MirBSD manpage: pread(2), preadv(2), read(2), readv(2)

READ(2)                    BSD Programmer's Manual                     READ(2)


     read, readv, pread, preadv - read input


     #include <unistd.h>

     read(int d, void *buf, size_t nbytes);

     pread(int d, void *buf, size_t nbytes, off_t offset);

     #include <sys/uio.h>
     #include <unistd.h>

     readv(int d, const struct iovec *iov, int iovcnt);

     preadv(int d, const struct iovec *iov, int iovcnt, off_t offset);


     read() attempts to read nbytes of data from the object referenced by the
     descriptor d 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 po-
     sition in the file without modifying the file pointer.

     For readv() and preadv(), the iovec structure is defined as:

           struct iovec {
                   void *iov_base;
                   size_t iov_len;

     Each iovec entry specifies the base address and length of an area in
     memory where data should be placed. readv() will always fill an area com-
     pletely before proceeding to the next.

     On objects capable of seeking, the read() starts at a position given by
     the pointer associated with d (see lseek(2)). 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 po-
     sition. The value of the pointer associated with such an object is unde-

     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 refer-
     ences a normal file that has that many bytes left before the end-of-file,
     but in no other case.

     Note that readv() and preadv() will fail if the value of iovcnt exceeds
     the constant IOV_MAX.


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


     read(), readv(), pread(), and preadv() will succeed unless:

     [EBADF]       d is not a valid file or socket descriptor open for read-

     [EFAULT]      buf points outside the allocated address space.

     [EIO]         An I/O error occurred while reading from the filesystem, or
                   the process is a member of a background process attempting
                   to read from its controlling terminal, the process is ig-
                   noring or blocking the SIGTTIN signal or the process group
                   is orphaned.

     [EINTR]       A read from a slow device was interrupted before any data
                   arrived by the delivery of a signal.

     [EINVAL]      The pointer associated with d was negative.

     [EAGAIN]      The file was marked for non-blocking I/O, and no data were
                   ready to be read.

     In addition, read() and pread() may return the following error:

     [EINVAL]      nbytes was larger than SSIZE_MAX.

     Also, readv() and preadv() may return one of the following errors:

     [EINVAL]      iovcnt was less than or equal to 0, or greater than

     [EINVAL]      The sum of the iov_len values in the iov array overflowed
                   an ssize_t.

     [EFAULT]      Part of the iov points outside the process's allocated ad-
                   dress space.


     dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socket(2),


     The read() function conforms to IEEE Std 1003.1-1990 ("POSIX.1"). The
     readv() and pread() functions conform to X/Open Portability Guide Issue
     4, Version 2 ("XPG4.2").


     The preadv() function first appeared in OpenBSD 2.7. The pread() function
     appeared in AT&T System V.4 UNIX. The readv() function call appeared in
     4.2BSD. The read() function call appeared in Version 2 AT&T UNIX.


     Error checks should explicitly test for -1. Code such as

             while ((nr = read(fd, buf, sizeof(buf))) > 0)

     is not maximally portable, as some platforms allow for nbytes to range
     between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an
     error-free read() may appear as a negative number distinct from -1. Prop-
     er loops should use

             while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)

MirBSD #10-current            November 29, 2021                              1

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