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diff --git a/man-pages-posix-2013/man3p/write.3p b/man-pages-posix-2013/man3p/write.3p new file mode 100644 index 0000000..5a07dd4 --- /dev/null +++ b/man-pages-posix-2013/man3p/write.3p @@ -0,0 +1,735 @@ +'\" et +.TH WRITE "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual" +.SH PROLOG +This manual page is part of the POSIX Programmer's Manual. +The Linux implementation of this interface may differ (consult +the corresponding Linux manual page for details of Linux behavior), +or the interface may not be implemented on Linux. + +.SH NAME +pwrite, +write +\(em write on a file +.SH SYNOPSIS +.LP +.nf +#include <unistd.h> +.P +ssize_t pwrite(int \fIfildes\fP, const void *\fIbuf\fP, size_t \fInbyte\fP, + off_t \fIoffset\fP); +ssize_t write(int \fIfildes\fP, const void *\fIbuf\fP, size_t \fInbyte\fP); +.fi +.SH DESCRIPTION +The +\fIwrite\fR() +function shall attempt to write +.IR nbyte +bytes from the buffer pointed to by +.IR buf +to the file associated with the open file descriptor, +.IR fildes . +.P +Before any action described below is taken, and if +.IR nbyte +is zero and the file is a regular file, the +\fIwrite\fR() +function may detect and return errors as described below. In the +absence of errors, or if error detection is not performed, the +\fIwrite\fR() +function shall return zero and have no other results. If +.IR nbyte +is zero and the file is not a regular file, the results are +unspecified. +.P +On a regular file or other file capable of seeking, the actual writing +of data shall proceed from the position in the file indicated by the +file offset associated with +.IR fildes . +Before successful return from +\fIwrite\fR(), +the file offset shall be incremented by the number of bytes actually +written. On a regular file, if the position of the last byte written +is greater than or equal to the length of the file, +the length of the file shall be set to this position plus one. +.P +On a file not capable of seeking, writing shall always take place +starting at the current position. The value of a file offset associated +with such a device is undefined. +.P +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 write +and no intervening file modification operation shall occur between +changing the file offset and the write operation. +.P +If a +\fIwrite\fR() +requests that more bytes be written than there is room for (for +example, +the file size limit of the process or +the physical end of a medium), only as many bytes as there is room for +shall be written. For example, suppose there is space for 20 bytes more +in a file before reaching a limit. A write of 512 bytes will return +20. The next write of a non-zero number of bytes would give a failure +return (except as noted below). +.P +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 to be written, +the request shall fail and the implementation shall generate the +SIGXFSZ signal for the thread. +.P +If +\fIwrite\fR() +is interrupted by a signal before it writes any data, it shall +return \(mi1 with +.IR errno +set to +.BR [EINTR] . +.P +If +\fIwrite\fR() +is interrupted by a signal after it successfully writes some data, it +shall return the number of bytes written. +.P +If the value of +.IR nbyte +is greater than +{SSIZE_MAX}, +the result is implementation-defined. +.P +After a +\fIwrite\fR() +to a regular file has successfully returned: +.IP " *" 4 +Any successful +\fIread\fR() +from each byte position in the file that was modified by that write +shall return the data specified by the +\fIwrite\fR() +for that position until such byte positions are again modified. +.IP " *" 4 +Any subsequent successful +\fIwrite\fR() +to the same byte position in the file shall overwrite that file data. +.br +.P +Write requests to a pipe or FIFO shall be handled in the same way +as a regular file with the following exceptions: +.IP " *" 4 +There is no file offset associated with a pipe, hence each write +request shall append to the end of the pipe. +.IP " *" 4 +Write requests of +{PIPE_BUF} +bytes or less shall not be interleaved with data from other processes +doing writes on the same pipe. 