Question

I need help with debugging this piece of code. I know the problem is in malloc and free but can’t find exactly where, why and how to fix it. Please don’t answer: “Use gdb” and that’s it. I would use gdb to debug it, but I still don’t know much about it and am still learning it, and would like to have, in the meanwhile, another solution.

Thanks.

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

#define MAX_COMMAND_LENGTH  256
#define MAX_ARGS_NUMBER     128
#define MAX_HISTORY_NUMBER  100

#define PROMPT ">>> "

int num_elems;

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_ARGS_NUMBER];    

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

    num_elems = i;precisa em free_mem
    if (num_elems == 0)
        return 0;

    cmd_info->arg = (char **) ( malloc(num_elems * sizeof(char *)) );
    cmd_info->infile = NULL;
    cmd_info->outfile = NULL;
    cmd_info->background = 0;

    bool b_infile = false;
    bool b_outfile = false;

    int iarg = 0;
    for (i = 0; i < num_elems; i++)
    {                   
        if ( !strcmp(args[i], "<") )
        {               
            if ( b_infile || i == num_elems-1 || !strcmp(args[i+1], "<") || !strcmp(args[i+1], ">") || !strcmp(args[i+1], "&") )
                return -1;                      

            i++;
            cmd_info->infile = malloc(strlen(args[i]) * sizeof(char));
            strcpy(cmd_info->infile, args[i]);
            b_infile = true;
        }

        else if (!strcmp(args[i], ">"))
        {
            if ( b_outfile || i == num_elems-1 || !strcmp(args[i+1], ">") || !strcmp(args[i+1], "<") || !strcmp(args[i+1], "&") )
                return -1;

            i++;    
            cmd_info->outfile = malloc(strlen(args[i]) * sizeof(char));
            strcpy(cmd_info->outfile, args[i]);
            b_outfile = true;
        }

        else if (!strcmp(args[i], "&"))
        {
            if ( i == 0 || i != num_elems-1 || cmd_info->background )
                return -1;

            cmd_info->background = true;
        }

        else
        {
            cmd_info->arg[iarg] = malloc(strlen(args[i]) * sizeof(char));
            strcpy(cmd_info->arg[iarg], args[i]);
            iarg++;
        }
    }

    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';
}

pid_t exec_simple(Command_Info *cmd_info)
{
    pid_t pid = fork();

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

    if (pid == 0)
    {
        if ( (execvp(cmd_info->arg[0], cmd_info->arg)) == -1)
        {
            perror(cmd_info->arg[0]);
            exit(1);
        }
    }

    return pid;
}

void type_prompt(void)
{
    printf("%s", PROMPT);
}

void syntax_error(void)
{
    printf("msh syntax error\n");
}

void free_mem(Command_Info *cmd_info)
{
    int i;
    for (i = 0; cmd_info->arg[i] != NULL; i++)
        free(cmd_info->arg[i]);
    free(cmd_info->arg);
    free(cmd_info->infile);
    free(cmd_info->outfile);
}

int main(int argc, char* argv[])
{
    char cmd_line[MAX_COMMAND_LENGTH];
    Command_Info cmd_info;
    //char* history[MAX_HISTORY_NUMBER];

    while (true)
    {   
        type_prompt();

        get_cmd(cmd_line);

        if ( parse_cmd(cmd_line, &cmd_info) == -1)
        {
            syntax_error();
            continue;
        }

        if (!strcmp(cmd_line, ""))
            continue;

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

        pid_t pid = exec_simple(&cmd_info);

        waitpid(pid, NULL, 0);

        free_mem(&cmd_info);
    }

    return 0;
} 

Answer 1

Since strings in C are null-terminated, their actuall size in memory is length+1, so instead of

cmd_info->infile = malloc(strlen(args[i]) * sizeof(char));

You should have

cmd_info->infile = malloc((strlen(args[i])+1) * sizeof(char));

EDIT: As Aeth said, you need to change every single occurence of malloc to contain space for that additional null character:

cmd_info->arg = (char **) ( malloc(num_elems * sizeof(char *)) ); //this one can stay, since it determines number of strings, not string length
cmd_info->outfile = malloc((strlen(args[i])+1) * sizeof(char));
cmd_info->arg[iarg] = malloc((strlen(args[i])+1) * sizeof(char));