Writes data to a file.

int _write(
   int fd,
   const void *buffer,
   unsigned int count 


  • fd
    File descriptor of file into which data is written.

  • buffer
    Data to be written.

  • count
    Number of bytes.

Return Value

If successful, _write returns the number of bytes actually written. If the actual space remaining on the disk is less than the size of the buffer the function is trying to write to the disk, _write fails and does not flush any of the buffer's contents to the disk. A return value of –1 indicates an error. If invalid parameters are passed, this function invokes the invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, the function returns -1 and errno is set to one of three values: EBADF, which means the file descriptor is invalid or the file is not opened for writing; ENOSPC, which means there is not enough space left on the device for the operation; or EINVAL, which means that buffer was a null pointer.

See _doserrno, errno, _sys_errlist, and _sys_nerr for more information on these, and other, return codes.

If the file is opened in text mode, each linefeed character is replaced with a carriage return – linefeed pair in the output. The replacement does not affect the return value.


The _write function writes count bytes from buffer into the file associated with fd. The write operation begins at the current position of the file pointer (if any) associated with the given file. If the file is open for appending, the operation begins at the current end of the file. After the write operation, the file pointer is increased by the number of bytes actually written.

When writing to files opened in text mode, _write treats a CTRL+Z character as the logical end-of-file. When writing to a device, _write treats a CTRL+Z character in the buffer as an output terminator.



Required header



For additional compatibility information, see Compatibility in the Introduction.


// crt__write.c
// This program opens a file for output and uses _write to write
// some bytes to the file.

#include <io.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <share.h>

char buffer[] = "This is a test of '_write' function";

int main( void )
   int         fileHandle = 0;
   unsigned    bytesWritten = 0;

   if ( _sopen_s(&fileHandle, "write.o", _O_RDWR | _O_CREAT,
                  _SH_DENYNO, _S_IREAD | _S_IWRITE) )
      return -1;

   if (( bytesWritten = _write( fileHandle, buffer, sizeof( buffer ))) == -1 )
         case EBADF:
            perror("Bad file descriptor!");
         case ENOSPC:
            perror("No space left on device!");
         case EINVAL:
            perror("Invalid parameter: buffer was NULL!");
            // An unrelated error occured 
            perror("Unexpected error!");
      printf_s( "Wrote %u bytes to file.\n", bytesWritten );
   _close( fileHandle );

Wrote 36 bytes to file.

See Also


Low-Level I/O


_open, _wopen