Code won't execute in do while loop in C

Question

Here is a snippet of my code:

printf("\nCommand? ");
ret = scanf("%c", &command);
do
{
    // printf("Command? ");
    // ret = scanf("%c", &command);
    if (ret != 1)
    {
        fprintf(stderr, "Invalid input!\n");
    }

    if (command == 'd')
    {
        result = dequeue(&queue1, &entry);
        if (result == 1)
            printf("%d was dequeued\n", entry);
        else if (result == 0)
            fprintf(stderr, "ERROR: attempt to dequeue from an empty"
                    " queue\n");
    }
    else if (command == 'e')
    {
        ret = scanf("%d", &add);
        result = enqueue(q, add);
    }
    else if (command == 'q')
        break;
    else
        fprintf(stderr, "Invalid command!\n");

    printf("Queue:");
    for (int i = 0; i < q->end; ++i)
    {
        printf("%d", q->element[i]);
    }
    printf("\nCommand? ");
    scanf("%c", &command);
} while (command != 'q');

Then here is the partial GDB log:

146             printf("Command? ");
(gdb)
147             ret = scanf("%c", &command);
(gdb)
Command? d
148             if (ret != 1)
(gdb)
153             if (command == 'd')
(gdb)
155                 result = dequeue(&queue1, &entry);
(gdb)
156                 if (result == 1)
(gdb)
158                 else if (result == 0)
(gdb)
159                     fprintf(stderr, "ERROR: attempt to dequeue from an empty"
(gdb)
ERROR: attempt to dequeue from an empty queue
172             printf("Queue:");
(gdb)
173             for (int i = 0; i < q->end; ++i)
(gdb)
177             printf("\nCommand? ");
(gdb)
Queue:
178             scanf("%c", &command);
(gdb)
179         } while (command != 'q');
(gdb)

as you can see, the line 172 printf("Queue:"); won't get executed, as well as the rest of the code. I cannot figure out why.

I typed d into command

Could someone help me explain this? Thanks.

Answer 1

I think your concern is that the printf is traced in the debugger but no output is produced. This is probably because printf calls send output to stdout , which is usually buffered, so output may not appear until later when running in gdb. In some systems, the buffer is flushed when a newline is seen. So you might try adding \\n to the end of "Queue:" . Or fflush(stdout); after the print will definitely cause the printf to work. Output to stderr is not buffered. That's why you see that output immediately.

Answer 2

It is executed, as you can see in your single stepping of the code in the debugger. It doesn't print right away because printf() output is kept in a buffer until the buffer is full or until a newline is encountered. Either put a newline at the end or fflush(stdout) afterwards if you need to see the output immediately.

Answer 3

检查完代码后，我唯一能想到的解决方案是for循环q->end是<= 0

Answer 4

all.

I think I've figure it out how: just to write another function to print out all the results.

So the modified code is like this:

do
{
    ret = fgets(buf, BUF_LENGTH, "%c %d", &command, &add);
    if (ret != 1 && ret != 2)
    {
        fprintf(stderr, "Invalid input!\n");
        continue;
    }

    if (command == 'd')
    {
        ...
    }
    else if (command == 'e')
    {
         ...
    }
    else if (command == 'q')
        break;
    else
        fprintf(stderr, "Invalid command!\n");

    /* Printing out the queue elements */
    print_element(q);

    printf("Command? ");
} while (command != 'q');

I know my code is messy, I am still a beginner of C programming language. I am in the progress of learning pointers.

There are some modifications in the "..." part of code, but I believe the changes are not related to I/O.

Thank you all for suggestions.