为什么变量声明的位置很重要？

Question

The main task for my code is to take out data from outside files and then ascribe a number at the beginning. The problem is that the code below works just fine.

#include <stdio.h>

int main(void) {
    char name[20], surname[30], group[2];
    puts("Now i download data from file");
    int n = 1, number;
    while (!feof(stdin)) {
        scanf ("%s %s %d %s", name, surname, &number, group);
        printf("%d. %s | %s | %d | %s", n, name, surname, number, group);
    }
    return 0;
}

But when I change the position of one line it sets the value of n to 0 and doesn't rise.

#include <stdio.h>

int main(void) {
    int n = 1, number;
    char name[20], surname[30], group[2];
    puts("Now I download data from file");
    while (!feof(stdin)) {
        scanf ("%s %s %d %s", name, surname, &number, group);
        printf("%d. %s | %s | %d | %s", n, name, surname, number, group);
    }
    return 0;
}

Answer 1

Your code has potential undefined behavior: if the data file contains fields that exceed the maximum length storable in the destination arrays, fscanf() will cause a buffer overrun and overwrite something else in memory, possibly another local variable.... Changing the order of the local variable definitions may change the side effects of this bug. Undefined behavior can lead to anything, from no visible effect to the program crash or even a market crash.

Also note that while (!feof(stdin)) is always wrong .

Here is how you can modify your code to avoid this undefined behavior and verify that the data file is correctly converted:

#include <stdio.h>

int main(void) {
    char buf[128];
    char name[20], surname[30], group[2];
    int n = 1, number;
    char dummy;

    puts("Now I download data from file");

    while (fgets(buf, sizeof buf, stdin)) {
        if (sscanf("%19s %29s %d %1s %c", name, surname, &number, group, &dummy) == 4) {
            printf("%d. %s | %s | %d | %s\n", n++, name, surname, number, group);
        } else {
            printf("Invalid format: %s", buf);
        }
    }
    printf("%d records\n", n);
    return 0;
}

Any records with a group larger than a single character will cause a format error. If the group may have 2 characters, you should define the array as char group[3] and convert it with %2s . The same remark applies to all string fields.