man pread (2): read input

Other Alias

read, readv, preadv

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In unistd.h Ft ssize_t Fn read int fd void *buf size_t nbytes Ft ssize_t Fn pread int fd void *buf size_t nbytes off_t offset In sys/uio.h Ft ssize_t Fn readv int fd const struct iovec *iov int iovcnt Ft ssize_t Fn preadv int fd const struct iovec *iov int iovcnt off_t offset

DESCRIPTION

The Fn read system call attempts to read Fa nbytes of data from the object referenced by the descriptor Fa fd into the buffer pointed to by Fa buf . The Fn readv system call performs the same action, but scatters the input data into the Fa iovcnt buffers specified by the members of the Fa iov array: iov[0], iov[1], ..., iov[iovcnt-1]. The Fn pread and Fn preadv system calls perform the same functions, but read from the specified position in the file without modifying the file pointer.

For Fn readv and Fn preadv , the Fa iovec structure is defined as:

struct iovec {
        void   *iov_base;  /* Base address. */
        size_t iov_len;    /* Length. */
};

Each Fa iovec entry specifies the base address and length of an area in memory where data should be placed. The Fn readv system call will always fill an area completely before proceeding to the next.

On objects capable of seeking, the Fn read starts at a position given by the pointer associated with Fa fd (see lseek(2)). Upon return from Fn read , the pointer is incremented by the number of bytes actually read.

Objects that are not capable of seeking always read from the current position. The value of the pointer associated with such an object is undefined.

Upon successful completion, Fn read , Fn readv , Fn pread and Fn preadv return the number of bytes actually read and placed in the buffer. The system guarantees to read the number of bytes requested if the descriptor references a normal file that has that many bytes left before the end-of-file, but in no other case.

RETURN VALUES

If successful, the number of bytes actually read is returned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The Fn read , Fn readv , Fn pread and Fn preadv system calls will succeed unless:

Bq Er EBADF: The Fa fd argument is not a valid file or socket descriptor open for reading.
Bq Er ECONNRESET: The Fa fd argument refers to a socket, and the remote socket end is forcibly closed.
Bq Er EFAULT: The Fa buf argument points outside the allocated address space.
Bq Er EIO: An I/O error occurred while reading from the file system.
Bq Er EBUSY: Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped
Bq Er EINTR: A read from a slow device (i.e. one that might block for an arbitrary amount of time) was interrupted by the delivery of a signal before any data arrived.
Bq Er EINVAL: The pointer associated with Fa fd was negative.
Bq Er EAGAIN: The file was marked for non-blocking I/O, and no data were ready to be read.
Bq Er EISDIR: The file descriptor is associated with a directory residing on a file system that does not allow regular read operations on directories (e.g. NFS).
Bq Er EOPNOTSUPP: The file descriptor is associated with a file system and file type that do not allow regular read operations on it.
Bq Er EOVERFLOW: The file descriptor is associated with a regular file, Fa nbytes is greater than 0, Fa offset is before the end-of-file, and Fa offset is greater than or equal to the offset maximum established for this file system.
Bq Er EINVAL: The value Fa nbytes is greater than INT_MAX

In addition, Fn readv and Fn preadv may return one of the following errors:

Bq Er EINVAL: The Fa iovcnt argument was less than or equal to 0, or greater than IOV_MAX
Bq Er EINVAL: One of the Fa iov_len values in the Fa iov array was negative.
Bq Er EINVAL: The sum of the Fa iov_len values in the Fa iov array overflowed a 32-bit integer.
Bq Er EFAULT: Part of the Fa iov array points outside the process's allocated address space.

The Fn pread and Fn preadv system calls may also return the following errors:

Bq Er EINVAL: The Fa offset value was negative.
Bq Er ESPIPE: The file descriptor is associated with a pipe, socket, or FIFO.

STANDARDS

The Fn read system call is expected to conform to St -p1003.1-90 . The Fn readv and Fn pread system calls are expected to conform to St -xpg4.2 .

HISTORY

The Fn preadv system call appeared in Fx 6.0 . The Fn pread function appeared in AT&T System V.4 . The Fn readv system call appeared in BSD 4.2 The Fn read function appeared in AT&T System v6 .