NAME

write − write on a file

SYNOPSIS

int write (fildes, buf, nbyte)
int fildes;
char ∗buf;
unsigned nbyte;

DESCRIPTION

fildes is a file descriptor obtained from a creat(2), open(2), dup(2), fcntl(2), or pipe(2) system call.

write attempts to write nbyte bytes from the buffer pointed to by buf to the file associated with the fildes.

On devices capable of seeking, the actual writing of data proceeds from the position in the file indicated by the file pointer.  Upon return from write, the file pointer is incremented by the number of bytes actually written.

On devices incapable of seeking, writing always takes place starting at the current position.  The value of a file pointer associated with such a device is undefined. 

If the O_APPEND flag of the file status flags is set, the file pointer is set to the end of the file prior to each write. 

For regular files, if the O_SYNC flag of the file status flags is set, the write does not return until both the file data and file status have been physically updated.  This function is for special applications that require extra reliability at the cost of performance.  For block special files, if O_SYNC is set, the write does not return until the data has been physically updated. 

A write to a regular file is blocked if mandatory file/record locking is set [see chmod(2)], and there is a record lock owned by another process on the segment of the file to be written. If O_NDELAY is not set, the write sleeps until the blocking record lock is removed. 

For STREAMS [see intro(2)] files, the operation of write is determined by the values of the minimum and maximum nbyte range ("packet size") accepted by the stream.  These values are contained in the topmost stream module.  Unless the user pushes [see I_PUSH in streamio(7)] the topmost module, these values can not be set or tested from user level.  If nbyte falls within the packet size range, nbyte bytes are written.  If nbyte does not fall within the range and the minimum packet size value is zero, write 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 nbyte does not fall within the range and the minimum value is non-zero, write fails with errno set to ERANGE.  Writing a zero-length buffer (nbyte is zero) sends zero bytes with zero returned. 

For STREAMS files, if O_NDELAY is not set and the stream can not accept data (the stream write queue is full due to internal flow control conditions), write blocks until data can be accepted.  O_NDELAY prevents a process from blocking due to flow control conditions.  If O_NDELAY is set and the stream can not accept data, write fails.  If O_NDELAY is set and part of the buffer has been written when a condition in which the stream can not accept additional data occurs, write terminates and returns the number of bytes written. 

write fails and the file pointer does not change if one or more of the following are true:

­[EAGAIN] Mandatory file/record locking was set, O_NDELAY was set, and there was a blocking record lock. 

­[EAGAIN] Total amount of system memory available when reading via raw I/O is temporarily insufficient. 

­[EAGAIN] Attempt to write to a stream that can not accept data with the O_NDELAY flag set. 

­[EBADF] fildes is not a valid file descriptor open for writing. 

­[EDEADLK] The write was going to go to sleep and cause a deadlock situation to occur. 

­[EFAULT] buf points outside the process’s allocated address space. 

­[EFBIG] An attempt was made to write a file that exceeds the process’s file size limit or the maximum file size [see ulimit(2)].

­[EINTR] A signal was caught during the write system call. 

­[EINVAL] Attempt to write to a stream linked below a multiplexor. 

­[ENOLCK] The system record lock table was full, so the write could not go to sleep until the blocking record lock was removed. 

­[ENOLINK] fildes is on a remote machine and the link to that machine is no longer active. 

­[ENOSPC] During a write to an ordinary file, there is no free space left on the device. 

­[ENXIO] A hangup occurred on the stream being written to. 

[EPIPE and SIGPIPE signal ] An attempt is made to write to a pipe that is not open for reading by any process. 

­[ERANGE] Attempt to write to a stream with nbyte outside specified minimum and maximum write range, and the minimum value is non-zero. 

If a write requests that more bytes be written than there is room for (e.g., the ulimit [see ulimit(2)] or the physical end of a medium), only as many bytes as there is room for are written. For example, suppose there is space for 20 bytes more in a file before reaching a limit. A write of 512-bytes returns 20. The next write of a non-zero number of bytes gives a failure return (except as noted below).

If the file being written is a pipe (or FIFO) and the O_NDELAY flag of the file flag word is set, then write to a full pipe (or FIFO) returns a count of 0.  Otherwise ( O_NDELAY clear), writes to a full pipe (or FIFO) blocks until space becomes available. 

A write to a STREAMS file can fail if an error message has been received at the stream head.  In this case, errno is set to the value included in the error message. 

SEE ALSO

creat(2), dup(2), fcntl(2), intro(2), lseek(2), open(2), pipe(2), ulimit(2). 

DIAGNOSTICS

Upon successful completion the number of bytes actually written is returned.  Otherwise, −1 is returned and errno is set to indicate the error. 

March 13, 1992