Read size-limited input line ignoring the tail part

Question

Is there a library function to read a line of input from stdin with the following requirements?

1. I have a limited static buffer of specific size (size may be a known constant).
2. No dynamic allocation allowed. So the library functions like getline() cannot be used.
3. For lines whose length is beyond the buffer size, the unread tail part of the line is to be ignored.

My solution to read a line is using fgets and a loop to read and ignore the tail part. The code is as below

char buffer[80], tail_buffer[80];
char * line, * tail;
line = tail = fgets(buffer, 80, stdin);
/* Read the tail part to ignore it */
while (tail != NULL && tail[strlen(tail)-1] != '\n')
{
    tail = fgets(tail_buffer, 80, stdin);
}
/* Use 'line' as needed */

Answer 1

After looking into the documentation of scanf, I found a solution myself.

char buffer[80];
scanf(" %79[^\n]%*[^\n]", buffer);

EDIT: With the comments from @chux that this has some limitations in parsing the blank lines and initial spaces, and with the solution from @user3386109, I enhance this as below to scan all the lines till EOF.

char buffer[80] = "";

while (scanf("%79[^\n]%*[^\n]", buffer) != EOF)
{
    /* Process the line in buffer */

    if (feof(stdin)) break;
    getchar(); /* Remove end of line */
    buffer[0] = 0;
}

Answer 2

fgets() has corner cases that preclude using it at a complete solution to OP's goal.
Simply loop using fgetc().

// Return count of `char` read - not including potential \n.
int read_line(char *dest, int size) {
  int i = 0;
  if (size > 0) {
    size--;
    int ch;
    while ((ch = fgetc(stdin)) != '\n' && ch != EOF) {
      if (i < size) {
        dest[i++] = ch;
      }
    }
    dest[i] = '\0';
    if (ch == EOF && i == 0) return EOF;
    }
  return i;
}

---

A forced use of fgets() looks like

bool was_there_extra(char *buf, size_t size) {
  char *lf = strchr(buf, '\n');
  if (lf) {
    *lf = '\0';  // optional: lop off potential trailing \n
    return false;
  }
  int ch;
  bool extra = false;
  while ((ch = fgetc(stdin)) != '\n' && ch != EOF) {
    extra = true;
  }
  return extra;
}

while (fgets(buf, sizeof buf, stdin)) {
  if (was_there_extra(buf, sizeof buf)) ...
  else ...
}

This approach does get fooled if code reads a '\0'.

Answer 3

An alternative solution using scanf to read until the newline is found, and the getchar to read the newline.

char buffer[80];
if ( fgets( buffer, sizeof buffer, stdin ) != NULL )
{
    if ( strchr( buffer, '\n' ) == NULL )   // if the buffer does not contain the newline
    {
        scanf( "%*[^\n]" );                 // read up to the newline
        getchar();                          // read the newline
    }
}