NAME

fsetpos — set current file position

SYNOPSIS

#include <stdio.h>

int fsetpos(FILE *stream, const fpos_t *pos);


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 fsetpos() function shall set the mbstate_t object (if any) and file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which the application shall ensure is a value obtained from an earlier call to fgetpos() on the same stream. If a read or write error occurs, the error indicator for the stream shall be set and fsetpos() fails.

A successful call to the fsetpos() function shall undo any effects of the ungetc() function on the stream, clear the end-of-file indicator for the stream, and then establish the new parse state and position. After a successful fsetpos() call, the next operation on an update stream can be either input or output.

^[CX] The behavior of fsetpos() on devices which are incapable of seeking is implementation-defined. The value of the file offset associated with such a device is undefined.

If the stream has an underlying file description and is writable, and buffered data has not been written to the underlying file, fsetpos() shall cause the unwritten data to be written to the file and shall mark the last data modification and last file status change timestamps of the file for update.

If the stream was created by fmemopen(), open_memstream(), or open_wmemstream() and the stream is writable, and if the stream is buffered and data in the stream's buffer has not been written to the underlying memory buffer, fsetpos() shall cause the unwritten data to be written to the underlying memory buffer.

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

RETURN VALUE

The fsetpos() function shall return 0 if it succeeds; otherwise, it shall return a non-zero value and set errno to indicate the error.

ERRORS

The fsetpos() function shall fail if, ^[CX]  either the stream is unbuffered or the stream's buffer needed to be flushed, and the call to fsetpos() causes an underlying lseek() or write() to be invoked, and:

[EAGAIN]

^[CX] The O_NONBLOCK flag is set for the file descriptor and the thread would be delayed in the write operation.

[EBADF]

^[CX] The file descriptor underlying the stream file is not open for writing or the stream's buffer needed to be flushed and the file is not open.

[EFBIG]

^[CX] An attempt was made to write a file that exceeds the maximum file size.

[EFBIG]

^[CX] An attempt was made to write a file that exceeds the file size limit of the process.
^[XSI] A SIGXFSZ signal shall also be generated for the thread.

[EFBIG]

^[CX] The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream.

[EINTR]

^[CX] The write operation was terminated due to the receipt of a signal, and no data was transferred.

[EIO]

^[CX] A physical I/O error has occurred, or the process is a member of a background process group attempting to perform a write() to its controlling terminal, TOSTOP is set, the calling thread is not blocking SIGTTOU, the process is not ignoring SIGTTOU, and the process group of the process is orphaned. This error may also be returned under implementation-defined conditions.

[ENOMEM]

^[CX] The stream was created by open_memstream() or open_wmemstream() and insufficient memory is available.

[ENOSPC]

^[CX] There was no free space remaining on the device containing the file or in the buffer used by the fmemopen() function.

[EPIPE]

^[CX] An attempt was made to write to a pipe or FIFO that is not open for reading by any process; a SIGPIPE signal shall also be sent to the thread.

[ESPIPE]

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

The fsetpos() function may fail if:

[ENXIO]

^[CX] A request was made of a nonexistent device, or the request was outside the capabilities of the device.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

The ERRORS section does not include an [EINVAL] error equivalent to the one for fseek() because applications are required to obtain the fpos_t value using fgetpos(), in which case the file position to be set will always be valid. Directly manipulating the fpos_t object to set a position results in undefined behavior. However, if an implementation detects that the requested file position would be a negative value, or would be beyond the end of the underlying memory buffer of a stream that was created by fmemopen(), it is recommended that fsetpos() returns a non-zero value and sets errno to [EINVAL].

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, fopen, fmemopen, ftell, lseek, open_memstream, rewind, ungetc, write

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 4. Derived from the ISO C standard.

Issue 6

Extensions beyond the ISO C standard are marked.

The normative text is updated to avoid use of the term "must" for application requirements.

The DESCRIPTION is updated to clarify that the error indicator is set for the stream on a read or write error. This is for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/21 is applied, deleting an erroneous [EINVAL] error case from the ERRORS section.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/43 is applied, updating the [EAGAIN] error in the ERRORS section from "the process would be delayed" to "the thread would be delayed".

Issue 7

SD5-XSH-ERN-220 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0192 [105], XSH/TC1-2008/0193 [79], XSH/TC1-2008/0194 [225], XSH/TC1-2008/0195 [450], XSH/TC1-2008/0196 [450], and XSH/TC1-2008/0197 [14] are applied.

Issue 8

Austin Group Defect 308 is applied, clarifying the handling of [EFBIG] errors.

Austin Group Defect 1225 is applied, aligning the CX requirements with fseek().

Austin Group Defect 1249 is applied, correcting some text mismatches with the ISO C standard.

Austin Group Defect 1669 is applied, removing XSI shading from part of the [EFBIG] error relating to the file size limit for the process.