Myshell Segmentation Fault, Possible issue with getting input? - c++

Question

I have been working on this project for a while. The purpose is to make a functioning shell that can do pretty much all the shell commands (except cd). It does almost everything I want it to do, except for a couple things. The first is that when I put an '&' to signify background processing, it does it, but then doesn't print another myshell> line. I can still input something, but the myshell> never shows up, no matter where I put another cout<<"myshell> ";.
Another issue is if I press enter, making myString empty, many times, it crashes the program with a seg fault. Also after I do the '&' background processing and press enter to get the myshell> to come back up, it prints one myshell> but then seg faults on the next hit of enter. I'm sorry if I didn't explain this well, but it is really driving me crazy. Please let me know if you have any suggestions.
#include <stdio.h>
#include <iostream>
#include <unistd.h>
#include <cstdlib>
#include <cstring>
#include <sys/types.h>
#include <cstdio>
#include <sys/wait.h>
#include <stdio.h>
/*Function that parses the command the user inputs.
It takes myArgv and myString as inputs.
It returns the value of exitcond, which is used to see if the user wants to exit or not.
Also, this is where myString is tokenized using strok()*/
int parseCommand(char *myArgv[10], char myString[255])
{
int exitcond=0;
if((strcmp(myArgv[0], "exit") == 0)||(strcmp(myArgv[0], "quit")==0))
{
exitcond = 1;
return exitcond;
}
int i;
char *token;
token = strtok(myString," ");
i=0;
while (token != NULL)
{
myArgv[i] = token;
token = strtok(NULL," ");
i++;
}
/*
* Set the last entry our new argv to a null char
* (see man execvp to understand why).
*/
myArgv[i] = '\0';
return exitcond;
}
/*Function that gets the command from the user and sees if they want
background processing or not (presence of '&').
It takes inputs of choose and myString. choose is the variable for
whether background processing is necessary or not, while myString is
an empty character array.
It outputs the value of the choose variable for lter use.*/
int getCommand(int choose, char myString[255])
{
int i;
choose=0;
fgets(myString, 256, stdin);
if (myString[0]=='\0')
{
choose=0;
return choose;
}
for (i=0; myString[i]; i++)
{
if (myString[i]== '&')
{
choose=1;
myString[i]=' ';
}
if (myString[i] == '\n')
{
myString[i] = '\0';
}
}
return choose;
}
/*Main function where all the calling of other functions and processes
is done. This is where the user enters and exits the shell also. All
usage of fork, pid, waitpid and execvp is done here.*/
int main()
{
using namespace std;
int exitCondition=0, i=0, status;
char myString[255];
char *token, *myArgv[10];
pid_t pid, waiting;
int bg=0;
while (!exitCondition)
{
/* print a prompt and allow the user to enter a stream of characters */
cout << "myshell> ";
bg=0;
int choose=0;
bg=getCommand(choose,myString);
exitCondition=parseCommand(myArgv,myString);
if(exitCondition==1)
{
cout<<"Thank you for using my shell.\n";
}
else {
/* while (myString[0]=='\0')
{
cout<<"myshell> ";
bg=getCommand(choose,myString);
}*/
/* The user has a command, so spawn it in a child process */
pid = fork();
if (pid == -1)
{
/* to understand why this is here, see man 2 fork */
cout << "A problem arose, the shell failed to spawn a child process" << endl;
return(1);
}
else if (pid == 0)
{
// Child process
execvp(myArgv[0],myArgv);
cout << "Bad command or file name, please try again!\n" << endl;
return 0;
} else {
/* This makes sure that the spawned process is run in the foreground,
because the user did not choose background */
if(bg==0)
{
waitpid(pid,NULL,0);
}
}
}
}
return 0;
}

