fsetpos man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

fsetpos — set current file position


#include <stdio.h>

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


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.1‐2008 defers to the ISO C standard.

The fsetpos() function shall set the file position and state indicators 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 clear the end-of-file indicator for the stream and undo any effects of ungetc() on the same stream. After an fsetpos() call, the next operation on an update stream may be either input or output.

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.

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.


The fsetpos() function shall fail if, 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:

The O_NONBLOCK flag is set for the file descriptor and the thread would be delayed in the write operation.
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.
An attempt was made to write a file that exceeds the maximum file size.
An attempt was made to write a file that exceeds the file size limit of the process.
The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream.
The write operation was terminated due to the receipt of a signal, and no data was transferred.
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.
There was no free space remaining on the device containing the file.
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.
The file descriptor underlying stream is associated with a pipe, FIFO, or socket.

The fsetpos() function may fail if:

A request was made of a nonexistent device, or the request was outside the capabilities of the device.

The following sections are informative.

See Also

Section 2.5, Standard I/O Streams, fopen(), ftell(), lseek(), rewind(), ungetc(), write()

The Base Definitions volume of POSIX.1‐2008, <stdio.h>