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/*-------------------------------------------------------------------------
*
* preadv.c
* Implementation of preadv(2) for platforms that lack one.
*
* Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
*
* IDENTIFICATION
* src/port/preadv.c
*
* Note that this implementation changes the current file position, unlike
* the POSIX-like function, so we use the name pg_preadv().
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#ifdef WIN32
#include <windows.h>
#else
#include <unistd.h>
#endif
#include "port/pg_iovec.h"
ssize_t
pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
{
#ifdef HAVE_READV
if (iovcnt == 1)
return pread(fd, iov[0].iov_base, iov[0].iov_len, offset);
if (lseek(fd, offset, SEEK_SET) < 0)
return -1;
return readv(fd, iov, iovcnt);
#else
ssize_t sum = 0;
ssize_t part;
for (int i = 0; i < iovcnt; ++i)
{
part = 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 (part < iov[i].iov_len)
return sum;
}
return sum;
#endif
}
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