This is part of Family API which allow to create dual-os version of program runs under OS/2 and DOS

Note: This is legacy API call. It is recommended to use 32-bit equivalent

2021/09/17 04:47 · prokushev · 0 Comments
2021/08/20 03:18 · prokushev · 0 Comments


This call transfers the specified number of bytes from a buffer to the specified file, synchronously with respect to the requesting process's execution.


DosWrite (FileHandle, BufferArea, BufferLength, BytesWritten)


  • FileHandle (HFILE) - input : File handle from DosOpen.
  • BufferArea (PVOID) - input : Address of the output buffer.
  • BufferLength (USHORT) - input : Number of bytes to write.
  • BytesWritten (PUSHORT) - output : Address of the number of bytes written.

Return Code

rc (USHORT) - return

Return code descriptions are:

  • 0 NO_ERROR


On output, BytesWritten is the number of bytes actually written. If BytesWritten is different from BufferLength, this usually indicates insufficient disk space.

A BufferLength value of 0 is not considered an error. No data transfer occurs. There is no effect on the file or the file pointer.

Buffers that are multiples of the hardware's base physical unit for data written to the file on these base boundaries, are written directly to the device. (The base physical unit is defined as the smallest block that can be physically written to the device.) Other buffer sizes force some I/O to go through an internal system buffer and greatly reduce the efficiency of I/O operation.

The file pointer is moved by read and write operations. It can be moved to a desired position by calling DosChgFilePtr.

If the file is read-only, the write to the file is not performed.

Family API Considerations

Some options operate differently in the DOS mode than in OS/2 mode. Therefore, the following restriction applies to DosWrite when coding for the DOS mode.

  • Only single-byte DosWrite requests can be made to the COM device, because the COM device driver for the DOS environment does not support multiple-byte I/O.

Named Pipe Considerations

DosWrite is also used to write bytes or messages to a named pipe.

Each write to a message pipe writes a message whose size is the length of the write; DosWrite automatically encodes message lengths in the pipe, so applications need not encode this information in the buffer being written.

Writes in blocking mode always write all requested bytes before returning. In non-blocking mode, if the message size is bigger than the buffer size, the write blocks. If the message size is smaller than the pipe but not enough space is left in the pipe, the write returns immediately with a value of zero, indicating no bytes were written.

In the case of a byte pipe, if the number of bytes to be written exceeds the space available in the pipe, DosWrite writes as many bytes as it can and returns with the number of bytes actually written.

An attempt to write to a pipe whose other end has been closed returns ERROR_BROKEN_PIPE.

Example Code

C Binding

USHORT  rc = DosWrite(FileHandle, BufferArea, BufferLength, BytesWritten);
HFILE            FileHandle;    /* File handle */
PVOID            BufferArea;    /* User buffer */
USHORT           BufferLength;  /* Buffer length */
PUSHORT          BytesWritten;  /* Bytes written (returned) */
USHORT           rc;            /* return code */

This example writes to a file.

#define OPEN_FILE 0x01
#define CREATE_FILE 0x10
#define FILE_ARCHIVE 0x20
#define DASD_FLAG 0
#define INHERIT 0x80
#define WRITE_THRU 0
#define FAIL_FLAG 0
#define SHARE_FLAG 0x10
#define ACCESS_FLAG 0x02
#define FILE_NAME "test.dat"
#define FILE_SIZE 800L
#define RESERVED 0L
HFILE   FileHandle;
USHORT  Wrote;
USHORT  Action;
PSZ     FileData[100];
   Action = 2;
   strcpy(FileData, "Data...");
   if(!DosOpen(FILE_NAME,                /* File path name */
                &FileHandle,             /* File handle */
                &Action,                 /* Action taken */
                FILE_SIZE,               /* File primary allocation */
                FILE_ATTRIBUTE,          /* File attribute */
                FILE_EXISTS | FILE_NOEXISTS,    /* Open function type */
                DASD_FLAG | INHERIT |           /* Open mode of the file */
                WRITE_THRU | FAIL_FLAG |
                SHARE_FLAG | ACCESS_FLAG,
                RESERVED))               /* Reserved (must be zero) */
      rc = DosWrite(FileHandle,           /* File handle */
                   (PVOID) FileData,     /* User buffer */
                   sizeof(FileData),     /* Buffer length */
                   &Wrote);              /* Bytes written */

MASM Binding

PUSH   WORD    FileHandle    ;File handle
PUSH@  OTHER   BufferArea    ;User buffer
PUSH   WORD    BufferLength  ;Buffer length
PUSH@  WORD    BytesWritten  ;Bytes written (returned)
CALL   DosWrite

Returns WORD