Write bytes to a file


#include <unistd.h>

ssize_t write( int fildes,
               const void* buf,
               size_t nbytes );


The file descriptor for the file you want to write in.
A pointer to a buffer that contains the data you want to write.
The number of bytes to write.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The write() function attempts to write nbytes bytes to the file associated with the open file descriptor, fildes, from the buffer pointed to by buf.

If nbytes is zero, write() returns zero, and has no other effect.

On a regular file or other file capable of seeking, and if O_APPEND isn't set, write() starts at a position in the file given by the file offset associated with fildes. If O_APPEND is set, the file offset is set to the end of file before each write operation. Before successfully returning from write(), the file offset is incremented by the number of bytes actually written. On a regular file, if this incremented file offset is greater than the length of the file, the length of the file is set to this file offset.

Note that the write() call ignores advisory locks that may have been set by the fcntl() function.

On a file not capable of seeking, write() starts at the current position.

If write() requests that more bytes be written than there's room for (for example, all blocks on a disk are already allocated), only as many bytes as there's room for are written. For example, if there's only room for 80 more bytes in a file, a write of 512 bytes would return 80. The next write of a nonzero number of bytes would give a failure return (except as noted below).

When write() returns successfully, its return value is the number of bytes actually written to the file. This number is never greater then nbytes, although it may be less than nbytes under certain circumstances detailed below.

If write() is interrupted by a signal before it has written any data, it returns a value of -1, and errno is set to EINTR. However, if write() is interrupted by a signal after it has successfully written some data, it returns the number of bytes written.

If the value of nbytes is greater than INT_MAX, write() returns -1 and sets errno to EINVAL. See <limits.h>.

Write requests to a pipe (or FIFO) are handled the same as a regular file, with the following exceptions:

  • There's no file offset associated with a pipe, therefore each write request appends to the end of the pipe.
  • Write requests of PIPE_BUF bytes or less aren't 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 is set.
  • If the O_NONBLOCK flag is clear, a write request may cause the process to block, but on normal completion it returns nbytes.
  • If the O_NONBLOCK flag is set, write requests are handled differently, in the following ways:
    • The write() function doesn't block the process.
    • Write requests for PIPE_BUF bytes or less either succeed completely and return nbytes, or return -1 and errno is set to EAGAIN.

    If you call write() with nbytes greater than PIPE_BUF bytes, it either transfers what it can and returns the number of bytes written, or transfers no data, returning -1 and setting errno to EAGAIN. Also, if nbytes is greater than PIPE_BUF bytes and all data previously written to the pipe has been read (that is, the pipe is empty), write() transfers at least PIPE_BUF bytes.

When attempting to write to a file (other than a pipe or FIFO) that supports nonblocking writes and can't accept the data immediately:

  • If the O_NONBLOCK flag is clear, write() blocks until the data can be accepted.
  • If the O_NONBLOCK flag is set, write() doesn't block the process. If some data can be written without blocking the process, write() transfers what it can and returns the number of bytes written. Otherwise, it returns -1 and sets errno to EAGAIN.

If write() is called with the file offset beyond the end-of-file, the file is extended to the current file offset with the intervening bytes filled with zeroes. This is a useful technique for pregrowing a file.

If write() succeeds, the st_ctime and st_mtime fields of the file are marked for update.


The number of bytes written, or -1 if an error occurred (errno is set).


The O_NONBLOCK flag is set for the file descriptor, and the process would be delayed in the write operation.
The file descriptor, fildes, isn't a valid file descriptor open for writing.
A write was attempted on a socket that isn't connected.
One of the following occurred:
  • An attempt was made to write a file that exceeds the maximum file size or the process's file size limit, and there was no room for any bytes to be written.
  • The file is a regular file, nbytes 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 fildes.
The write operation was interrupted by a signal, and either no data was transferred, or the resource manager responsible for that file doesn't report partial transfers.
One of the following:
  • The process is a member of a background process group attempting to write to its controlling terminal, TOSTOP is set, the process is neither ignoring nor blocking SIGTTOU, and the process group of the process is orphaned.
  • A physical I/O error occurred (for example, a bad block on a disk). The precise meaning is device-dependent.
  • The filesystem resides on a removable media device, and the media has been forcibly removed.
A write was attempted on a socket and the local network interface used to reach the destination is down.
A write was attempted on a socket and no route to the network is present.
There's no free space remaining on the device containing the file.
The write() function isn't implemented for the filesystem specified by filedes.
One of the following occurred:
  • A request was made of a nonexistent device, or the request was outside the capabilities of the device.
  • A hangup occurred on the STREAM being written to.
One of the following occurred:
  • An attempt was made to write to a pipe (or FIFO) that isn't open for reading by any process, or that has only one end open. A SIGPIPE signal is also sent to the process.
  • 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 is delivered to the calling process.
The transfer request size was outside the range supported by the STREAMS file associated with fildes.


#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>

char buffer[] = { "A text record to be written" };

int main( void )
    int  fd;
    int  size_written;

    /* open a file for output          */
    /* replace existing file if it exists */
    fd = creat( "myfile.dat", S_IRUSR | S_IWUSR );

    /* write the text              */
    size_written = write( fd, buffer,
              sizeof( buffer ) );

    /* test for error              */
    if( size_written != sizeof( buffer ) ) {
        perror( "Error writing myfile.dat" );
        return EXIT_FAILURE;

    /* close the file              */
    close( fd );

    return EXIT_SUCCESS;


POSIX 1003.1 XSI

Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes