如果n <= 0,返回NULL
如果n == 1,返回""，也就是一個空串。
如果成功返回值等于string，也就是獲得的字符串的首地址。
如果出錯，或者讀到FILE的結尾，返回NULL.

以下是這個函數的實現：

/***
*fgets.c - get string from a file
*
* Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
* defines fgets() - read a string from a file
*
*******************************************************************************/

#include <cruntime.h>
#include <stdio.h>
#include <dbgint.h>
#include <file2.h>
#include <internal.h>
#include <mtdll.h>
#include <tchar.h>

/***
*char *fgets(string, count, stream) - input string from a stream
*
*Purpose:
* get a string, up to count-1 chars or '\n', whichever comes first,
* append '\0' and put the whole thing into string. the '\n' IS included
* in the string. if count<=1 no input is requested. if EOF is found
* immediately, return NULL. if EOF found after chars read, let EOF
* finish the string as '\n' would.
*
*Entry:
* char *string - pointer to place to store string
* int count - max characters to place at string (include \0)
* FILE *stream - stream to read from
*
*Exit:
* returns string with text read from file in it.
* if count <= 0 return NULL
* if count == 1 put null string in string
* returns NULL if error or end-of-file found immediately
*
*Exceptions:
*
*******************************************************************************/

_TSCHAR * __cdecl _fgetts (
_TSCHAR *string,
int count,
FILE *str
)
{
REG1 FILE *stream;
REG2 _TSCHAR *pointer = string;
_TSCHAR *retval = string;
int ch;

_VALIDATE_RETURN(( string != NULL ) || ( count == 0 ), EINVAL, NULL);
_VALIDATE_RETURN(( count >= 0 ), EINVAL, NULL);
_VALIDATE_RETURN(( str != NULL ), EINVAL, NULL);

if (count == 0)
{
return NULL;
}

/* The C Standard states the input buffer should remain
unchanged if EOF is encountered immediately. Hence we
do not blank out the input buffer here */

/* Init stream pointer */
stream = str;

_lock_str(stream);
__try {

#ifndef _UNICODE
_VALIDATE_STREAM_ANSI_SETRET(stream, EINVAL, retval, NULL);
#endif /* _UNICODE */
if(retval!=NULL)
{
while (--count)
{
if ((ch = _fgettc_nolock(stream)) == _TEOF)
{
if (pointer == string) {
retval=NULL;
goto done;
}

break;
}

if ((*pointer++ = (_TSCHAR)ch) == _T('\n'))
break;
}
*pointer = _T('\0');
}


/* Common return */
done:

; }
__finally {
_unlock_str(stream);
}

return(retval);
}