pwrite(), pwrite64()

QNX SDP8.0C Library ReferenceAPIDeveloper

Write into a file without changing the file pointer

Synopsis:

#include <unistd.h>

ssize_t pwrite( int filedes,
                const void* buff,
                size_t nbytes,
                off_t offset );

ssize_t pwrite64( int filedes,
                  const void* buff,
                  size_t nbytes,
                  off64_t offset );

Arguments:

filedes: The file descriptor for the file you want to write in.
buff: A pointer to a buffer that contains the data you want to write.
nbytes: The number of bytes that you want to write. This amount must not exceed SSIZE_MAX - sizeof(io_write_t) - sizeof(struct _xtype_offset) (see <limits.h>), or the function fails and sets errno to EOVERFLOW.
offset: The desired position inside the file.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The pwrite() and pwrite64() functions perform the same action as write(), except that they write into a given position without changing the file pointer.

Note:

The pwrite64() function is a large-file support version of pwrite() provided for backwards compatibility. If you're using large-file support functions and data types, you should define _LARGEFILE64_SOURCE with a value of 1 to ensure they appear in the name space. For more information, see Classification in What's in a Function Description?.

Returns:

The number of bytes actually written, or -1 if an error occurred (errno is set).

Errors:

EAGAIN

The O_NONBLOCK flag is set for the file descriptor, and the process would be delayed in the write operation.

EBADF

The file descriptor, filedes, isn't a valid file descriptor open for writing.

EFBIG

One of the following occurred:

An attempt was made to write a file that exceeds the maximum file size and there was no room for any bytes to be written.
The file is a regular file, nbytes is greater than 0, and the starting position is greater than or equal to the offset maximum established in the open file description associated with filedes.

EINTR

The write operation was interrupted by a signal, and either no data was transferred, or the resource manager responsible for that file doesn't report partial transfers.

EINVAL

The offset argument is invalid; the value is negative.

EIO

One of the following occurred:

The process is a member of a background process group attempting to write to its controlling terminal, TOSTOP is set, the process is neither ignoring nor blocking SIGTTOU, and the process group of the process is orphaned.
A physical I/O error occurred (for example, a bad block on a disk). The precise meaning is device-dependent.

ENOBUFS

Insufficient resources were available in the system to perform the operation.

ENOSPC

There's no free space remaining on the device containing the file.

ENOSYS

The pwrite() function isn't implemented for the device specified by filedes.

ENXIO

One of the following occurred:

A request was made of a nonexistent device, or the request was outside the capabilities of the device.
A hangup occurred on the STREAM being written to.

EOVERFLOW

An attempt was made to write an amount of data that when added to the sizes of the write message and extra type offset structures exceeds the allowable limit.

EPIPE

An attempt was made to write to a pipe (or FIFO) that isn't open for reading by any process. A SIGPIPE signal is also sent to the process.

ERANGE

The transfer request size was outside the range supported by the STREAMS file associated with filedes.

ESPIPE

The filedes argument is associated with a pipe, FIFO, or socket. The file pointer remains unchanged.

Classification:

pwrite() is POSIX 1003.1; pwrite64() is Large-file support


Safety:
Cancellation point	Yes
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025