Simulating the UNIX type shell

Question

I am trying to implement the UNIX type shell. I have not implemented the shell creating a child process to run each command part, but this is just a code to break the string entered at the command prompt by the user into the appropriate arguments by using a space as a delimiter, which will then be passed to the file corresponding to the command (say ls -l) as command line arguments.

It runs alright on my Windows 7 OS, but when I run it on Ubuntu with my input string as "ABC XYZ WGH", I get an error, saying "Undefined symbol 'printf' version ABC." Please help me understand, where I am going wrong?

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int arguments(char str[],char *** args)
{
    char *s;
    int num;
    int i;
    s=(char*)calloc(strlen(str)+1,sizeof(char));
    strcpy(s,str);
    if(strtok(s," ")==NULL)
    num=0;
    else
    {
        num=1;
        while(strtok(NULL," ")!=NULL)
        num++;
    }
    strcpy(s,str);
    **args=strtok(s," ");
    if(num)
    {
       for(i=1;i<num;i++)
       *((*args)+i)=strtok(NULL," ");
    }
    return num;
}


int main()
{
    char buffer[256];
    char **arg;
    int args;
    int i;
    while(true)
    {
           gets(buffer);
           if(!strcmp(buffer,"STOP"))
           break;
           args=arguments(buffer,&arg);
           printf("%d\n",args);
           for(i=0;i<args;i++)
           printf("%s\n",arg[i]);

    }
    return 0;
}

[do not cast the return value of standard allocator functions](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858). — , Jan 23 '14 at 15:20
Can you copy and paste the exact error message, along with whatever command you typed at the terminal? — John Kugelman, Jan 23 '14 at 15:23
@JohnKugelman: The error i am getting is: "./a.out: symbol lookup error: ./a.out undefined symbol: printf, version ABC. — amiageek, Jan 23 '14 at 15:36
conio.h is windows specific so I guess the code above is different from the ubuntun code — Fredrik Pihl, Jan 23 '14 at 15:37
@Dipto: I am using the gnu gcc comipler available with Ubuntu installation. — amiageek, Jan 23 '14 at 15:42
run it through gdb and you'll get the complete backtrace when segfault occurs. Something like `gcc -g main.c; gdb ./a.out --args ABC XYZ WGH` Then from the gdb-prompt, type `run` — Fredrik Pihl, Jan 23 '14 at 15:47
@FredrikPihl After running this in GDB i got: "Inferior 1 (Process ####) exited with code 0177" — amiageek, Jan 23 '14 at 16:08

score 1 · Answer 1 · answered Jan 23 '14 at 17:11

Under gdb, I get:

Program received signal SIGSEGV, Segmentation fault. 0x00000000004007a8 in arguments (str=0x7fffffffe010 "", args=0x7fffffffe000) at u.c:20 20 **args=strtok(s," ");

i.e, you use a pointer that points to a pointer that points to nothing usable: this is undefined behaviour, and it may be why you get strange things like ""Undefined symbol 'printf' version ABC.", or segmentation fault under gdb.

I suggest you rewrite the fonction arguments() with less indirection levels (and a better use of strtok() =), but for now this code may do what you want:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 32

int arguments(char str[],char **args)
{
    char *s;
    int num;
    int i;
    s=malloc(strlen(str)+1);
    strcpy(s,str);
    if(strtok(s," ")==NULL)
        num=0;
    else {
        num=1;
        while(strtok(NULL," ")!=NULL)
        num++;
    }
    strcpy(s,str);
//  **args=strtok(s," ");
    *args=strtok(s," ");
    if(num)
    {
       for(i=1;i<num;i++)
//         *((*args)+i)=strtok(NULL," ");
           args[i] = strtok(NULL," ");
    }
    return num;
    // note. 's' is not free()'d ..
}

int main()
{
    char buffer[256];
    char *arg[MAX];
    int args;
    int i;

    while(1)
    {
           gets(buffer);
           if(!strcmp(buffer,"STOP"))
               break;
           args=arguments(buffer, arg);
           printf("%d\n",args);
           for(i=0;i<args;i++)
               printf("%s\n",arg[i]);

    }
    return 0;
}

Simulating the UNIX type shell

1 Answers1