Only use the "p" variants if you have good use for reading at random offsets avoiding seeks and allowing concurrent access via one file handle , which often the case with some kind of database files record-oriented with records at known offsets and rarely in other applications.
It's usually an universal truth that the less system calls program issues the more efficient it is. Python Javascript Linux Cheat sheet Contact. Only n-1 characters can be entered, and any entries that are exceeded will be discarded. Success: return the number of bytes read; error: return -1;. Reason: Read and write multiple non-contiguous buffers in one function call, but these buffers have been expressed with iovec.
Reduced the number of system calls. Reasons: Pipes, FIFOs, and some devices, especially terminals, networks, and STREAMS devices have the following two properties: First, the data returned by a read operation may be less than the required data, even if the end of the file has not been reached yet This may be the case. This is not an error and the device should continue to be read. The second is that the value returned by a write operation may also be less than the specified number of output bytes, which may be caused by several factors.
These are not errors, and you should continue to write the remaining data to the device. Usually this write-back only occurs for non-blocking descriptors or when a signal is caught. But when reading and writing to the disk, such a situation is rarely encountered.
So this function actually calls read and write as many times as needed until N bytes of data are read and written, which we call it: return until the collection is complete. In the message queue:. Success: the length of the data part of the returned message; error: -1;.
Supplement: nbytes indicates the length of the data buffer. Used to construct mymesg. If this flag is not set and the message is too long, an error will return E2BIG the message is still in the queue. We can specify the type of message we want with the parameter type. Success: return 0; error: -1;. Supplement: Each message is composed of three parts, they are: positive long integer type field, actual data bytes the two pairs are myseq structure , non-negative length nbytes.
Messages are always placed at the end of the queue. The ptr parameter points to a long integer, which contains a positive integer message type, followed by the message data.
The recipient can use the message type to fetch the messages in a non-first-in first-out order. Success: the length of the message in bytes; error: -1; no message available or the other party has ended in sequence: 0;. Supplement: if addr is not empty, it will contain the socket endpoint address of the data sender. When recvfrom is called, the addrlen parameter needs to be set to point to an integer containing the byte size of the socket buffer pointed to by addr.
On return, the integer is set to the actual byte size of the address. Because the address of the sender can be obtained, recvfrom is usually used for connectionless sockets. Added: The structure msghdr is used by recvmsg to specify the input buffer for receiving data. Success: return the number of bytes sent; error: -1;. Supplement: If the send is successful, it does not necessarily mean that the process connecting to the other end receives the data.
The only guarantee is that when send returns successfully, the data has been sent to the network without errors. Supplement: It is applicable to connectionless sockets. Send cannot be used unless the target address is set in advance when connect is called, or sendto is used to provide another way of sending messages. Supplement: You can call sendmsg with the msghdr structure to specify multiple buffers to transfer data, much like writev. Transfer file descriptors omitted, self-implemented.
We are not currently using this method, though we have experimented with it in the past. Before our recent changes to reduce write latency, we probably didn't notice the overhead of the extra system call due to the background noise on the system.
Since improvements have been made, it is worth re-testing using the pwrite method to see if there is any measurable benefit. We'll get to the numbers later, but first let's just have a look at the difference between the two approaches.
If you are averse to JDK source code, you can skip down to "Enough code already". The API of RandomAccessFile requires that the programmer set the write position on the file, then call the write method with the data that is to be written. The call starts in RandomAccessFile. See man lseek for more details. This delegates to the native function RandomAccessFile.
No, pwrite does not call fsync. See pwrite 3 : on file - Linux man page :. The pwrite function shall be equivalent to write , except that it writes into a given position without changing the file pointer. Also fsync write data from kernel buffers to disk, so which system call read data from disk to kernel buffers? To write data from kernel buffers to disk , fsync can be called, but it doesn't have to, if it suffices that the buffers are flushed eventually, which will happen anyway sooner or later, unless the system crashes or is reset.
To read data from disk to kernel buffers , having a dedicated system call is not needed 1. The system knows from the read call which data are to be read, and data must be read before the call returns unless they are already buffered. Allows an application to to tell the kernel how it expects to use a file handle, so that the kernel can choose appropriate read-ahead and caching techniques for access to the corresponding file.
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