Capturing a variable length string from the command-line in C

Question

I\'ve looked everywhere for an answer to my question, but I have yet to find a solid answer to my problem.

I\'m currently in the process of writing a program in C, s

Answer 1

I think the answer about assuming a maximum command length is the correct one: typically you'll want to keep commands within a reasonable length.

But if you really can't make assumptions about the maximum length of a command, then you'll need to buffer.

Keep:

• a fixed buffer that you always fgets the same number of characters into, and
• a command that you append to, and realloc when necessary.

The following code probably lacks some error handling:

#define BUFFER_SIZE 20
#define COMMAND_BLOCK_SIZE 50

void get_command()
{
    char *buffer = malloc(sizeof(char) * (BUFFER_SIZE + 1));

    char *command = malloc(sizeof(char) * (COMMAND_BLOCK_SIZE + 1));
    int commandSize = 50;

    // tmp pointer for realloc:
    char *tmp = NULL;
    char *retval = NULL;

    if ((buffer == NULL) || (command == NULL))
        return_error("Error allocating memory");

    retval = fgets(buffer, BUFFER_SIZE, stdin);

    while (retval != NULL)
    {
        if (strlen(buffer) + strlen(command) > commandSize)
        {
            tmp = realloc(command, commandSize + (COMMAND_BLOCK_SIZE + 1));
            if (tmp == NULL)
                return_error("Error allocating memory");
            else
            {
                 command = tmp;
                 commandSize += COMMAND_BLOCK_SIZE;
            }
        }

        // not using strncat because the check above should guarantee that
        //    we always have more than BUFFER_SIZE more bytes in command 
        strcat(command, buffer);

        if (buffer[strlen(buffer) - 1] == '\n')
            break;

        retval = fgets(buffer, BUFFER_SIZE, stdin);
    }

    printf("COMMAND: %s\n", command);
    free(buffer);
}

Also note that:

• we don't do anything useful with command there, you'll probably want to pass in a char ** so that you can get command out of this function, and free it in the calling code, e.g. in your main loop.
• that the '\n' is retained in command: you may want to discard that.