2 * Copyright 2014 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 #ifndef FOLLY_FILEUTIL_H_
18 #define FOLLY_FILEUTIL_H_
20 #include "folly/Portability.h"
21 #include "folly/ScopeGuard.h"
26 #include <sys/types.h>
34 * Convenience wrappers around some commonly used system calls. The *NoInt
35 * wrappers retry on EINTR. The *Full wrappers retry on EINTR and also loop
36 * until all data is written. Note that *Full wrappers weaken the thread
37 * semantics of underlying system calls.
39 int openNoInt(const char* name, int flags, mode_t mode=0644);
40 int closeNoInt(int fd);
42 int dup2NoInt(int oldfd, int newfd);
43 int fsyncNoInt(int fd);
44 int fdatasyncNoInt(int fd);
45 int ftruncateNoInt(int fd, off_t len);
46 int truncateNoInt(const char* path, off_t len);
47 int flockNoInt(int fd, int operation);
48 int shutdownNoInt(int fd, int how);
50 ssize_t readNoInt(int fd, void* buf, size_t n);
51 ssize_t preadNoInt(int fd, void* buf, size_t n, off_t offset);
52 ssize_t readvNoInt(int fd, const iovec* iov, int count);
54 ssize_t writeNoInt(int fd, const void* buf, size_t n);
55 ssize_t pwriteNoInt(int fd, const void* buf, size_t n, off_t offset);
56 ssize_t writevNoInt(int fd, const iovec* iov, int count);
59 * Wrapper around read() (and pread()) that, in addition to retrying on
60 * EINTR, will loop until all data is read.
62 * This wrapper is only useful for blocking file descriptors (for non-blocking
63 * file descriptors, you have to be prepared to deal with incomplete reads
64 * anyway), and only exists because POSIX allows read() to return an incomplete
65 * read if interrupted by a signal (instead of returning -1 and setting errno
68 * Note that this wrapper weakens the thread safety of read(): the file pointer
69 * is shared between threads, but the system call is atomic. If multiple
70 * threads are reading from a file at the same time, you don't know where your
71 * data came from in the file, but you do know that the returned bytes were
72 * contiguous. You can no longer make this assumption if using readFull().
73 * You should probably use pread() when reading from the same file descriptor
74 * from multiple threads simultaneously, anyway.
76 * Note that readvFull and preadvFull require iov to be non-const, unlike
77 * readv and preadv. The contents of iov after these functions return
80 ssize_t readFull(int fd, void* buf, size_t n);
81 ssize_t preadFull(int fd, void* buf, size_t n, off_t offset);
82 ssize_t readvFull(int fd, iovec* iov, int count);
84 ssize_t preadvFull(int fd, iovec* iov, int count, off_t offset);
88 * Similar to readFull and preadFull above, wrappers around write() and
89 * pwrite() that loop until all data is written.
91 * Generally, the write() / pwrite() system call may always write fewer bytes
92 * than requested, just like read(). In certain cases (such as when writing to
93 * a pipe), POSIX provides stronger guarantees, but not in the general case.
94 * For example, Linux (even on a 64-bit platform) won't write more than 2GB in
95 * one write() system call.
97 * Note that writevFull and pwritevFull require iov to be non-const, unlike
98 * writev and pwritev. The contents of iov after these functions return
101 ssize_t writeFull(int fd, const void* buf, size_t n);
102 ssize_t pwriteFull(int fd, const void* buf, size_t n, off_t offset);
103 ssize_t writevFull(int fd, iovec* iov, int count);
104 #if FOLLY_HAVE_PWRITEV
105 ssize_t pwritevFull(int fd, iovec* iov, int count, off_t offset);
109 * Read entire file (if num_bytes is defaulted) or no more than
110 * num_bytes (otherwise) into container *out. The container is assumed
111 * to be contiguous, with element size equal to 1, and offer size(),
112 * reserve(), and random access (e.g. std::vector<char>, std::string,
115 * Returns: true on success or false on failure. In the latter case
116 * errno will be set appropriately by the failing system primitive.
118 template <class Container>
119 bool readFile(const char* file_name, Container& out,
120 size_t num_bytes = std::numeric_limits<size_t>::max()) {
121 static_assert(sizeof(out[0]) == 1,
122 "readFile: only containers with byte-sized elements accepted");
125 const auto fd = open(file_name, O_RDONLY);
126 if (fd == -1) return false;
128 size_t soFar = 0; // amount of bytes successfully read
130 assert(out.size() >= soFar); // resize better doesn't throw
132 // Ignore errors when closing the file
138 if (fstat(fd, &buf) == -1) return false;
139 // Some files (notably under /proc and /sys on Linux) lie about
140 // their size, so treat the size advertised by fstat under advise
141 // but don't rely on it. In particular, if the size is zero, we
142 // should attempt to read stuff. If not zero, we'll attempt to read
144 constexpr size_t initialAlloc = 1024 * 4;
147 buf.st_size ? buf.st_size + 1 : initialAlloc,
150 while (soFar < out.size()) {
151 const auto actual = readFull(fd, &out[soFar], out.size() - soFar);
156 if (soFar < out.size()) {
160 // Ew, allocate more memory. Use exponential growth to avoid
161 // quadratic behavior. Cap size to num_bytes.
162 out.resize(std::min(out.size() * 3 / 2, num_bytes));
170 #endif /* FOLLY_FILEUTIL_H_ */