NAME

ftell, ftello — return a file offset in a stream

SYNOPSIS

#include <stdio.h>

long ftell(FILE *stream);


[CX] off_t ftello(FILE *stream);

DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The ftell() function shall obtain the current value of the file-position indicator for the stream pointed to by stream.

The ftell() function shall not change the setting of errno if successful.

^[CX] The ftello() function shall be equivalent to ftell(), except that the return value is of type off_t and the ftello() function may change the setting of errno if successful.

RETURN VALUE

Upon successful completion, ftell() ^[CX]  and ftello()  shall return the current value of the file-position indicator for the stream measured in bytes from the beginning of the file, ^[CX]  except in the case of streams opened with open_wmemstream() for which the position shall be measured in wide characters.

Otherwise, ftell() and ftello() shall return -1, and set errno to indicate the error.

ERRORS

The ftell() ^[CX]  and ftello() functions shall fail if:

[EBADF]

^[CX] The file descriptor underlying stream is not an open file descriptor.

[EOVERFLOW]

^[CX] For ftell(), the current file offset cannot be represented correctly in an object of type long.

[EOVERFLOW]

^[CX] For ftello(), the current file offset cannot be represented correctly in an object of type off_t.

[ESPIPE]

^[CX] The file descriptor underlying stream is associated with a pipe, FIFO, or socket.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

For all streams other than those opened by open_wmemstream(), ftell() and fseek() operate on byte offsets. The behavior with open_wmemstream() streams is intentionally different—ftell() and fseek() operate on wide character offsets. This is because those streams are unique in that the backing storage is not a multibyte representation but a wide character array, and it is useful to be able to use the output of ftell() to index into that array.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, fgetpos, fopen, fseek, lseek, open_memstream

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Large File Summit extensions are added.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The ftello() function is added.

• The [EOVERFLOW] error conditions are added.

An additional [ESPIPE] error condition is added for sockets.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0204 [105], XSH/TC1-2008/0205 [421], XSH/TC1-2008/0206 [122], XSH/TC1-2008/0207 [122], and XSH/TC1-2008/0208 [14] are applied.

Issue 8

Austin Group Defect 1027 is applied, specifying that for streams opened with open_wmemstream() the position is measured in wide characters, not bytes.