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. +.IP " *" 4 +If the O_NONBLOCK flag is clear, a write request may cause the thread +to block, but on normal completion it shall return +.IR nbyte . +.IP " *" 4 +If the O_NONBLOCK flag is set, +\fIwrite\fR() +requests shall be handled differently, in the following ways: +.RS 4 +.IP -- 4 +The +\fIwrite\fR() +function shall not block the thread. +.IP -- 4 +A write request for +{PIPE_BUF} +or fewer bytes shall have the following effect: if there is sufficient +space available in the pipe, +\fIwrite\fR() +shall transfer all the data and return the number of bytes requested. +Otherwise, +\fIwrite\fR() +shall transfer no data and return \(mi1 with +.IR errno +set to +.BR [EAGAIN] . +.IP -- 4 +A write request for more than +{PIPE_BUF} +bytes shall cause one of the following: +.RS 4 +.IP -- 4 +When at least one byte can be written, transfer what it can and return +the number of bytes written. When all data previously written to the +pipe is read, it shall transfer at least +{PIPE_BUF} +bytes. +.IP -- 4 +When no data can be written, transfer no data, and return \(mi1 with +.IR errno +set to +.BR [EAGAIN] . +.RE +.RE +.P +When attempting to write to a file descriptor (other than a pipe or +FIFO) that supports non-blocking writes and cannot accept the data +immediately: +.IP " *" 4 +If the O_NONBLOCK flag is clear, +\fIwrite\fR() +shall block the calling thread until the data can be accepted. +.IP " *" 4 +If the O_NONBLOCK flag is set, +\fIwrite\fR() +shall not block the thread. If some data can be written without +blocking the thread, +\fIwrite\fR() +shall write what it can and return the number of bytes written. +Otherwise, it shall return \(mi1 and set +.IR errno +to +.BR [EAGAIN] . +.P +Upon successful completion, where +.IR nbyte +is greater than 0, +\fIwrite\fR() +shall mark for update the last data modification and last file +status change timestamps of the file, and if the file is a regular file, +the S_ISUID and S_ISGID bits of the file mode may be cleared. +.P +For regular files, no data transfer shall occur past the offset maximum +established in the open file description associated with +.IR fildes . +.P +If +.IR fildes +refers to a socket, +\fIwrite\fR() +shall be equivalent to +\fIsend\fR() +with no flags set. +.P +If the O_DSYNC bit has been set, +write I/O operations on the file descriptor shall complete as defined +by synchronized I/O data integrity completion. +.P +If the O_SYNC bit has been set, write I/O operations on the file +descriptor shall complete as defined by synchronized I/O file +integrity completion. +.P +If +.IR fildes +refers to a shared memory object, the result of the +\fIwrite\fR() +function is unspecified. +.P +If +.IR fildes +refers to a typed memory object, the result of the +\fIwrite\fR() +function is unspecified. +.P +If +.IR fildes +refers to a STREAM, the operation of +\fIwrite\fR() +shall be determined by the values of the minimum and maximum +.IR nbyte +range (packet size) accepted by the STREAM. These values are determined +by the topmost STREAM module. If +.IR nbyte +falls within the packet size range, +.IR nbyte +bytes shall be written. If +.IR nbyte +does not fall within the range and the minimum packet size value is 0, +\fIwrite\fR() +shall break the buffer into maximum packet size segments prior to +sending the data downstream (the last segment may contain less than the +maximum packet size). If +.IR nbyte +does not fall within the range and the minimum value is non-zero, +\fIwrite\fR() +shall fail with +.IR errno +set to +.BR [ERANGE] . +Writing a zero-length buffer (\c +.IR nbyte +is 0) to a STREAMS device sends 0 bytes with 0 returned. However, +writing a zero-length buffer to a STREAMS-based pipe or FIFO sends no +message and 0 is returned. The process may issue I_SWROPT +\fIioctl\fR() +to enable zero-length messages to be sent across the pipe or FIFO. +.P +When writing to a STREAM, data messages are created with a priority +band of 0. When writing to a STREAM that is not a pipe or FIFO: +.IP " *" 4 +If O_NONBLOCK is clear, and the STREAM cannot accept data (the STREAM +write queue is full due to internal flow control conditions), +\fIwrite\fR() +shall block until data can be accepted. +.IP " *" 4 +If O_NONBLOCK is set and the STREAM cannot accept data, +\fIwrite\fR() +shall return \(mi1 and set +.IR errno +to +.BR [EAGAIN] . +.IP " *" 4 +If O_NONBLOCK is set and part of the buffer has been written while a +condition in which the STREAM cannot accept additional data occurs, +\fIwrite\fR() +shall terminate and return the number of bytes written. +.P +In addition, +\fIwrite\fR() +shall fail if the STREAM head has processed an asynchronous error +before the call. In this case, the value of +.IR errno +does not reflect the result of +\fIwrite\fR(), +but reflects the prior error. +.P +The +\fIpwrite\fR() +function