分段错误 - strcpy（） - C.

Question

I'm implementing a history feature for a command line shell. I've implemented a circular array to hold to ten most recent commands. Each command is also labeled by an integer specifying which total command is. For Example, if 30 total commands were entered, the ten commands in the circular array would be numbered (30, 29, 28, 27,...,21).

If a user were to insert the command "r" followed by a number labeling one of the ten instructions then that instruction is supposed to run. I keep running into a seg fault when trying to ensure that a two word command is accepted properly. Can anyone help point out what the problem is.

int main(void)
{
    char inputBuffer[MAX_LINE]; /* buffer to hold the command entered */
    int background;             /* equals 1 if a command is followed by '&' */
    char *args[MAX_LINE/2+1];/* command line (of 80) has max of 40 arguments */

    int position, count, rnum = 0;
    char historyArray[10][MAX_LINE];
    char *holder[MAX_LINE]={0};

    while (1){            /* Program terminates normally inside setup */
        background = 0;
        printf("COMMAND->");
        fflush(0);

        setup(inputBuffer, args, &background);       /* get next command */

        position = (count % MOD_VAL);
        strcpy(historyArray[position],args[0]);

        if(!strcmp("rr",args[0]))
        {
            strcpy(historyArray[position],historyArray[((position-1)+MOD_VAL)%MOD_VAL]);
            printf("%i",count);
            printf("%c",'.');
            printf("%c",' ');
            printf("%s",historyArray[position]);
            printf("%c",'\n');
            strcpy(args[0],historyArray[position]);
        }

        else if(!strcmp("r",args[0])) //SEG FAULT OCCURING IN THIS ELSE-IF BLOCK!
        {
            //args[1] will hold given number
            printf("%c",'\n');
            printf("%s",args[0]);
            printf("%s",args[1]);
            printf("%s",args[2]);
            printf("%c",'\n'); //PRINT STATEMENTS FOR DEBUGGING

            strncpy(holder[0], args[2], MAX_LINE - 1); //SEG FAULT

            rnum = atoi(args[1]);
            strcpy(historyArray[position],historyArray[((position-(count-rnum))+MOD_VAL)%MOD_VAL]);
            strcpy(args[0],historyArray[position]); //CHANGES VALUES OF args[1], args[2]

            if(holder[0] != NULL)
            {
                strncpy(args[1],holder[0],MAX_LINE-1);
                args[2] = NULL;
            }
            else
            {
                args[1] = NULL;
            }

            printf("%c",'\n');
            printf("%s",args[0]);
            printf("%s",args[1]);
            printf("%s",args[2]);
            printf("%c",'\n');
        }

        else if(!(strcmp("h",args[0]))||!(strcmp("history",args[0])))
        {
            int counter = 0;
            while(counter < 10)
            {
                printf("%i",(count - counter));
                printf("%c",'.');
                printf("%c",' ');
                printf("%s", historyArray[((position - counter + MOD_VAL)%MOD_VAL)]);
                printf("%c",' ');
                printf("%c",'\n');
                counter ++;

                if(counter > count)
                    break;
            }
        }
        count++;

        pid_t pid1; //Initialize pid_t variable to hold process identifier
        pid1 = fork(); //Fork process and assign process identifier to "pid1"

        if (pid1 == 0) //Child process
        {
            //Child process executes the command specified by the user and
            //then quits.
            execvp(args[0], args);
            exit(0);
        }
        else //Parent process
        {
            if (background != 1)//Check for inclusion of '&' in command 
            {
                wait(NULL); //Wait for child process to finish executing
            }
        } 

        /* the steps are:
         (1) fork a child process using fork()
         (2) the child process will invoke execvp()
         (3) if background == 0, the parent will wait, 
         otherwise returns to the setup() function. */
    }
}

Any assistance is appreciated!

-MATT

Answer 1

Here your args is the array of character pointers.

But strcpy requires two arguments - that should be array or character pointer to which memory allocated by malloc

But your strcpy(historyArray[position],args[0]); takes one argument as character pointer which will not be accepted.

so you can either change the args[] to args[][] or args[0] = malloc(some_no) , segfault will be removed.

Answer 2

You note that the crash occurs on the line

else if(!strcmp("r",args[0]))

If I were you, I would load the core file in a debugger and see what the value of args[0] is when passed to strcmp() .

I expect you have compiler warnings about type mismatch between char and char* . You declare args as char* . That means args[0] is a char , not a char* . To compare the single character, just use a character instead of strcmp() :

else if ('r' != args[0])

---

Some notes on pitfalls with C's string handling:

• strcmp() isn't safe with respect to array boundaries when its arguments are not correctly NUL-terminated
  • use strncmp() to provide a limit to the number of characters compared
• although strncpy() guards against array boundaries, it does not guarantee to NUL-terminate the destination string
• strcpy() does not respect array boundaries; it is your responsibility to ensure the destination array is large enough to receive the string being copied to it

Answer 3

You are missing to allocated memory the the char pointers held in args and holder .

So referring to those as pointers to 0-terminated character arrays ("strings") via the str*() family of functions, leads to undefined bahaviour, as the str*() function try to derefernce those pointers not point to valid memory.