linux读写二进制大文件

https://raw.githubusercontent.com/ceph/ceph/master/src/common/safe_io.c

一般而言，用pwrite和pread读写磁盘文件不需要用循环，但是在读写超大的文件时就一定需要循环，保证正确读写

// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
// vim: ts=8 sw=2 smarttab
/*
* Ceph - scalable distributed file system
*
* Copyright (C) 2011 New Dream Network
*
* This is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License version 2.1, as published by the Free Software
* Foundation.  See file COPYING.
*
*/

#define _XOPEN_SOURCE 500

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>

#include "common/safe_io.h"
#include "include/compat.h"

ssize_t safe_read(int fd, void *buf, size_t count)
{
size_t cnt = 0;

while (cnt < count) {
ssize_t r = read(fd, buf, count - cnt);
if (r <= 0) {
if (r == 0) {
// EOF
return cnt;
}
if (errno == EINTR)
continue;
return -errno;
}
cnt += r;
buf = (char *)buf + r;
}
return cnt;
}

ssize_t safe_read_exact(int fd, void *buf, size_t count)
{
ssize_t ret = safe_read(fd, buf, count);
if (ret < 0)
return ret;
if ((size_t)ret != count)
return -EDOM;
return 0;
}

ssize_t safe_write(int fd, const void *buf, size_t count)
{
while (count > 0) {
ssize_t r = write(fd, buf, count);
if (r < 0) {
if (errno == EINTR)
continue;
return -errno;
}
count -= r;
buf = (char *)buf + r;
}
return 0;
}

ssize_t safe_pread(int fd, void *buf, size_t count, off_t offset)
{
size_t cnt = 0;
char *b = (char*)buf;

while (cnt < count) {
ssize_t r = pread(fd, b + cnt, count - cnt, offset + cnt);
if (r <= 0) {
if (r == 0) {
// EOF
return cnt;
}
if (errno == EINTR)
continue;
return -errno;
}

cnt += r;
}
return cnt;
}

ssize_t safe_pread_exact(int fd, void *buf, size_t count, off_t offset)
{
ssize_t ret = safe_pread(fd, buf, count, offset);
if (ret < 0)
return ret;
if ((size_t)ret != count)
return -EDOM;
return 0;
}

ssize_t safe_pwrite(int fd, const void *buf, size_t count, off_t offset)
{
while (count > 0) {
ssize_t r = pwrite(fd, buf, count, offset);
if (r < 0) {
if (errno == EINTR)
continue;
return -errno;
}
count -= r;
buf = (char *)buf + r;
offset += r;
}
return 0;
}

#ifdef CEPH_HAVE_SPLICE
ssize_t safe_splice(int fd_in, loff_t *off_in, int fd_out, loff_t *off_out,
size_t len, unsigned int flags)
{
size_t cnt = 0;

while (cnt < len) {
ssize_t r = splice(fd_in, off_in, fd_out, off_out, len - cnt, flags);
if (r <= 0) {
if (r == 0) {
// EOF
return cnt;
}
if (errno == EINTR)
continue;
return -errno;
}
cnt += r;
}
return cnt;
}

ssize_t safe_splice_exact(int fd_in, loff_t *off_in, int fd_out,
loff_t *off_out, size_t len, unsigned int flags)
{
ssize_t ret = safe_splice(fd_in, off_in, fd_out, off_out, len, flags);
if (ret < 0)
return ret;
if ((size_t)ret != len)
return -EDOM;
return 0;
}
#endif

int safe_write_file(const char *base, const char *file,
const char *val, size_t vallen)
{
int ret;
char fn[PATH_MAX];
char tmp[PATH_MAX];
int fd;

// does the file already have correct content?
char oldval[80];
ret = safe_read_file(base, file, oldval, sizeof(oldval));
if (ret == (int)vallen && memcmp(oldval, val, vallen) == 0)
return 0;  // yes.