shall be equivalent to +\fIwrite\fR(), +except that it writes into a given position and does not change the +file offset (regardless of whether O_APPEND is set). The first three +arguments to +\fIpwrite\fR() +are the same as +\fIwrite\fR() +with the addition of a fourth argument +.IR offset +for the desired position inside the file. An attempt to perform a +\fIpwrite\fR() +on a file that is incapable of seeking shall result in an error. +.SH "RETURN VALUE" +Upon successful completion, these functions shall return the number of +bytes actually written to the file associated with +.IR fildes . +This number shall never be greater than +.IR nbyte . +Otherwise, \(mi1 shall be returned and +.IR errno +set to indicate the error. +.SH ERRORS +These functions shall fail if: +.TP +.BR EAGAIN +The file is neither a pipe, nor a FIFO, nor a socket, the O_NONBLOCK flag +is set for the file descriptor, and the thread would be delayed in the +\fIwrite\fR() +operation. +.TP +.BR EBADF +The +.IR fildes +argument is not a valid file descriptor open for writing. +.TP +.BR EFBIG +An attempt was made to write a file that exceeds the +implementation-defined maximum file size +or the file size limit of the process, +and there was no room for any bytes to be written. +.TP +.BR EFBIG +The file is a regular file, +.IR nbyte +is greater than 0, and the starting position is greater than or equal +to the offset maximum established in the open file description +associated with +.IR fildes . +.TP +.BR EINTR +The write operation was terminated due to the receipt of a signal, and +no data was transferred. +.TP +.BR EIO +The process is a member of a background process group attempting to +write to its controlling terminal, TOSTOP is set, the calling thread +is not blocking SIGTTOU, the process is not ignoring SIGTTOU, +and the process group of the process is orphaned. This error may also +be returned under implementation-defined conditions. +.TP +.BR ENOSPC +There was no free space remaining on the device containing the file. +.TP +.BR ERANGE +The transfer request size was outside the range supported by the +STREAMS file associated with +.IR fildes . +.P +The +\fIpwrite\fR() +function shall fail if: +.TP +.BR EINVAL +The file is a regular file or block special file, and the +.IR offset +argument is negative. The file pointer shall remain unchanged. +.TP +.BR ESPIPE +The file is a pipe, FIFO, or socket. +.P +The +\fIwrite\fR() +function shall fail if: +.TP +.BR EAGAIN +The file is a pipe or FIFO, the O_NONBLOCK flag is set for the file +descriptor, and the thread would be delayed in the write operation. +.TP +.BR EAGAIN " or " EWOULDBLOCK +.br +The file is a socket, the O_NONBLOCK flag is set for the file +descriptor, and the thread would be delayed in the write operation. +.TP +.BR ECONNRESET +A write was attempted on a socket that is not connected. +.TP +.BR EPIPE +An attempt is made to write to a pipe or FIFO that is not open for +reading by any process, or that only has one end open. A SIGPIPE signal +shall also be sent to the thread. +.TP +.BR EPIPE +A write was attempted on a socket that is shut down for writing, or is +no longer connected. In the latter case, if the socket is of type +SOCK_STREAM, a SIGPIPE signal shall also be sent to the thread. +.P +These functions may fail if: +.TP +.BR EINVAL +The STREAM or multiplexer referenced by +.IR fildes +is linked (directly or indirectly) downstream from a multiplexer. +.TP +.BR EIO +A physical I/O error has occurred. +.TP +.BR ENOBUFS +Insufficient resources were available in the system to perform the +operation. +.TP +.BR ENXIO +A request was made of a nonexistent device, or the request was outside +the capabilities of the device. +.TP +.BR ENXIO +A hangup occurred on the STREAM being written to. +.P +A write to a STREAMS file may fail if an error message has been +received at the STREAM head. In this case, +.IR errno +is set to the value included in the error message. +.br +.P +The +\fIwrite\fR() +function may fail if: +.TP +.BR EACCES +A write was attempted on a socket and the calling +process does not have appropriate privileges. +.TP +.BR ENETDOWN +A write was attempted on a socket and the local network interface used +to reach the destination is down. +.TP +.BR ENETUNREACH +.br +A write was attempted on a socket and no route to the network is +present. +.LP +.IR "The following sections are informative." +.SH EXAMPLES +.SS "Writing from a Buffer" +.P +The following example writes data from the buffer pointed to by +.IR buf +to