pg_iovec.h

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/*-------------------------------------------------------------------------
 *
 * pg_iovec.h
 *    Header for vectored I/O functions, to use in place of <sys/uio.h>.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/port/pg_iovec.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef PG_IOVEC_H
#define PG_IOVEC_H
 
#ifndef WIN32
 
#include <limits.h>
#include <sys/uio.h>            /* IWYU pragma: export */
#include <unistd.h>
 
#else
 
/* POSIX requires at least 16 as a maximum iovcnt. */
#define IOV_MAX 16
 
/* Define our own POSIX-compatible iovec struct. */
struct iovec
{
    void       *iov_base;
    size_t      iov_len;
};
 
#endif
 
/*
 * Define a reasonable maximum that is safe to use on the stack in arrays of
 * struct iovec and other small types.  The operating system could limit us to
 * a number as low as 16, but most systems have 1024.
 */
#define PG_IOV_MAX Min(IOV_MAX, 128)
 
/*
 * Like preadv(), but with a prefix to remind us of a side-effect: on Windows
 * this changes the current file position.
 */
static inline ssize_t
pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
{
#if HAVE_DECL_PREADV
    /*
     * Avoid a small amount of argument copying overhead in the kernel if
     * there is only one iovec.
     */
    if (iovcnt == 1)
        return pread(fd, iov[0].iov_base, iov[0].iov_len, offset);
    else
        return preadv(fd, iov, iovcnt, offset);
#else
    ssize_t     sum = 0;
    ssize_t     part;
 
    for (int i = 0; i < iovcnt; ++i)
    {
        part = pg_pread(fd, iov[i].iov_base, iov[i].iov_len, offset);
        if (part < 0)
        {
            if (i == 0)
                return -1;
            else
                return sum;
        }
        sum += part;
        offset += part;
        if ((size_t) part < iov[i].iov_len)
            return sum;
    }
    return sum;
#endif
}
 
/*
 * Like pwritev(), but with a prefix to remind us of a side-effect: on Windows
 * this changes the current file position.
 */
static inline ssize_t
pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
{
#if HAVE_DECL_PWRITEV
    /*
     * Avoid a small amount of argument copying overhead in the kernel if
     * there is only one iovec.
     */
    if (iovcnt == 1)
        return pwrite(fd, iov[0].iov_base, iov[0].iov_len, offset);
    else
        return pwritev(fd, iov, iovcnt, offset);
#else
    ssize_t     sum = 0;
    ssize_t     part;
 
    for (int i = 0; i < iovcnt; ++i)
    {
        part = pg_pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
        if (part < 0)
        {
            if (i == 0)
                return -1;
            else
                return sum;
        }
        sum += part;
        offset += part;
        if ((size_t) part < iov[i].iov_len)
            return sum;
    }
    return sum;
#endif
}
 
#endif                          /* PG_IOVEC_H */