The Open Group Base Specifications Issue 8
IEEE Std 1003.1-2024
Copyright © 2001-2024 The IEEE and The Open Group

NAME

ftell, ftello — return a file offset in a stream

SYNOPSIS

#include <stdio.h>

long ftell(FILE *
stream);

[CX] [Option Start] off_t ftello(FILE *stream); [Option End]

DESCRIPTION

[CX] [Option Start] 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. [Option End]

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] [Option Start] 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. [Option End]

RETURN VALUE

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

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

ERRORS

The ftell() [CX] [Option Start]  and ftello() [Option End] functions shall fail if:

[EBADF]
[CX] [Option Start] The file descriptor underlying stream is not an open file descriptor. [Option End]
[EOVERFLOW]
[CX] [Option Start] For ftell(), the current file offset cannot be represented correctly in an object of type long. [Option End]
[EOVERFLOW]
[CX] [Option Start] For ftello(), the current file offset cannot be represented correctly in an object of type off_t. [Option End]
[ESPIPE]
[CX] [Option Start] The file descriptor underlying stream is associated with a pipe, FIFO, or socket. [Option End]

The following sections are informative.

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:

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.

End of informative text.

 

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