Question

I am implementing a simple version of a linux shell in c.

I have succesfully written the parser, but I am having some trouble forking out the child process. However, I think the problem is due to arrays, pointers and such, because just started C with this project and am not still very knowledgable with them.

I am getting a segmentation fault and don’t know where from. Any help is greatly appreciated.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/types.h>

#define MAX_COMMAND_LENGTH 250
#define MAX_ARG_LENGTH 250

typedef enum {false, true} bool;

typedef struct {
    char **arg;     
    char *infile;   
    char *outfile;  
    int background; 
} Command_Info;

int parse_cmd(char *cmd_line, Command_Info *cmd_info)
{
    char *arg;
    char *args[MAX_ARG_LENGTH]; 

    int i = 0;
    arg = strtok(cmd_line, " ");
    while (arg != NULL) {
        args[i] = arg;
        arg = strtok(NULL, " ");
        i++;
    }

    int num_elems = i;
    if (num_elems == 0)
        return -1;

    cmd_info->infile = NULL;
    cmd_info->outfile = NULL;
    cmd_info->background = 0;

    int iarg = 0;
    for (i = 0; i < num_elems-1; i++)
    {                   
        if (!strcmp(args[i], "<"))
        {
            if (args[i+1] != NULL)
                cmd_info->infile = args[++i];
            else
                return -1;                      
        }

        else if (!strcmp(args[i], ">"))
        {
            if (args[i+1] != NULL)
                cmd_info->outfile = args[++i];
            else 
                return -1;                          
        }

        else
            cmd_info->arg[iarg++] = args[i];
    }

    if (!strcmp(args[i], "&"))
        cmd_info->background = true;
    else
        cmd_info->arg[iarg++] = args[i];

    cmd_info->arg[iarg] = NULL; 

    return 0;   
}


void print_cmd(Command_Info *cmd_info)
{
    int i;  
    for (i = 0; cmd_info->arg[i] != NULL; i++)
        printf("arg[%d]=\"%s\"\n", i, cmd_info->arg[i]);
    printf("arg[%d]=\"%s\"\n", i, cmd_info->arg[i]);    
    printf("infile=\"%s\"\n", cmd_info->infile);
    printf("outfile=\"%s\"\n", cmd_info->outfile);
    printf("background=\"%d\"\n", cmd_info->background);
}

void get_cmd(char* str)
{
    fgets(str, MAX_COMMAND_LENGTH, stdin);
    str[strlen(str)-1] = '\0'; //apaga o '\n' do fim
}

pid_t exec_simple(Command_Info *cmd_info)

{
    pid_t pid = fork();


    if (pid < 0)
    {
        perror("Fork Error");
        return -1;
    }



    if (pid == 0)

    {
        execvp(cmd_info->arg[0], cmd_info->arg);

        perror(cmd_info->arg[0]);
        exit(1);
    }


    return pid;

}

int main(int argc, char* argv[])
{

    while (true)
    {
        char cmd_line[MAX_COMMAND_LENGTH];
        Command_Info cmd_info;

        printf(">>> ");

        get_cmd(cmd_line);

        if ( (parse_cmd(cmd_line, &cmd_info) == -1) )   
            return -1;

        parse_cmd(cmd_line, &cmd_info);



        if (!strcmp(cmd_info.arg[0], "exit"))
            exit(0);

        pid_t pid = exec_simple(&cmd_info);

        waitpid(pid, NULL, 0);  
    }

    return 0;
} 

Thanks.

Answer 1

The problem is with cmd_info->arg, which is declared as char **arg and you never allocate memory for it.

So, when you try to access it to save the arguments like this cmd_info->arg[iarg++] = args[i], you are dereferencing an uninitialized pointer, causing the segmentation fault.

A solution would be to change the Command_Info structure to declare arg like this :

char *arg[MAX_ARG_LENGTH];