ftell、_ftelli64

项目
06/09/2015

Gets the current position of a file pointer.

long ftell( 
   FILE *stream 
);
__int64 _ftelli64( 
   FILE *stream 
);

参数

stream
Target FILE structure.

返回值

ftell and _ftelli64 return the current file position. The value returned by ftell and _ftelli64 may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return–linefeed translation. Use ftell with fseekor_ftelli64with_fseeki64 to return to file locations correctly. On error, ftelland_ftelli64 invoke the invalid parameter handler, as described in 参数验证. If execution is allowed to continue, these functions return –1L and set errno to one of two constants, defined in ERRNO.H. The EBADF constant means the stream argument is not a valid file pointer value or does not refer to an open file. EINVAL means an invalid stream argument was passed to the function. On devices incapable of seeking (such as terminals and printers), or when stream does not refer to an open file, the return value is undefined.

有关这些内容的详细信息以及其他返回代码，请参见 _doserrno、errno、_sys_errlist 和 _sys_nerr。

备注

The ftell and _ftelli64functions retrieve the current position of the file pointer (if any) associated with stream*.*The position is expressed as an offset relative to the beginning of the stream.

Note that when a file is opened for appending data, the current file position is determined by the last I/O operation, not by where the next write would occur. For example, if a file is opened for an append and the last operation was a read, the file position is the point where the next read operation would start, not where the next write would start.(When a file is opened for appending, the file position is moved to end of file before any write operation.)If no I/O operation has yet occurred on a file opened for appending, the file position is the beginning of the file.

In text mode, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing, fopen and all related routines check for a CTRL+Z at the end of the file and remove it if possible. 这是因为使用 ftell 和 fseek的结合或_ftelli64 和 _fseeki64的结合，在以 CTRL+Z 结尾的文件中移动时，可能导致 ftell 或 _ftelli64 在文件末尾错误运行。

该函数在运行期间锁定调用线程，因此线程安全的。有关非固定版本，请参见 _ftell_nolock。

要求

功能	必需的标头	可选标头
ftell	<stdio.h>	<errno.h>
_ftelli64	<stdio.h>	<errno.h>

有关其他兼容性信息，请参见“简介”中的兼容性。

示例

// crt_ftell.c
// This program opens a file named CRT_FTELL.C
// for reading and tries to read 100 characters. It
// then uses ftell to determine the position of the
// file pointer and displays this position.
 

#include <stdio.h>

FILE *stream;

int main( void )
{
   long position;
   char list[100];
   if( fopen_s( &stream, "crt_ftell.c", "rb" ) == 0 )
   {
      // Move the pointer by reading data: 
      fread( list, sizeof( char ), 100, stream );
      // Get position after read: 
      position = ftell( stream );
      printf( "Position after trying to read 100 bytes: %ld\n",
              position );
      fclose( stream );
   }
}