snprintf(fn, sizeof(fn), "%s/%s", base, file);
snprintf(tmp, sizeof(tmp), "%s/%s.tmp", base, file);
fd = open(tmp, O_WRONLY|O_CREAT|O_TRUNC, 0644);
if (fd < 0) {
ret = errno;
return -ret;
}
ret = safe_write(fd, val, vallen);
if (ret) {
VOID_TEMP_FAILURE_RETRY(close(fd));
return ret;
}

ret = fsync(fd);
if (ret < 0) ret = -errno;
VOID_TEMP_FAILURE_RETRY(close(fd));
if (ret < 0) {
unlink(tmp);
return ret;
}
ret = rename(tmp, fn);
if (ret < 0) {
ret = -errno;
unlink(tmp);
return ret;
}

fd = open(base, O_RDONLY);
if (fd < 0) {
ret = -errno;
return ret;
}
ret = fsync(fd);
if (ret < 0) ret = -errno;
VOID_TEMP_FAILURE_RETRY(close(fd));

return ret;
}

int safe_read_file(const char *base, const char *file,
char *val, size_t vallen)
{
char fn[PATH_MAX];
int fd, len;

snprintf(fn, sizeof(fn), "%s/%s", base, file);
fd = open(fn, O_RDONLY);
if (fd < 0) {
return -errno;
}
len = safe_read(fd, val, vallen);
if (len < 0) {
VOID_TEMP_FAILURE_RETRY(close(fd));
return len;
}
// close sometimes returns errors, but only after write()
VOID_TEMP_FAILURE_RETRY(close(fd));

return len;
}

下面是自己写的代码，为了验证read/writeBlock的正确性，分两批写入文件，然后又分两批读入文件，实际工程中只要一次读写就行了

#include <unistd.h>
#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <string>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <fstream>
#include <vector>
#include <sstream>
#include <errno.h>

using namespace std;

bool writeBlock(int fd, int64_t &fdOft,int8_t *arr, int64_t len) {
int64_t num = 0;
int64_t oft = 0;
int8_t *arr1 = (int8_t*)arr;
while (oft < len) {
num = pwrite(fd, arr1+oft, len-oft, fdOft+oft);
if (num == -1) {
return false;
}
oft += num;
}
fdOft += oft;

return true;
}

bool readBlock(int fd, int64_t &fdOft, void *arr, int64_t len) {
int64_t num = 0;
int64_t oft = 0;
int8_t *b = (int8_t*)arr;
while (oft < len) {
num = pread(fd, b + oft, len - oft, fdOft + oft);
if (num <= 0) {
if (num == 0) {
if (oft != len) return false;
else return true;
}
if (errno == EINTR) {
continue;//the call was interrupted by a signal befor any data was read
}
return false;
}

oft += num;
}
fdOft += oft;

return true;
}

int main(int argc, char *argv[]) {
int64_t len = atol(argv[1]);
cout << "len = "<<len <<endl;
int fd = open(argv[2], O_RDWR|O_CREAT|O_TRUNC, S_IRWXU);
if (fd == -1) {
cout << "not open\n";
return -1;
}

int64_t total = sizeof(double)*len;
cout << "total = " << total <<endl;
double *arr = (double*)malloc(total);
if (arr == NULL) {
cout << "malloc err\n";
return -1;
}

arr[0] = 0.001;
for (int64_t i = 1; i < len; ++i) {
arr[i] = arr[i-1] + 0.001;
}

//int64_t num = pwrite(fd,arr,total,0);
int64_t fdOft = 0;
int64_t first = total/2;
int64_t second = total - first;
bool flag = writeBlock(fd, fdOft,(int8_t*)arr, first);
if (!flag) {
cout << "first no\n";
}
else {
cout << "first yes\n";
}

flag = writeBlock(fd, fdOft, (int8_t*)arr + first, second);
//cout << num<<endl;
if (!flag) {
cout << "second no\n";
}
else {
cout << "second yes\n";
}

close(fd);

int fdr = open(argv[2], O_RDONLY);
double *arr1 = (double*)malloc(total);
if (arr1 == NULL) {
cout << "malloc error\n";
return -1;
}

int64_t readOft = 0;
if (readBlock(fdr, readOft, arr1, first)) {
cout << "first read yes\n";
}

if (readBlock(fdr, readOft, (int8_t*)arr1 + first, second)) {
cout << "second read yes\n";
}

flag = true;
for (int64_t i = 0; i < len; ++i) {
if (arr1[i] != arr[i]) {
flag = false;
cout << i << " no equal\n";
}

}

if (flag) cout << "equal\n";

close(fdr);

return 1;
}