the file associated with the file descriptor +.IR fd . +.sp +.RS 4 +.nf +\fB +#include <sys/types.h> +#include <string.h> +\&... +char buf[20]; +size_t nbytes; +ssize_t bytes_written; +int fd; +\&... +strcpy(buf, "This is a test\en"); +nbytes = strlen(buf); +.P +bytes_written = write(fd, buf, nbytes); +\&... +.fi \fR +.P +.RE +.SH "APPLICATION USAGE" +None. +.SH RATIONALE +See also the RATIONALE section in +\fIread\fR(). +.P +An attempt to write to a pipe or FIFO has several major +characteristics: +.IP " *" 4 +\fIAtomic/non-atomic\fP: A write is atomic if the whole amount written +in one operation is not interleaved with data from any other process. +This is useful when there are multiple writers sending data to a single +reader. Applications need to know how large a write request can be +expected to be performed atomically. This maximum is called +{PIPE_BUF}. +This volume of POSIX.1\(hy2008 does not say whether write requests for more than +{PIPE_BUF} +bytes are atomic, but requires that writes of +{PIPE_BUF} +or fewer bytes shall be atomic. +.IP " *" 4 +\fIBlocking/immediate\fP: Blocking is only possible with O_NONBLOCK +clear. If there is enough space for all the data requested to be +written immediately, the implementation should do so. Otherwise, the +calling thread may block; that is, pause until enough space is +available for writing. The effective size of a pipe or FIFO (the +maximum amount that can be written in one operation without blocking) +may vary dynamically, depending on the implementation, so it is not +possible to specify a fixed value for it. +.IP " *" 4 +\fIComplete/partial/deferred\fP: A write request: +.RS 4 +.sp +.RS 4 +.nf +\fB +int fildes; +size_t nbyte; +ssize_t ret; +char *buf; +.P +ret = write(fildes, buf, nbyte); +.fi \fR +.P +.RE +.P +may return: +.IP Complete 10 +\fIret\fP=\fInbyte\fP +.IP Partial 10 +\fIret\fP<\fInbyte\fP +.RS 10 +.P +This shall never happen if +.IR nbyte \(<=\c +{PIPE_BUF}. +If it does happen (with +.IR nbyte >\c +{PIPE_BUF}), +\&this volume of POSIX.1\(hy2008 does not guarantee atomicity, even if +.IR ret \(<=\c +{PIPE_BUF}, +because atomicity is guaranteed according to the amount +.IR requested , +not the amount +.IR written . +.RE +.IP Deferred: 10 +\fIret\fP=\(mi1, \fIerrno\fP=[EAGAIN] +.RS 10 +.P +This error indicates that a later request may succeed. It does not +indicate that it +.IR shall +succeed, even if +.IR nbyte \(<=\c +{PIPE_BUF}, +because if no process reads from the pipe or FIFO, the write never +succeeds. An application could usefully count the number of times +.BR [EAGAIN] +is caused by a particular value of +.IR nbyte >\c +{PIPE_BUF} +and perhaps do later writes with a smaller value, on the assumption +that the effective size of the pipe may have decreased. +.RE +.P +Partial and deferred writes are only possible with O_NONBLOCK set. +.RE +.P +The relations of these properties are shown in the following tables: +.TS +center box tab(!); +cB s s s +cB | cB cB c +l1 | lw(1.25i)1 lw(1.25i)1 lw(1.25i). +Write to a Pipe or FIFO with O_NONBLOCK \fIclear\fP +_ +Immediately Writable:!None!Some!\fInbyte\fP +_ +\fInbyte\fP\(<={PIPE_BUF}!Atomic blocking!Atomic blocking!Atomic immediate +!\fInbyte\fP!\fInbyte\fP!\fInbyte\fP +_ +\fInbyte\fP>{PIPE_BUF}!Blocking \fInbyte\fP!Blocking \fInbyte\fP!Blocking \fInbyte\fP +.TE +.P +If the O_NONBLOCK flag is clear, a write request shall block if the +amount writable immediately is less than that requested. If the flag is +set (by +\fIfcntl\fR()), +a write request shall never block. +.TS +center box tab(!); +cB s s s +cB | cB cB c +l1 | lw(1.25i)1 lw(1.25i)1 lw(1.25i). +Write to a Pipe or FIFO with O_NONBLOCK \fIset\fP +_ +Immediately Writable:!None!Some!\fInbyte\fP +_ +\fInbyte\fP\(<={PIPE_BUF}!\(mi1, [EAGAIN]!\(mi1, [EAGAIN]!Atomic \fInbyte\fP +_ +\fInbyte\fP>{PIPE_BUF}!\(mi1, [EAGAIN]!<\fInbyte\fP or \(mi1,!