem with readlink()?

Hello...

I am working on a program and I don't quite understand why my last line "Child completed" is not printed out. My idea is to output for the main process in the task, (i) the executable and its pathname used and (ii) the command line used.

I have the following codes:

Code:

int main() {
        int pid, child_pid;
	char *pid_string;
	char *buf;
	char *temp = "/proc";
	pid = fork();
	if(pid > 0) {	
		printf("Parent\n");
		wait(NULL);
	}
	
	else if(pid == 0) {
		printf("Child\n");
		child_pid = getpid();
		printf("temp1: %s", temp);
		
		sprintf(pid_string, "%d", child_pid); // Convert integer to string
		printf("pid_string %s", pid_string);
		
		strcat(temp, pid_string);
		strcat(temp, "/exe");
		printf("\ntemp3: %s\n", temp);
		
		int len = readlink(temp, buf, sizeof(buf)-1);
		
	 	if (len == -1) {
			// The call failed.
			if (errno == EINVAL) {
				// It's not a symbolic link; report that.
				fprintf (stderr, "%s is not a symbolic link\n", temp);
			}
			
			else {
				// Some other problem occurred; print the generic message.
				perror("readlink");
				return 1;
			}
			
		}
		
		else {
			// NUL-terminate the target path.
			buf[len] = '\0';
			// Print it.
			printf ("%s\n", buf);
			return 0;
		}
		
		printf("\nChild completed\n");
	}
	
	else {
		perror(" fork failed...");
		exit(1);
	}
	
	return 0;
}

My problem is, I don't get the words "Child completed" printed at the end when the child finishes. However, if I commet away the codes from sprintf to the bracket just before the printf for "Child completed", I get it printed out.

Is there something wrong with readlink, or is there already something wrong with the conversion of the integer of pid to string?

I know readlink would get me the path using readlink on the file exe in the /proc folder, exit status is from the waitpid(). What about the processes and commandlines (which i omitted for this code)? Should I readlink the cmdline in the /proc/[pid] folder as well?

Thanks a lot... =)

Thanks a lot Salem. Tried your suggestions, and it worked. So happy... However, the last line "Child Completed" still doesn't printe out. Humm...

Now I try to include the execvp to run ./busywait and use readlink to get the path of this ./busywait using the child_pid. ./busywait is a program which just performs some 'useless' computations for around 1 second, by default if no arguments put in. Upon completion, it will print out the time taken to run the codes.

If I leave the readlink alone, ./busywait will finish in 1 sec as per coded. However, if I put in the readlink, be it above or below the execvp, it ended up finishing in 0 sec. The readlink also has got no effect, i.e., it doesn't show the path of the ./busywait in the child and also in parent neither.

Here's the modification of the previous codes:

Code:

int main(){
	int pid, child_pid;
	char *pid_string;
	char buf[50];
	char temp[50];
	int len;
	int execStatus;
	int status;
	char *parameter[10];
	
	pid = fork();
	
	if(pid > 0) {	
		printf("Parent\n");
                //len = readlink(temp, buf, sizeof(buf) - 1);
		waitpid(-1, &status,0);
		printf("\nexit status: %d\n", status);
	}
	
	else if(pid == 0) {
		printf("Child\n");
		child_pid = getpid();
		sprintf(temp, "/proc/%d/exe", child_pid);
		parameter[0] = "./busywait";
		parameter[1] = '\0';
		
		//len = readlink(temp, buf, sizeof(buf) - 1);
		
		execStatus = execvp("./busywait", parameter);
		if (execStatus < 0) {
			perror("Error in executing the process");	
		}
		
		//len = readlink(temp, buf, sizeof(buf) - 1);
		
	 	if (len == -1) {
			// The call failed.
			if (errno == EINVAL) {
				// It's not a symbolic link; report that.
				fprintf (stderr, "%s is not a symbolic link\n", temp);
			}
			
			else {
				// Some other problem occurred; print the generic message.
				perror("readlink error");
				return 1;
			}
			
		}
		
		else {
			buf[len] = '\0';
			printf ("%s\n", buf);
			return 0;
		}
		
		printf("\nChild completed\n");
	}
	
	else {
		perror("fork fail...");
		exit(1);
	}
	
	return 0;
}

What could be the problem? Thank you very much!

Oh I see. But how come the readlink before the execvp couldn't work?

The earlier version too, the "Child finished" also not pritned.

Thank you very much. =)

Yes, I did. Included my codes at the earlier post, thought could save some space... Copy & paste here agian:

I've tried to execvp in the current child, and do the readlink in the parent after the waitpid. busywait program ran, but it didn't run for the expected result of 1 seconds. Any possible reasons? Also, the readlink didn't work, saying that it couldn't get the directory. Seems like it couldn't get the PID in time and read the /proc directory. Same results if I put the readlink above the waitpid

Code:

int main(){
	int pid, child_pid;
	char buf[50];
	char temp[50];
	int len;
	int execStatus;
	int status;
	char *parameter[10];
	
	setenv("PATH", "home/QuietBoi/Desktop/TestSysCommand", 1);
	pid = fork();
	
	if(pid > 0) {
		printf("Parent\n");
		waitpid(-1, &status, 0);
		
		sprintf(temp, "/proc/%d/exe", child_pid);
		len = readlink(temp, buf, sizeof(buf) - 1);
		
	 	if (len == -1) {
			// The call failed.
			if (errno == EINVAL) {
				// It's not a symbolic link; report that.
				fprintf (stderr, "%s is not a symbolic link\n", temp);
			}
			
			else {
				// Some other problem occurred; print the generic message.
				perror("readlink error");
				return 1;
			}
			
		}
		
		else {
			buf[len] = '\0';
			printf ("%s\n", buf);
		}
		
		printf("\nexit status: %d\n", status);
	}
	
	else if(pid == 0) {
		printf("Child\n");
		child_pid = getpid();
		
		parameter[0] = "busywait";
		parameter[1] = '\0';
		
		execStatus = execvp("busywait", parameter);
		if (execStatus < 0) {
			perror("Error in executing the process");	
		}
		
		printf("\nChild completed\n");
	}
	
	else {
		perror("oops why did fork fail?");
		exit(1);
	}
	
	return 0;
}

Result I get:

Code:

Child
Parent


busywait program finished in 0 secs


readlink error: No such file or directory

Anywhere could have gone wrong?

Thanks a lot. =)

Thanks a lot Salem, just figured it out during the numerous tryouts just now. Didn't know that it's just that simple. Haha... Guess I'm still very new in C.

But then, readlink still can't get the folder right. it still has the

Code:

readlink error: No such file or directory.

Tried before the waitpid and after waitpid also the same error. Where else could I put the readlink so that i can get the path of busywait? Maybe I should sleep for a short while in parent before I readlink? But what happens if busywait takes a very short time, say 0.1ms? After I wait, the process is gone also.

Thank you very much. =)

Thank you very much Salem. Finally got it working. Million thanks!

Now another problem: To get the commands busywait invoked.

Take example, busywait creates a child process with the command line gcc test.c
How could I capture the command line and it's path of the program:

/usr/bin/gcc: gcc test.c

I think that it has got to do with cmdLine in the /proc. How can I use this? readlink cmdLine doesn't work. Any other special functions which I could use?