pread.c

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/*-------------------------------------------------------------------------
 *
 * pread.c
 *    Implementation of pread(2) for platforms that lack one.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/port/pread.c
 *
 * Note that this implementation changes the current file position, unlike
 * the POSIX function, so we use the name pg_pread().
 *
 *-------------------------------------------------------------------------
 */


#include "postgres.h"

#ifdef WIN32
#include <windows.h>
#else
#include <unistd.h>
#endif

ssize_t
pg_pread(int fd, void *buf, size_t size, off_t offset)
{
#ifdef WIN32
    OVERLAPPED  overlapped = {0};
    HANDLE      handle;
    DWORD       result;

    handle = (HANDLE) _get_osfhandle(fd);
    if (handle == INVALID_HANDLE_VALUE)
    {
        errno = EBADF;
        return -1;
    }

    overlapped.Offset = offset;
    if (!ReadFile(handle, buf, size, &result, &overlapped))
    {
        if (GetLastError() == ERROR_HANDLE_EOF)
            return 0;

        _dosmaperr(GetLastError());
        return -1;
    }

    return result;
#else
    if (lseek(fd, offset, SEEK_SET) < 0)
        return -1;

    return read(fd, buf, size);
#endif
}