/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2018.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU Lesser General Public License as published   *
* by the Free Software Foundation, either version 3 or (at your option)   *
* any later version. This program is distributed without any warranty.    *
* See the files COPYING.lgpl-v3 and COPYING.gpl-v3 for details.           *
\*************************************************************************/

/* Listing 59-1 */

/* read_line.c

   Implementation of readLine().
*/
#include <unistd.h>
#include <errno.h>
#include "read_line.h"                  /* Declaration of readLine() */

/* Read characters from 'fd' until a newline is encountered. If a newline
  character is not encountered in the first (n - 1) bytes, then the excess
  characters are discarded. The returned string placed in 'buf' is
  null-terminated and includes the newline character if it was read in the
  first (n - 1) bytes. The function return value is the number of bytes
  placed in buffer (which includes the newline character if encountered,
  but excludes the terminating null byte). */

ssize_t
readLine(int fd, void *buffer, size_t n)
{
    ssize_t numRead;                    /* # of bytes fetched by last read() */
    size_t totRead;                     /* Total bytes read so far */
    char *buf;
    char ch;

    if (n <= 0 || buffer == NULL) {
        errno = EINVAL;
        return -1;
    }

    buf = buffer;                       /* No pointer arithmetic on "void *" */

    totRead = 0;
    for (;;) {
        numRead = read(fd, &ch, 1);

        if (numRead == -1) {
            if (errno == EINTR)         /* Interrupted --> restart read() */
                continue;
            else
                return -1;              /* Some other error */

        } else if (numRead == 0) {      /* EOF */
            if (totRead == 0)           /* No bytes read; return 0 */
                return 0;
            else                        /* Some bytes read; add '\0' */
                break;

        } else {                        /* 'numRead' must be 1 if we get here */
            if (totRead < n - 1) {      /* Discard > (n - 1) bytes */
                totRead++;
                *buf++ = ch;
            }

            if (ch == '\n')
                break;
        }
    }

    *buf = '\0';
    return totRead;
}