本文共 5268 字，大约阅读时间需要 17 分钟。
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#include #include #include #include #include #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_SPLICEssize_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;}#endifint 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#include #include #include #include #include #include #include #include #include #include #include #include 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 = "< <
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