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pread(), pread64()

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.

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() function performs the same action as read(), except that it reads from a given position in the file without changing the file pointer.

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.

The pread64() function is a 64-bit version of pread().

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, fildes, 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.

EIO

A physical I/O error occurred (for example, a bad block on a disk). The precise meaning is device-dependent.

ENOSYS

The pread() function isn't implemented for the filesystem specified by filedes.

Classification:

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

Safety:
Cancellation point	Yes
Interrupt handler	No
Signal handler	Yes
Thread	Yes

See also:

close(), creat(), dup(), dup2(), errno, fcntl(), lseek(), open(), pipe(), pwrite(), read(), readblock(), readv(), select(), write(), writeblock(), writev()

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