C-fgets如何与多个电话一起使用

Question

I have the following code:

int i = 0;
char ar[500];

while(i < 20)
{
    printf("Type a line: ");
    fgets(ar, 500, stdin);
    fprintf(fp,"%s", ar);
    i++;
}

Basically, i am trying to open a file, then take 20 lines of input from the user and store it in an array, then print it to the file. It works fine but i don't understand how fgets works in the while loop. As i understand, when i = 0, fgets stores the 1st line to ar[500] and fprintf prints it out to the file, when i = 1, fgets stores the 2nd line to ar[500], but now ar[500] has 2 lines, but why fprintf only prints the 2nd line to the file but not all the 2 lines and so on when i increment by 1.

Answer 1

fgets always start populating the array at arr[0]. At every iteration, the previous line is overwritten. fgets will add the null terminating character for you in arr, so only the currently read line will be outputed to FILE pointed to by fp.

Answer 2

The description of char *fgets(char * restrict s, int n, FILE * restrict stream); is (C11 7.21.7p2) :

2 The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s . No additional characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array.

The first character read by fgets will be always stored at s[0] and so forth up until at most s[n - 1] or the first newline character encountered (it being the last character stored). The element after the last character read will be set to '\\0' , therefore terminating the string.

Ie effectively a fgets call will overwrite k + 1 first elements in the array where k is either the length of the line or n , whichever is smaller, and 1 means the null termination.

To store 20 lines, you'd need an array of arrays:

char ar[20][500];

then read into ar[i] .

Answer 3

The buffer ar will be overwritten each time.

fgets has the prototype char *fgets(char *str, int n, FILE *stream) . It will read a maximum of n-1 characters from stream and write them to str and then adds the '\\0' character.

Answer 4

regarding: "but now ar[500] has 2 lines,"

NO, the function fgets() does NOT append.

Rather, it overlays.

In the posted code, always starting at ar[0]