ftell()

(PHP 4, PHP 5, PHP 7)

返回文件指针读/写的位置

说明

返回由$handle指定的文件指针的位置，也就是文件流中的偏移量。

参数

文件指针必须是有效的，且必须指向一个通过fopen()或popen()成功打开的文件。在附加模式（加参数"a"打开文件）中ftell()会返回未定义错误。

返回值

Returns the position of the file pointer referenced by$handleas an integer; i.e., its offset into the file stream.

如果出错，返回FALSE。

范例

Example #1ftell()例子

参见

• fopen() 打开文件或者 URL
• popen() 打开进程文件指针
• fseek() 在文件指针中定位
• rewind() 倒回文件指针的位置

When opening a file for reading and writing via fopen('file','a+') the file pointer should be at the end of the file. However ftell() returns int(0) even if the file is not empty. Also it seems that there is two pointers, first for reading and second for writing, because it acts differently on first operation (reading or writing).
Example:

Result:
0 "123456789" 9
0 "123456789" 9
2 "3456789" 9
2 "" 2
0 "rr3456789aa" 11

When opening a file to append via fopen('file','ab') the file pointer should be at the end of the file. However ftell() returns int(0) even if the file is not empty and even after writing some text into the file.

In response to php at michielvleugel dot com:
This does not seem to be the case with PHP 5.2.0 and FreeBSD 5.4.
#!/usr/local/bin/php

root@localhost:/home/david# echo Hello World | ./test.php
int(0)
root@localhost:/home/david# ./test.php
int(6629927)
When something is piped to the script, it returns an integer value of 0, however, it also returns an integer when nothing is piped to the script.
The code should be modified to this:
#!/usr/local/bin/php

And the result is:
root@localhost:/home/david# echo Hello World | ./test.php
Something was piped: Hello World
root@localhost:/home/david# ./test.php
Nothing was piped

Attention! If you open a file with the "text"-modifier (e.g. 'rt') and the file contains \r\n as line-endings, ftell() returns the position as if there were only \n as line-endings.
Example:
If the first line only contains 1 char followed by \r\n, the start of the second line should be position 3. (1char + \r + \n = 3 bytes) But ftell() will return 2 - ignoring one byte. If you call ftell() in line 3, the value will differ from the real value by 2 bytes. The error gets greater with every line.
(Watched this behavior in PHP 5.0.4 for Windows.)
BUT: fseek() works as expected - using the true byte values.

Actually, ftell() gives more than an undefined result for append only streams; it gives the offset from the end of the file as defined before any data was appended. So if you open a file that had 3017 characters, and append 41 characters, and then execute ftell(), the value returned will be 41.

When trying to determine whether or not something was piped into a command line script, it is not smart to do a fgets(STDIN), because it will wait indefenitely if nothing is piped. Instead, I found ftell on STDIN to be very handy: it will return an integer of zero when something was piped, and nothing if nothing was piped to the script.
#!/usr/bin/php4 -q 

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