Answer 1

Okay, you had three bugs, one of which caused the segfault. Of the others, one would put a garbage argument in the array passed to execvp and the other would leak zombie processes for background jobs.
I've corrected the code and annotated it with where the bugs were along with the fixes [please pardon the gratuitous style cleanup]:
#include <cstdio>
#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#define AVCOUNT 100
#define STRBUFLEN 2000
/*Function that parses the command the user inputs.
It takes myArgv and myString as inputs.
It returns the value of exitcond, which is used to see if the user wants to
exit or not.
Also, this is where myString is tokenized using strok()*/
int
parseCommand(char **myArgv, char *myString)
{
char *token;
char *bp;
int exitcond = 0;
int i;
// NOTE/BUG: original check for exit/quit was here -- at this point
// myArgv is undefined (hence the segfault)
// NOTE/BUG: your original loop -- at the end i was one beyond where it
// should have been so that when myArgv gets passed to execvp it would
// have an undefined value at the end
#if 0
token = strtok(myString, " ");
i = 0;
while (token != NULL) {
myArgv[i] = token;
token = strtok(NULL, " ");
i++;
}
#endif
// NOTE/BUGFIX: here is the corrected loop
i = 0;
bp = myString;
while (1) {
token = strtok(bp, " ");
bp = NULL;
if (token == NULL)
break;
myArgv[i++] = token;
}
/*
* Set the last entry our new argv to a null pointer
* (see man execvp to understand why).
*/
// NOTE/BUG: with your code, i was one too high here
myArgv[i] = NULL;
// NOTE/BUGFIX: moved exit/quit check to here now that myArgv is valid
token = myArgv[0];
if (token != NULL) {
if ((strcmp(token, "exit") == 0) || (strcmp(token, "quit") == 0))
exitcond = 1;
}
return exitcond;
}
/*Function that gets the command from the user and sees if they want
background processing or not (presence of '&').
It takes inputs of choose and myString. choose is the variable for
whether background processing is necessary or not, while myString is
an empty character array.
It outputs the value of the choose variable for lter use.*/
int
getCommand(int choose, char *myString)
{
int i;
choose = 0;
fgets(myString, STRBUFLEN, stdin);
if (myString[0] == '\0') {
choose = 0;
return choose;
}
for (i = 0; myString[i]; i++) {
if (myString[i] == '&') {
choose = 1;
myString[i] = ' ';
}
if (myString[i] == '\n') {
myString[i] = '\0';
break;
}
}
return choose;
}
/*Main function where all the calling of other functions and processes
is done. This is where the user enters and exits the shell also. All
usage of fork, pid, waitpid and execvp is done here.*/
int
main()
{
using namespace std;
int exitCondition = 0;
int status;
char myString[STRBUFLEN];
char *myArgv[AVCOUNT];
pid_t pid;
int bg = 0;
while (!exitCondition) {
// NOTE/BUGFIX: without this, any background process that completed
// would become a zombie because it was never waited for [again]
// reap any finished background jobs
while (1) {
pid = waitpid(0,&status,WNOHANG);
if (pid < 0)
break;
}
/* print a prompt and allow the user to enter a stream of characters */
cout << "myshell> ";
bg = 0;
int choose = 0;
bg = getCommand(choose, myString);
exitCondition = parseCommand(myArgv, myString);
if (exitCondition == 1) {
cout << "Thank you for using my shell.\n";
break;
}
/* while (myString[0]=='\0') { cout<<"myshell> "; bg=getCommand(choose,myString); } */
/* The user has a command, so spawn it in a child process */
pid = fork();
if (pid == -1) {
/* to understand why this is here, see man 2 fork */
cout << "A problem arose, the shell failed to spawn a child process" << endl;
return 1;
}
if (pid == 0) {
// Child process
execvp(myArgv[0], myArgv);
cout << "Bad command or file name, please try again!\n" << endl;
return 1;
}
/* This makes sure that the spawned process is run in the
foreground, because the user did not choose background */
if (bg == 0)
waitpid(pid, &status, 0);
}
return 0;
}