Question

I am writing a simple shell program that must take input from a file one character at a time using the read() function in C. The read() function is the only way I want to take input from the user. I can get this part to operate properly in a very simple like this one:

int main( int argc, char *argv[] )
{
int error = 0;
int input_result = 1; /*Integer to track the result of inputs.*/
int input_exit = 0;
char input_buffer[100];
char *buffer_pointer = &input_buffer[0];
char *input;

write( 1, "# ", 2 ); /*Display # to signal the user to input data.*/

int input_counter = 0; /*Int that tracks the number of input characters.*/

/*While loop to continually take input from the user.*/
/*Step one of the programming assignment.*/
while( input_exit == 0 && input_result == 1 && input_counter < 15 && error == 0)
{
    input_result = read( 0, input, (size_t) 1 );

    input_buffer[input_counter] = *input;

    printf( "%c - %d\n", input_buffer[input_counter], input_result );

    input_counter++;
} /*End while for user input*/

write( 1, input_buffer, (size_t) input_counter );
}

Using my input file, which is also very simple I get the proper output with no errors what so ever. Once I start to make the code more complicated and add some nested while loops I start to receive the “Bad address” error. The code that is throwing the error can be seen below. As a side note, I do realize my C programming is not the greatest overall and there are things I could change, however I would like to just focus on the issues with read. Thank you in advance for your assistance it is greatly appreciated.

int main( int argc, char *argv[] )
{
/*Infinite loop to keep the shell running until it is implicitly ended by the user.*/
while( 1 )
{
    int error = 0; /*Int to keep track if an error occurred.*/

    char input_buffer[101]; /*Array of chars to hold the input.*/
    char *input_bufferp = &input_buffer[0]; /*Pointer to the first element of the char array.*/
    char *input; /*Char pointer to hold the read input*/
    char *newline = "\n";
    int buffer_counter = 0; /*Int that tracks the number of input characters.*/
    int input_result = 1; /*Int to hold the result of the read.*/

    char input_string[17][64]; /*Array to the parsed input data.*/
    char **input_stringp; /*Pointer to the first element of the string array.*/
    char *input_strings; /*Holds the parsed information before it is organized.*/
    int string_counter = 0; /*Int to track the number of strings.*/


    write( 1, "# ", 2 ); /*Display # to signal the user to input data.*/

    /*While loop to parse the information into separate strings.*/
    /*This while loop contains steps 1 and 2.*/
    while( string_counter == 0 && error == 0)
    {
        /*While to take in information via read.*/
        while( buffer_counter < 100 && input_result == 1 )
        {
            input_result = read( 0, input, (size_t) 1 ); /*Read one char from the user.*/

            /*If statement to signal an input error.*/
            if( input_result == -1 )
            {
                error = 1; /*Signal the error*/
                printf( "\nInput errno: %s\n", (char *)strerror(errno) ); /*Inform the user of the error.*/

                exit(0);
            }/*End if to signal an error.*/

            /*If to handle the end of the file.*/
            else if( input_result == 0 )
            {
                input_buffer[buffer_counter] = '\0'; /*Place the escape char.*/
            }/*End if for end of file.*/

            /*If statement handles a proper read from the user.*/
            else if( input_result == 1 )
            {
                /*If statement to check for a new line.*/
                if( strcmp( input_bufferp[buffer_counter], newline ) == 0 )
                {
                    input_result = 0; /*Set variable to exit the while loop.*/
                    input_buffer[buffer_counter] = '\0'; /*Place the escape char.*/
                }/*End new line if.*/

                /*Else statement to put input into the buffer.*/
                else
                {
                    input_buffer[buffer_counter++] = *input; /*Place the input into the buffer.*/
                }/*End buffer else.*/
            } /*End good input read.*/
        } /*End input gather while loop.*/

        write( 1, input_bufferp, (size_t) buffer_counter ); /*Echo the input to the user*/

        input_strings = strtok( input_bufferp, " \t" ); /*Split the input into tokens.*/

        /*While loop to tokenize the rest of the data.*/
        while( input_strings != NULL )
        {
            input_stringp[string_counter++] = input_strings; /*Store the tokenized string.*/
            input_strings = strtok( NULL, " \t" ); /*Spilt the remain data into tokens.*/
        } /*End tokenizer while loop.*/

        input_stringp[string_counter--][0] = '\0'; /*Please the escape char.*/
    } /*End parsing while loop.*/

    /*Check if the user wants to exit.*/
    if( strncmp( *input_stringp, "exit", 4 ) == 0 )
    {
        exit( 0 ); /*Exit the shell.*/
    } /*End if to exit program.*/


} /*End of infinite while loop.*/
}

Answer 1

You cannot use the value of a variable until you set it to some value. You never initialize input to point to anything and then you pass its value to read, reading data to no place in particular.

The second parameter to read tells read where to store the data. It must point to some space that you have allocated to store one or more characters, which your code never does.

Change input to be a char, not a char *. That will allocate space to store ono character. Then pass the address of input to read.