\(<=\fInbyte\fP or \(mi1, +!![EAGAIN]![EAGAIN] +.TE +.P +There is no exception regarding partial writes when O_NONBLOCK is set. +With the exception of writing to an empty pipe, this volume of POSIX.1\(hy2008 does not specify +exactly when a partial write is performed since that would require +specifying internal details of the implementation. Every application +should be prepared to handle partial writes when O_NONBLOCK is set and +the requested amount is greater than +{PIPE_BUF}, +just as every application should be prepared to handle partial writes +on other kinds of file descriptors. +.P +The intent of forcing writing at least one byte if any can be written +is to assure that each write makes progress if there is any room in the +pipe. If the pipe is empty, +{PIPE_BUF} +bytes must be written; if not, at least some progress must have been +made. +.P +Where this volume of POSIX.1\(hy2008 requires \(mi1 to be returned and +.IR errno +set to +.BR [EAGAIN] , +most historical implementations return zero (with the O_NDELAY +flag set, which is the historical predecessor of O_NONBLOCK, but is not +itself in this volume of POSIX.1\(hy2008). The error indications in this volume of POSIX.1\(hy2008 were chosen so that an +application can distinguish these cases from end-of-file. While +\fIwrite\fR() +cannot receive an indication of end-of-file, +\fIread\fR() +can, and the two functions have similar return values. Also, some +existing systems (for example, Eighth Edition) permit a write of zero +bytes to +mean that the reader should get an end-of-file indication; for those +systems, a return value of zero from +\fIwrite\fR() +indicates a successful write of an end-of-file indication. +.P +Implementations are allowed, but not required, to perform error +checking for +\fIwrite\fR() +requests of zero bytes. +.P +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. +.P +See also the discussion of O_NONBLOCK in +\fIread\fR(). +.P +Writes can be serialized with respect to other reads and writes. If a +\fIread\fR() +of file data can be proven (by any means) to occur after a +\fIwrite\fR() +of the data, it must reflect that +\fIwrite\fR(), +even if the calls are made by different processes. A similar +requirement applies to multiple write operations to the same file +position. This is needed to guarantee the propagation of data from +\fIwrite\fR() +calls to subsequent +\fIread\fR() +calls. This requirement is particularly significant for networked file +systems, where some caching schemes violate these semantics. +.P +Note that this is specified in terms of +\fIread\fR() +and +\fIwrite\fR(). +The XSI extensions +\fIreadv\fR() +and +\fIwritev\fR() +also obey these semantics. A new ``high-performance'' write +analog that did not follow these serialization requirements would also +be permitted by this wording. This volume of POSIX.1\(hy2008 is also silent about any effects of +application-level caching (such as that done by +.IR stdio ). +.P +This volume of POSIX.1\(hy2008 does not specify the value of the file offset after an error is +returned; there are too many cases. For programming errors, such as +.BR [EBADF] , +the concept is meaningless since no file is involved. For errors that +are detected immediately, such as +.BR [EAGAIN] , +clearly the pointer should not change. After an interrupt or hardware +error, however, an updated value would be very useful and is the +behavior of many implementations. +.P +This volume of POSIX.1\(hy2008 does not specify behavior of concurrent writes to a file from +multiple processes. Applications should use some form of concurrency +control. +.P +This volume of POSIX.1\(hy2008 intentionally does not specify any +\fIpwrite\fR() +errors related to pipes, FIFOs, and sockets other than +.BR [ESPIPE] . +.SH "FUTURE DIRECTIONS" +None. +.SH "SEE ALSO" +.IR "\fIchmod\fR\^(\|)", +.IR "\fIcreat\fR\^(\|)", +.IR "\fIdup\fR\^(\|)", +.IR "\fIfcntl\fR\^(\|)", +.IR "\fIgetrlimit\fR\^(\|)", +.IR "\fIlseek\fR\^(\|)", +.IR "\fIopen\fR\^(\|)", +.IR "\fIpipe\fR\^(\|)", +.IR "\fIread\fR\^(\|)", +.IR "\fIulimit\fR\^(\|)", +.IR "\fIwritev\fR\^(\|)" +.P +The Base Definitions volume of POSIX.1\(hy2008, +.IR "\fB<limits.h>\fP", +.IR "\fB<stropts.h>\fP", +.IR "\fB<sys_uio.h>\fP", +.IR "\fB<unistd.h>\fP" +.SH COPYRIGHT +Portions of this text are reprinted and reproduced in electronic form +from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology +-- Portable Operating System Interface (POSIX), The Open Group Base +Specifications Issue 7, Copyright (C) 2013 by the Institute of +Electrical and Electronics Engineers, Inc and The Open Group. +(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the +event of any discrepancy between this version and the original IEEE and +The Open Group Standard, the original IEEE and The Open Group Standard +is the referee document. The original Standard can be obtained online at +http://www.unix.org/online.html . + +Any typographical or formatting errors that appear +in this page are most likely +to have been introduced during the conversion of the source files to +man page format. To report such errors, see +https://www.kernel.org/doc/man-pages/reporting_bugs.html . |