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86 lines
2.9 KiB
C
86 lines
2.9 KiB
C
/* Read data into multiple buffers. Base implementation for preadv
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and preadv64.
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Copyright (C) 2017-2025 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <unistd.h>
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#include <sys/uio.h>
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#include <sys/param.h>
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#include <errno.h>
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#include <malloc.h>
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#include <ldsodefs.h>
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#include <libc-pointer-arith.h>
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/* Read data from file descriptor FD at the given position OFFSET
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without change the file pointer, and put the result in the buffers
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described by VECTOR, which is a vector of COUNT 'struct iovec's.
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The buffers are filled in the order specified. Operates just like
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'pread' (see <unistd.h>) except that data are put in VECTOR instead
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of a contiguous buffer. */
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ssize_t
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PREADV (int fd, const struct iovec *vector, int count, OFF_T offset)
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{
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/* Find the total number of bytes to be read. */
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size_t bytes = 0;
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for (int i = 0; i < count; ++i)
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{
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/* Check for ssize_t overflow. */
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if (SSIZE_MAX - bytes < vector[i].iov_len)
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{
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__set_errno (EINVAL);
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return -1;
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}
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bytes += vector[i].iov_len;
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}
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/* Allocate a temporary buffer to hold the data. It could be done with a
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stack allocation, but due limitations on some system (Linux with
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O_DIRECT) it aligns the buffer to pagesize. A possible optimization
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would be querying if the syscall would impose any alignment constraint,
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but 1. it is system specific (not meant in generic implementation), and
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2. it would make the implementation more complex, and 3. it will require
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another syscall (fcntl). */
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void *buffer = __mmap (NULL, bytes, PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
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if (__glibc_unlikely (buffer == MAP_FAILED))
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return -1;
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ssize_t bytes_read = PREAD (fd, buffer, bytes, offset);
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if (bytes_read < 0)
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goto end;
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/* Copy the data from BUFFER into the memory specified by VECTOR. */
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bytes = bytes_read;
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void *buf = buffer;
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for (int i = 0; i < count; ++i)
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{
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size_t copy = MIN (vector[i].iov_len, bytes);
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memcpy (vector[i].iov_base, buf, copy);
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buf += copy;
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bytes -= copy;
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if (bytes == 0)
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break;
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}
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end:
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__munmap (buffer, bytes);
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return bytes_read;
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}
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