I am trying to scan unsigned integers until EOF (Ctrl-D on my platform). The integers scanned should be only positive and since 0 is considered unsigned I need to check if 0 was entered.

    int cnt1 = 0;
    unsigned *list1;
    list1 = malloc(sizeof(unsigned));

    printf("Enter positive integers to the first list:");
    while (scanf("%u", list1   cnt1)) {  /* getting the first list */
        if (*(list1   cnt1) == 0) {  /* checks to see if the number entered is 0 */
            printf("\nError - you must enter positive numbers");
            exit(-1);
        }
        cnt1  ;
        list1 = realloc(list1, (sizeof(unsigned)   cnt1 * sizeof(unsigned)));
    }
    printf("\n");

    free(list1);

My goal is that the user will type the integers until pressed Ctrl-D and all of the unsigned integers will be saved on the list1 pointer. But instead this happens:

Enter positive integers to the first list:23 10
Error - you must enter positive numbers

For some reason, the code only stops when I press Ctrl-D twice and it registers a number as 0.

CodePudding user response：

scanf("%u", list1 cnt1) returns:

• 1 if the conversion was successful
• 0 if the pending input cannot be converted to a number
• EOF if the stream is at end of file.

Hence you should write:

while (scanf("%u", list1   cnt1) == 1) {

Furthermore, it would be preferable to only reallocate the array if the number needs to be appended:

#include <stdio.h>
#include <stdlib.h>

unsigned *read_list(int *countp) {
    int count = 0;
    unsigned *list = NULL;
    unsigned *new_list;
    unsigned num;

    printf("Enter positive integers to the first list: ");
    while (scanf("%u", &num) == 1) {
        if (num == 0) {  /* checks to see if the number entered is 0 */
            printf("\nError - you must enter positive numbers\n");
            continue;
        }
        new_list = realloc(list, sizeof(*list) * (count   1));
        if (new_list == NULL) {
            printf("\nError - cannot allocate memory\n");
            free(list);
            *countp = -1;
            return NULL;
        }
        list = new_list;
        list[count  ] = num;
    }
    printf("\n");
    *countp = count;
    return list;
}