pread(), pread64()

QNX SDP8.0C Library ReferenceAPIDeveloper

Read from a file without moving the file pointer

Synopsis:

#include <unistd.h>

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

ssize_t pread64( int filedes,
                 void *buff,
                 size_t nbytes,
                 off64_t offset );

Arguments:

filedes: The descriptor of the file that you want to read from.
buff: A pointer to a buffer where the function can store the data that it reads.
nbytes: The number of bytes that you want to read. This amount must not exceed SSIZE_MAX (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 pread() and pread64() functions perform the same action as read(), except that they read from a given position in the file without changing the file pointer.

Note:

The pread64() function is a large-file support version of pread() 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?.

The pread() function reads up to the maximum offset value that can be represented in an off_t for regular files. An attempt to perform a pread() on a file that's incapable of seeking results in an error.

Returns:

The number of bytes actually read, 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 read operation.
EBADF: The file descriptor, filedes, isn't a valid file descriptor open for reading.
EINTR: The read operation was interrupted by a signal, and either no data was transferred, or the resource manager responsible for that file does not report partial transfers.
EINVAL: The offset argument is invalid; the value is negative.
EIO: A physical I/O error occurred (for example, a bad block on a disk). The precise meaning is device-dependent.
EISDIR: The file descriptor is for a directory.
ENOSYS: The pread() function isn't implemented for the filesystem specified by filedes.
EOVERFLOW: An attempt was made to read an amount of data that exceeds the allowable limit.

Classification:

pread() is POSIX 1003.1; pread64() is Large-file support


Safety:
Cancellation point	Yes
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025