The aim of the program is to fork a new child process and execute a process which also has command line arguments. If I enter /bin/ls --help, I get the error:
shadyabhi#shadyabhi-desktop:~/lab/200801076_lab3$ ./a.out
Enter the name of the executable(with full path)/bin/ls --help
Starting the executable as a new child process...
Binary file to be executed: /bin/ls
/bin/ls: unrecognized option '--help
'
Try `/bin/ls --help' for more information.
Status returned by Child process: 2
shadyabhi#shadyabhi-desktop:~/lab/200801076_lab3$
What would be the right argument to execve()?
#include<stdio.h>
#include<string.h> //strcpy() used
#include<malloc.h> //malloc() used
#include<unistd.h> //fork() used
#include<stdlib.h> //exit() function used
#include<sys/wait.h> //waitpid() used
int main(int argc, char **argv)
{
char command[256];
char **args=NULL;
char *arg;
int count=0;
char *binary;
pid_t pid;
printf("Enter the name of the executable(with full path)");
fgets(command,256,stdin);
binary=strtok(command," ");
args=malloc(sizeof(char*)*10);
args[0]=malloc(strlen(binary)+1);
strcpy(args[0],binary);
while ((arg=strtok(NULL," "))!=NULL)
{
if ( count%10 == 0) args=realloc(args,sizeof(char*)*10);
count++;
args[count]=malloc(strlen(arg));
strcpy(args[count],arg);
}
args[++count]=NULL;
if ((pid = fork()) == -1)
{
perror("Error forking...\n");
exit(1);
}
if (pid == 0)
{
printf("Starting the executable as a new child process...\n");
printf("Binary file to be executed: %s\n",binary);
execve(args[0],args,NULL);
}
else
{
int status;
waitpid(-1, &status, 0);
printf("Status returned by Child process: %d\n",WEXITSTATUS(status));
}
return 0;
}
The first entry in the args array should be the program name again. Your code calls /bin/ls with --help as the process name.
Please check to make sure args is not getting clobbered by the realloc call. See here on SO regarding realloc
Edit:
Also the loop looks funny....
You called strtok like this:
binary=strtok(command," ");
Change the loop construct to use binary instead as shown...
char *tmpPtr;
while (binary != NULL){
if ( count%10 == 0) tmpPtr=realloc(args,sizeof(char)*10);
if (tmpPtr != NULL) args = tmpPtr;
count++;
args[count-1]=malloc(strlen(binary)+1);
strcpy(args[count-1],binary);
binary = strtok(command, " ");
}
And use the binary for copying the string....
Hope this helps,
Best regards,
Tom.
Your program has some obvious errors. For instance, declaring char **args=NULL; and then args=realloc(args,sizeof(char)*10); (since it's char**, you should be alloc-ing to char*, no?..).
Since sizeof(char*) is usually 4 while sizeof(char) is usually 1, you end up with some serious memory management problems around there (you alloc less than you use, and you end up writing where you shouldn't). From there on, all hell breaks loose and you can't expect your program's behavior to make any sense.
I'd suggest that you run your program through an util such as Valgrind to figure out memory leaks and correct the program appropriately. Probably your execve problems will disappear as soon as the memory problems are corrected.
Related
Okay so overall im trying to complete a basic CLI C program which will complete functions such as clear, quit, cd, ls, help (bring up the unix man) etc.. i altered my code and so far i have this, im getting segmination error when trying to execute the cd command part of the program, (im very new to c btw);
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
int main (int argc, char *argv[])
{
char input[] = " ";
char *argument;
while(strcmp(input, "quit")!= 0)
{
printf("$");
scanf ("%s", input);
if(strcmp(input,"clear") == 0)
{
printf("\e[1;1H\e[2J");
}
else if(strcmp(argv[1],"cd") == 0)
{
if(chdir(argv[2]) == -1)
{
printf("\n directory does not exists");
}
}
else if(strcmp(input, "echo") == 0)
{
char str[50];
scanf("%[^\n]+", str);
printf(" %s", str);
}
}
}
input is declared as a ' ' (space) character. It will never match 'cd'.
