Linux的实现在toolchain里边，所以这里直接copy苹果的实现，这样对于内存分配处理就清楚了。
/* getline.c -- Replacement for GNU C library function getlineCopyright (C) 1993 Free Software Foundation, Inc.This program is free software; you can redistribute it and/ormodify it under the terms of the GNU General Public License aspublished by the Free Software Foundation; either version 2 of theLicense, or (at your option) any later version.This program is distributed in the hope that it will be useful, butWITHOUT ANY WARRANTY; without even the implied warranty ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNUGeneral Public License for more details.  *//* Written by Jan Brittenson, bson@gnu.ai.mit.edu.  */#ifdef HAVE_CONFIG_H#include <config.h>#endif#include <sys/types.h>#include <stdio.h>#include <assert.h>#include <errno.h>#if STDC_HEADERS#include <stdlib.h>#elsechar *malloc (), *realloc ();#endif/* Always add at least this many bytes when extending the buffer.  */#define MIN_CHUNK 64/* Read up to (and including) a TERMINATOR from STREAM into *LINEPTR   + OFFSET (and null-terminate it). *LINEPTR is a pointer returned from   malloc (or NULL), pointing to *N characters of space.  It is realloc‘d   as necessary.  Return the number of characters read (not including the   null terminator), or -1 on error or EOF.  On a -1 return, the caller   should check feof(), if not then errno has been set to indicate   the error.  */intgetstr (lineptr, n, stream, terminator, offset)     char **lineptr;     size_t *n;     FILE *stream;     char terminator;     int offset;{  int nchars_avail;		/* Allocated but unused chars in *LINEPTR.  */  char *read_pos;		/* Where we‘re reading into *LINEPTR. */  int ret;  if (!lineptr || !n || !stream)    {      errno = EINVAL;      return -1;    }  if (!*lineptr)    {      *n = MIN_CHUNK;      *lineptr = malloc (*n);      if (!*lineptr)	{	  errno = ENOMEM;	  return -1;	}    }  nchars_avail = *n - offset;  read_pos = *lineptr + offset;  for (;;)    {      int save_errno;      register int c = getc (stream);      save_errno = errno;      /* We always want at least one char left in the buffer, since we	 always (unless we get an error while reading the first char)	 NUL-terminate the line buffer.  */      assert((*lineptr + *n) == (read_pos + nchars_avail));      if (nchars_avail < 2)	{	  if (*n > MIN_CHUNK)	    *n *= 2;	  else	    *n += MIN_CHUNK;	  nchars_avail = *n + *lineptr - read_pos;	  *lineptr = realloc (*lineptr, *n);	  if (!*lineptr)	    {	      errno = ENOMEM;	      return -1;	    }	  read_pos = *n - nchars_avail + *lineptr;	  assert((*lineptr + *n) == (read_pos + nchars_avail));	}      if (ferror (stream))	{	  /* Might like to return partial line, but there is no	     place for us to store errno.  And we don‘t want to just	     lose errno.  */	  errno = save_errno;	  return -1;	}      if (c == EOF)	{	  /* Return partial line, if any.  */	  if (read_pos == *lineptr)	    return -1;	  else	    break;	}      *read_pos++ = c;      nchars_avail--;      if (c == terminator)	/* Return the line.  */	break;    }  /* Done - NUL terminate and return the number of chars read.  */  *read_pos = ‘\0‘;  ret = read_pos - (*lineptr + offset);  return ret;}intgetline (lineptr, n, stream)     char **lineptr;     size_t *n;     FILE *stream;{  return getstr (lineptr, n, stream, ‘\n‘, 0);}

getline c实现