Why does this code write input on second line?

Question

#include <stdio.h>
#include <string.h>


void main() {

    FILE* fp;
    char line[1024];
    char filename[1024];
    int length;
    int counter=0;

    while (1) {
       // fopen(filename, "w");
        int op = 0;
        printf("1. new file");
        scanf("%d", &op);
       
        switch (op) {

        case -1: break;
        case 1:
            
            printf("filename>>");
            scanf("%s", filename);
            length = strlen(filename);
            printf("length = %ld \n", strlen(filename));
            filename[length] = '.';
            filename[length + 1] = 't';
            filename[length + 2] = 'x';
            filename[length + 3] = 't';
            filename[length + 4] = '\0';
          
            fp=fopen(filename, "w");

            while (fgets(line, sizeof line, stdin) != NULL)
            { 
                if (line[0] == 'q' || line[0] == 'Q')
                {
                    printf("end of line \n");
                    fclose(fp);
                    break;
                }
                else
                    fputs(line, fp);
            }
        }
    }
}

I made this code to make text file with what I want. but the problem is, when I run this program, it writes input starting from the second line, not the first.

Why is this happening? I want to write input starting from the first line. How to change this code to write on the first line?

Answer 1

scanf("%s", filename);

When you input the name of file from keyboard, you type the filename then hit ENTER. The scanf function read filename but does not consume the ENTER character. Then when you use fgets, it will read this ENTER character then write a blank line into your file.

To solve this problem, you can use getchar() after scanf("%s", filename); to consume the enter character. You should add 1023 before %s in scanf function: Disadvantages of scanf.

scanf("%1023s", filename);
getchar();

Or you can use fgets instead:

fgets(filename, sizeof(filename), stdin);

One more thing, the break in switch statement does not help you to quit the first while loop (while (1) {...}). You should use one condition to exit this loop, for example:

char ch = 'a';
while (ch != 'q') {

  ...
  switch (op) {
      case -1: ch = 'q'; break; // type -1 will exit the first while loop.

The switch should have the default case for the exception.

 switch (op) {
    ...
    default:
       // handle the exception here.

Answer 2

Likely reason is that you are using fgets just after scanf on standard input.

See Using scanf and fgets in the same program?.