This is probably more along the lines of what you want to achieve, where the first parameter is the command (cd), and the second will be the directory:
int main (int argc, char *argv[])
{
char *argument;
if(strcmp(argv[1],"cd") == 0)
{
if(chdir(argv[2]) == -1)
{
printf("\n directory does not exists");
}
}
Edit Also please note that there is no need for the else satement. If chdir does not return an error, it will change the directory, thus no need to call it again in an else.
Additionally, another tip for using system calls in general, it would be of great help if you print the error number returned by the system upon a failure in system call. This will make things easier when things start going wrong. To do this simply include <errno.h>' and modify the printf to printerrno` which gives specific details about the error:
printf("Chdir error: %d", errno);
For instance chdir() does not only return an error when the directory does not exist, but also for example if you do not have permissions to view the contents of the directory. See the man page for a list of possible errors.
To implement your own shell, you need to take input directly from stdin, not from command-line arguments (argv) from another shell. The basic pattern is like this:
Read input
Execute command
Print results
Loop back to step 1
I am trying to create a simple c program that takes user input, passes it to underlying shell and return the ouput of the user command. For eg: pwd will give the present working directory.
I want to do this using fork() and exec() in an infinite loop but I am facing two problems:
My loop terminates after first run
It only takes first argument. 'ls -ltr' will give me the output of 'ls' and not 'ls -ltr'
int runit(char*);
void main() {
int pid=0;
char command[50];
while(1)
{
int d=0;
printf("Please enter your command!\n");
scanf("%s", &command);
switch (pid = fork())
{
case 0: // a fork returns 0 to the child
printf("Child process \n");
d=runit(command);
if(d==-1){
printf("command not found \n");}
break;
default:
wait(5); // a fork returns a pid to the parent
printf("Parent process \n");
break;
case -1: //if something went wrong
perror("fork");
exit(1);
}
}
}
int runit(char* command) { //executing the command
char path[50]="/bin/";
int d = execl(strcat(path,command),command,NULL,NULL);
return(d);
}
Can somebody tell me what I am doing wrong or guide me how to correct this.
This input reading
scanf("%s", &command);
would stop at the first whitespace (unless input failure occurs -- you should check the return value of scanf() in any case). So, when you input ls -ltr, command will have just ls. You need to read lines. For example, use fgets() to read input (and make sure to handle the trailing newline):
fgets(command, sizeof command, stdin);
command[strcspn(command, "\n")] = 0; /* to remove \n if present */
And the next problem is that execl() takes multiple arguments. That means
this usage:
int d = execl(strcat(path,command),command,NULL,NULL);
is broken. You need to split the input command into multiple strings and pass each arguments to execl(). A better option to store the individual strings in an array and use execv() instead.
Another thing you need to be aware of is exec*() family functions do not return on success. So, the return code check isn't very useful - instead you can just use perror() to know why it failed if it failed.
As many before me, I'm trying to implement a basic shell in C. Overall things are working nicely, and I'm now trying to add pipes and redirections.
I've read a lot about the pipe() function and have successfully written a side program to pipe a function's output into a second function's input.
Where I have trouble is when it comes to looping over an undetermined amount of functions.
Here's the last version of my function as well as the main I use to test it :
char **g_env;
int ft_pipeline(char **cmd, unsigned int pos)
{
int in;
int pfd[2];
pid_t pid;
char **cur_cmd;
in = 0;
while (cmd[pos])
{
if (pipe(pfd) != 0)
return (1);
close(pfd[0]);
dup2(pfd[1], in);
close(pfd[1]);
pid = fork();
if (pid == -1)
return (2);
if (pid == 0) //child
{
close(pfd[1]);
dup2(pfd[0], 0);
close(pfd[0]);
cur_cmd = ft_strsplit_blank(cmd[pos]);
execve(cur_cmd[0], cur_cmd, g_env);
}
wait(NULL);
in = pfd[0];
pos++;
}
return (0);
}
int main(void)
{
char **cmd = ft_strsplit("/bin/ls -l /dev | /bin/grep std", '|');
g_env = NULL;
ft_pipeline(cmd, 0);
return (0);
}
In this current form, ls is properly executed but written to stdout, and grep returns :
/bin/grep: (standard input): Bad file descriptor
This is the fifth time I rewrite my code, and I've tried to tweak it for a few days now. I've also read several other post here to try and grasp the logic behind this small program, to no avail.
