--- name: "pread" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/read/2" --- # pread(2) read input Part of [read(2)](/read/2.md). ## NAME **pread**, **read**, **preadv**, **readv** – read input ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** **#include \** **#include \** *ssize\_t*\ **pread**(*int d*, *void \*buf*, *size\_t nbyte*, *off\_t offset*); *ssize\_t*\ **read**(*int fildes*, *void \*buf*, *size\_t nbyte*); *ssize\_t*\ **preadv**(*int d*, *const struct iovec \*iov*, *int iovcnt*, *off\_t offset*); *ssize\_t*\ **readv**(*int d*, *const struct iovec \*iov*, *int iovcnt*); ## DESCRIPTION **read**() attempts to read *nbyte* bytes of data from the object referenced by the descriptor *fildes* into the buffer pointed to by *buf*. **readv**() performs the same action, but scatters the input data into the *iovcnt* buffers specified by the members of the *iov* array: iov\[0\], iov\[1\], ..., iov\[iovcnt-1\]. **pread**() and **preadv**() perform the same functions, but read from the specified position in the file without modifying the file pointer. For **readv**() and **preadv**(), the *iovec* structure is defined as: ``` struct iovec { char *iov_base; /* Base address. */ size_t iov_len; /* Length. */ }; ``` Each *iovec* entry specifies the base address and length of an area in memory where data should be placed. **readv**() and **preadv**() will always fill an area completely before proceeding to the next. On objects capable of seeking, the **read**() starts at a position given by the pointer associated with *fildes* (see [lseek(2)](/lseek/2.md)). Upon return from **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, **read**(), **readv**(), **pread**(), and **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. **read**() and **pread**() will fail if the parameter *nbyte* exceeds INT\_MAX, and they do not attempt a partial read. ## 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 **read**(), **readv**(), **pread**(), and **preadv**() calls will succeed unless: - \[EAGAIN\] The file was marked for non-blocking I/O, and no data were ready to be read. - \[EBADF\] *fildes* is not a valid file or socket descriptor open for reading. - \[EFAULT\] *Buf* points outside the allocated address space. - \[EINTR\] A read from a slow device was interrupted before any data arrived by the delivery of a signal. - \[EINVAL\] The pointer associated with *fildes* was negative. - \[EIO\] An I/O error occurred while reading from the file system. - \[EIO\] The process group is orphaned. - \[EIO\] The file is a regular file, *nbyte* is greater than 0, the starting position is before the end-of-file, and the starting position is greater than or equal to the offset maximum established for the open file descriptor associated with *fildes*. - \[EISDIR\] An attempt is made to read a directory. - \[ENOBUFS\] An attempt to allocate a memory buffer fails. - \[ENOMEM\] Insufficient memory is available. - \[ENXIO\] An action is requested of a device that does not exist. - \[ENXIO\] A requested action cannot be performed by the device. - \[ESTALE\] An attempt to read a remote file through NFS that has already been deleted in the server. - \[ETIMEDOUT\] The connection timed out while reading a remote file from a soft mounted NFS volume (see [mount\_nfs(8)](/mount_nfs/8.md)). - \[ETIMEDOUT\] The file is a “dataless” file that requires materialization and materialization timed out or encountered some other temporary failure. - \[EDEADLK\] The file is a “dataless” file that requires materialization and the I/O policy of the current thread or process disallows dataless file materialization (see [getiopolicy\_np(3)](/getiopolicy_np/3.md)). **pread**() and **preadv**() calls may also return the following errors: - \[EINVAL\] The specified file offset is invalid. - \[ESPIPE\] The file descriptor is associated with a pipe, socket, or FIFO. The **read**() call may also return the following errors: - \[ECONNRESET\] The connection is closed by the peer during a read attempt on a socket. - \[ENOTCONN\] A read is attempted on an unconnected socket. - \[ETIMEDOUT\] A transmission timeout occurs during a read attempt on a socket. The **read**() and **pread**() call may also return the following error: - \[EINVAL\] The value provided for *nbyte* exceeds INT\_MAX. The **readv**() and **preadv**() calls may also return one of the following errors: - \[EFAULT\] Part of the *iov* points outside the process's allocated address space. - \[EINVAL\] *Iovcnt* was less than or equal to 0, or greater than 16. - \[EINVAL\] One of the *iov\_len* values in the *iov* array was negative. - \[EINVAL\] The sum of the *iov\_len* values in the *iov* array overflowed a 32-bit integer. ## LEGACY SYNOPSIS **#include \** **#include \** **#include \** The include files ⟨sys/types.h⟩ and ⟨sys/uio.h⟩ are necessary for all functions. ## SEE ALSO [dup(2)](/dup/2.md), [fcntl(2)](/fcntl/2.md), [open(2)](/open/2.md), [pipe(2)](/pipe/2.md), [select(2)](/select/2.md), [socket(2)](/socket/2.md), [socketpair(2)](/socketpair/2.md), [compat(5)](/compat/5.md) ## STANDARDS The **read**() function call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”). The **readv**() and **pread**() functions are expected to conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”). **preadv**() is nonstandard. ## HISTORY The **pread**() function call appeared in AT&T System V Release 4 UNIX. The **readv**() function call appeared in 4.2BSD. A **read**() function call appeared in Version 6 AT&T UNIX.