Mac Write

[Nayme Cutforth]

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Before any action described below is taken, and if nbyte is zero and the file is a regular file, the write()function may detect and return errors as described below. In the absence of errors, or if error detection is not performed, thewrite() function shall return zero and have no other results. If nbyte is zero and the file is not a regular file,the results are unspecified.

On a regular file or other file capable of seeking, the actual writing of data shall proceed from the position in the fileindicated by the file offset associated with fildes. Before successful return from write(), the file offset shall beincremented by the number of bytes actually written. On a regular file, if the position of the last byte written is greater than orequal to the length of the file, the length of the file shall be set to this position plus one.

If the O_APPEND flag of the file status flags is set, the file offset shall be set to the end of the file prior to each writeand no intervening file modification operation shall occur between changing the file offset and the write operation.

If a write() requests that more bytes be written than there is room for (for example, [XSI] the filesize limit of the process or the physical end of a medium), only asmany bytes as there is room for shall be written. For example, suppose there is space for 20 bytes more in a file before reaching alimit. A write of 512 bytes will return 20. The next write of a non-zero number of bytes would give a failure return (except asnoted below).

[XSI] If the request would cause the file size to exceed the soft file size limit for the process and there is no room for any bytes tobe written, the request shall fail and the implementation shall generate the SIGXFSZ signal for the thread.

Any successful read() from each byte position in the file that was modified by thatwrite shall return the data specified by the write() for that position until such byte positions are again modified.

Write requests of PIPE_BUF bytes or less shall not be interleaved with data from other processes doing writes on the samepipe. Writes of greater than PIPE_BUF bytes may have data interleaved, on arbitrary boundaries, with writes by other processes,whether or not the O_NONBLOCK flag of the file status flags is set.

A write request for PIPE_BUF or fewer bytes shall have the following effect: if there is sufficient space available in thepipe, write() shall transfer all the data and return the number of bytes requested. Otherwise, write() shall transferno data and return -1 with errno set to [EAGAIN].

If the O_NONBLOCK flag is set, write() shall not block the thread. If some data can be written without blocking thethread, write() shall write what it can and return the number of bytes written. Otherwise, it shall return -1 and seterrno to [EAGAIN].

Upon successful completion, where nbyte is greater than 0, write() shall mark for update the last datamodification and last file status change timestamps of the file, and if the file is a regular file, the S_ISUID and S_ISGID bits ofthe file mode may be cleared.

[OB XSR] If fildes refers to a STREAM, the operation of write() shall be determined by the values of the minimum andmaximum nbyte range (packet size) accepted by the STREAM. These values are determined by the topmost STREAM module. Ifnbyte falls within the packet size range, nbyte bytes shall be written. If nbyte does not fall within therange and the minimum packet size value is 0, write() shall break the buffer into maximum packet size segments prior tosending the data downstream (the last segment may contain less than the maximum packet size). If nbyte does not fall withinthe range and the minimum value is non-zero, write() shall fail with errno set to [ERANGE]. Writing a zero-lengthbuffer (nbyte is 0) to a STREAMS device sends 0 bytes with 0 returned. However, writing a zero-length buffer to aSTREAMS-based pipe or FIFO sends no message and 0 is returned. The process may issue I_SWROPT ioctl() to enable zero-length messages to be sent across the pipe or FIFO.

If O_NONBLOCK is set and part of the buffer has been written while a condition in which the STREAM cannot accept additional dataoccurs, write() shall terminate and return the number of bytes written.

Upon successful completion, these functions shall return the number of bytes actually written to the file associated withfildes. This number shall never be greater than nbyte. Otherwise, -1 shall be returned and errno set toindicate the error.

Atomic/non-atomic: A write is atomic if the whole amount written in one operation is not interleaved with data from anyother process. This is useful when there are multiple writers sending data to a single reader. Applications need to know how largea write request can be expected to be performed atomically. This maximum is called PIPE_BUF. This volume of POSIX.1-2017 does notsay whether write requests for more than PIPE_BUF bytes are atomic, but requires that writes of PIPE_BUF or fewer bytes shallbe atomic.

Blocking/immediate: Blocking is only possible with O_NONBLOCK clear. If there is enough space for all the data requestedto be written immediately, the implementation should do so. Otherwise, the calling thread may block; that is, pause until enoughspace is available for writing. The effective size of a pipe or FIFO (the maximum amount that can be written in one operationwithout blocking) may vary dynamically, depending on the implementation, so it is not possible to specify a fixed value for it.

This error indicates that a later request may succeed. It does not indicate that it shall succeed, even ifnbyte PIPE_BUF and perhaps do laterwrites with a smaller value, on the assumption that the effective size of the pipe may have decreased.

There is no exception regarding partial writes when O_NONBLOCK is set. With the exception of writing to an empty pipe, thisvolume of POSIX.1-2017 does not specify exactly when a partial write is performed since that would require specifying internaldetails of the implementation. Every application should be prepared to handle partial writes when O_NONBLOCK is set and therequested amount is greater than PIPE_BUF, just as every application should be prepared to handle partial writes on other kindsof file descriptors.

The intent of forcing writing at least one byte if any can be written is to assure that each write makes progress if there isany room in the pipe. If the pipe is empty, PIPE_BUF bytes must be written; if not, at least some progress must have beenmade.

Where this volume of POSIX.1-2017 requires -1 to be returned and errno set to [EAGAIN], most historical implementationsreturn zero (with the O_NDELAY flag set, which is the historical predecessor of O_NONBLOCK, but is not itself in this volume ofPOSIX.1-2017). The error indications in this volume of POSIX.1-2017 were chosen so that an application can distinguish these casesfrom end-of-file. While write() cannot receive an indication of end-of-file, read() can, and the two functions have similar return values. Also, some existing systems (forexample, Eighth Edition) permit a write of zero bytes to mean that the reader should get an end-of-file indication; for thosesystems, a return value of zero from write() indicates a successful write of an end-of-file indication.

The concept of a PIPE_MAX limit (indicating the maximum number of bytes that can be written to a pipe in a single operation)was considered, but rejected, because this concept would unnecessarily limit application writing.

This volume of POSIX.1-2017 does not specify the value of the file offset after an error is returned; there are too many cases.For programming errors, such as [EBADF], the concept is meaningless since no file is involved. For errors that are detectedimmediately, such as [EAGAIN], clearly the pointer should not change. After an interrupt or hardware error, however, an updatedvalue would be very useful and is the behavior of many implementations.

This volume of POSIX.1-2017 does not specify the behavior of concurrent writes to a regular file from multiple threads, exceptthat each write is atomic (see Thread Interactions with Regular FileOperations). Applications should use some form of concurrency control.

The DESCRIPTION now states that if write() is interrupted by a signal after it has successfully written some data, itreturns the number of bytes written. In the POSIX.1-1988 standard, it was optional whether write() returned the number ofbytes written, or whether it returned -1 with errno set to [EINTR]. This is a FIPS requirement.

The nurse at the treatment center finds the wound several hours later. The center has no detox, names her too great a risk, and does not accept her. For the next five days, she is ours to love. We become her hospital and the possibility of healing fills our living room with life. It is unspoken and there are only a few of us, but we will be her church, the body of Christ coming alive to meet her needs, to write love on her arms.

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