I'd really like it if you could tell me where I'm making the mistake and how I could fix it.
Note : You will very likely find many ways to improve this code in its form. I know about it but that's something that I cannot do, in most cases. Although this is not homework, it is still something that I do for school (see it as a voluntary practice) and I have to respect standards in the way I write my code or the functions I use.
I need to implement a basic shell in C.
One of things I need is to implement a function that has a command and to execute it.
my code:
pID=fork();
if (pID == 0)
execvp(tmp[0], tmp);
else if (pID > 0)
{
printf("%d", pID);
wait(NULL);
}
else
printf("Failed to create proccess \n");
The problem is that no matter what is the command I put in tmp, the program shows me the prompt again, and do nothing except that.
For example if I write gedit (in order to open the gedit — a ntpad of Ubuntu), it doesn't open it, or if write ls -a it doesn't show me any output as the terminal of Ubuntu does.
execvp should work. As the others mentioned, you really need to show how you populate tmp. That said, I would guess that that's where the error is. tmp needs to be a null terminated array.
#include <stdio.h>
main( int argc, char * argv[] )
{
int pid = fork;
char * tmp[2];
memset( tmp, 0, sizeof(tmp) );
tmp[0] = argv[0];
if( 0 == pid )
{
if( -1 == execvp( tmp[0], tmp ) )
{
char errmsg[64];
snprintf( errmsg, sizeof(errmsg), "exec '%s' failed", tmp[0] );
perror( errmsg );
}
else if( 0 < pid )
{
printf("[%d] %s\n", pid, tmp[0]);
wait(NULL);
}
else
{
perror("fork failed");
}
}
Although you've failed to tell us what you're passing through the tmp variable to execvp, my psychic sense tells me that you forgot to null-terminate your argument list. A NULL argument tells execvp where the last argument is, and if you fail to put in a NULL, it will start reading random garbage off the stack.
If that random garbage points to large strings of non-zero data, it will run out of space to store the supposed arguments to the new process, which is typically a few hundred KB (see this page for some system-specific numbers, as well as various ways of getting your system's maximum arguments size).
When there's too much argument data, the system call execve(2) (called internally by execvp) fails with the error E2BIG.
So to see if this is what's happening to you, check the return value from execvp. If it even returns at all, it failed (if it succeeded, it wouldn't have returned since a new process would be executing!), so check the global value of errno to see why it failed:
if (pID == 0)
{
execvp(tmp[0], tmp);
printf("exec failed: %s\n", strerror(errno));
exit(1);
}
execvp() requires full path . If in tmp[0] isnt the full path of your executable file use execv()
execv(tmp[0], tmp);
It looks like this just sends a ping, but whats the point of that when you can just use ping?
/* WARNING: this is someone's attempt at writing a malware trojan. Do not
compile and *definitely* don't install. I added an exit as the
first line to avoid mishaps - msw */
int main (int argc, char *argv[])
{
exit(1);
unsigned int pid = 0;
char buffer[2];
char *args[] = {
"/bin/ping",
"-c",
"5",
NULL,
NULL
};
if (argc != 2)
return 0;
args[3] = strdup(argv[1]);
for (;;)
{
gets(buffer); /* FTW */
if (buffer[0] == 0x6e)
break;
switch (pid = fork())
{
case -1:
printf("Error Forking\n");
exit(255);
case 0:
execvp(args[0], args);
exit(1);
default:
break;
}
}
return 255;
}
It's a hack - or an attempt at a hack - to get arbitrary code run in a privileged mode. Ping needs to run SUID root to get a raw socket for an ICMP_ECHO_REQUEST and the intentional buffer overrun in gets(buffer) is intended to pass junk to ping.
I don't see how this could work in practice, but you shouldn't compile and run it.
It makes sure that ping is called with the arguments -c 5. Which is stupid, because a shell script or alias would be easier to read and faster to write.
This program basically emulates a simple shell program. A shell program is going to take the arguments of another program as input and launch that specified program in a new process. The program you have above is just hard coded for one specific program (ping in this case) and is very simple.
A shell program makes working with the operating system more user friendly by providing an interface to boot up programs.