Seg fault in custom C Shell

[Baelix]

Hey all. I'm writing a C Shell for my Operating Systems class, and was hoping you could help me find an error in my code. It executes all commands properly, and accounts for piping, input/output redirection, history, and CD commands. However, whenever I use the piping feature, it will execute all the commands I put into it, but it will terminate afterwards via a segmentation fault. I can't figure out where it is coming from. I have marked the area I think it's coming from with "XXXX Starts here" and "XXXX Ends here". I feel like the getchar call is casuing the error, but I'm not sure (that bit of code was provided by my instructor). Thanks in advance for your help!

My code:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <fcntl.h>

main()
{
   char *path, *argv[20], buf[80], n, *p;
   int m, status, inword, continu;
   int inFlag, outFlag, bgFlag, k, count, j, pipes, pid, aCount, dirCount,
      hCount;
   int r_tube[2], l_tube[2];
   char *hist;
   char *h1[20] = { 0 };
   aCount = 0;
   dirCount = 0;

   while (1)
   {
      inword = 0;
      p = buf;
      m = 0;
      continu = 0;
      inFlag = outFlag = bgFlag = k = count = j = pipes = pid = 0;
      hCount = 0;
      int loc[20] = { 0 };
      char currD[50];

      printf("\nshhh> ");

      while ((n = getchar()) != '\n' || continu)   //XXXXXXXXX Starts here
      {
         if (n == ' ')
         {
            if (inword)
            {
               inword = 0;
               *p++ = 0;
            }
         }
         else if (n == '\n')
            continu = 0;
         else if (n == '\\' && !inword)
            continu = 1;
         else
         {
            if (!inword)
            {
               inword = 1;
               argv[m++] = p;
               *p++ = n;
            }
            else
               *p++ = n;
         }
      }                                    //XXXXXXXX Ends here

      *p++ = 0;
      argv[m] = 0;

      getcwd(currD, 50);

      while (argv[k] != 0)
      {                         //stores all entered commands into history array
         h1[aCount] = strdup(argv[k]);
         aCount++;
         k++;
      }
      k = 0;

      if ((strcmp(argv[0], "history") == 0) ||
         (strcmp(argv[0], "History") == 0))
      {                         //tests to see if history is asked for
         printf("History: \n");
         for (k = 0; k < aCount; k++)
            printf("%s\n", h1[k]);
      }


      if (strcmp(argv[0], "cd") == 0)
      {                         //tests to see if CD is needed
         if (strcmp(argv[1], "..") == 0)
            chdir(currD);
         else
         {
            while (argv[dirCount] != 0)
            {
               getcwd(currD, 50);
               chdir(argv[dirCount]);
               dirCount++;
            }
         }
      }

      char *outFile = NULL;
      char *inFile = NULL;
      loc[0] = 0;

      while (argv[count] != 0)
      {  //while loop sets the flags for input redirection, 
         // output redirection, background operator, and piping
         if (strcmp(argv[count], "<") == 0)
         {
            inFile = strdup(argv[count + 1]);
            argv[count] = argv[count + 1] = 0;
            inFlag = 1;
         }
         else if (strcmp(argv[count], ">") == 0)
         {
            outFile = strdup(argv[count + 1]);
            argv[count] = argv[count + 1] = 0;
            outFlag = 1;
         }
         else if (strcmp(argv[count], "&") == 0)
         {
            argv[count] = 0;
            bgFlag = 1;
         }
         else if (strcmp(argv[count], "|") == 0)
         {
            argv[count] = 0;
            loc[pipes + 1] = count + 1;
            pipes++;
            printf("LOC0: %d ", loc[0]);
            printf("LOC1: %d ", loc[1]);
            printf("LOC2: %d ", loc[2]);
            printf("DONE WITH IF\n");
         }
         else
            loc[count] = count;

         count++;
      }

      for (k = 0; k <= pipes; k++)
      {                         //actual execution of commands
         printf("IN FOR\n");
         if (k < pipes)
         {
            pipe(r_tube);
            j++;
         }

         pid = fork();          //fork child every time to exec

         if (pid > 0)
         {
            if (j > 0)
            {
               close(l_tube[0]);
               close(l_tube[1]);
            }
            l_tube[0] = r_tube[0];
            l_tube[1] = r_tube[1];
         }
         else if (pid == 0)
         {
            if ((k == 0) && (inFlag == 1))
            {
               int n = open(inFile, O_RDONLY | O_CREAT);
               if (n == -1)
               {
                  printf("Couldn't open inFile!\n");
                  exit(1);
               }
               close(0);
               dup(n);
               close(n);
            }
            else if ((k == pipes) && (outFlag == 1))
            {
               int out = open(outFile, O_WRONLY | O_CREAT, 0666);
               if (out < 0)
               {
                  printf("Could'nt open outFile!\n");
                  exit(1);
               }
               close(1);
               dup(out);
               close(out);
            }
            printf("K: %d ", k);
            printf("PIPES: %d ", pipes);
            printf("PID: %d\n", pid);
            execvp(argv[loc[k]], &argv[loc[k]]);
         }
         printf("ONE\n");
         if (bgFlag == 0)
            wait(NULL);
      }
      printf("TWO\n");
      if (strcmp(argv[0], "quit") == 0)
         exit(0);               //tests for exit/quit to end program
      if (strcmp(argv[0], "exit") == 0)
         exit(0);
      printf("THREE\n");
      for (k = 0; k < 20; k++)  //reset all of argv to NULL
         argv[k] = 0;
      printf("FOUR\n");

      wait(&status);
   }
}

[Moschops]

Upon running, what input do you give that leads to the crash?

[modoran]

Does GCC compiles this code ? I think not.

[Moschops]

It's C. gcc will compile this as C code with a few warnings.

[ahmedkhan]

shhh>
Program received signal SIGSEGV, Segmentation fault.
0x08048772 in main () at sh.c:56
56 *p++ = n;
(gdb) x/c 0x08048772
0x8048772 <main+318>: -120 '\210'


Hope this should help you !!!

Note error occured while executing line 56 : *p++=n;

[Baelix]

@Moschops The input that leads to the error is anything to do with piping. I.e. ls -al | ls -al
Or ls | wc | more output.txt
Either of those commands will cause the seg fault.

@Modoran Yes, this code compiles perfectly fine (minus a few warnings) in a Unix/Linux environment.

@Ahmedkhan Thank you! This totally pinpoints where I believe the error to be. That segment of code was supplied by my professor, and I knew there was something wrong with it. What inputs did you use to produce that seg fault?

[ahmedkhan]

just use piping output of two or more linux cmds, I used "ls -l | sort" to trigger segfault