This example opens or creates and opens a normal file named "DOSTEST.DAT", writes to it, reads from it, and finally closes it.

#define INCL_DOSFILEMGR          /* File Manager values */
#define INCL_DOSERRORS           /* DOS Error values    */
#include <os2.h>
#include <stdio.h>
#include <string.h>

int main(void) {
   HFILE  hfFileHandle   = 0L;     /* Handle for file being manipulated */
   ULONG  ulAction       = 0;      /* Action taken by DosOpen */
   ULONG  ulBytesRead    = 0;      /* Number of bytes read by DosRead */
   ULONG  ulWrote        = 0;      /* Number of bytes written by DosWrite */
   ULONG  ulLocal        = 0;      /* File pointer position after DosSetFilePtr */
   UCHAR  uchFileName[20]  = "dostest.dat",     /* Name of file */
          uchFileData[100] = " ";               /* Data to write to file */
   APIRET rc             = NO_ERROR;            /* Return code */

   /* Open the file test.dat.  Use an existing file or create a new */
   /* one if it doesn't exist.                                      */
   rc = DosOpen(uchFileName,                    /* File path name */
                &hfFileHandle,                  /* File handle */
                &ulAction,                      /* Action taken */
                100L,                           /* File primary allocation */
                FILE_ARCHIVED | FILE_NORMAL,    /* File attribute */
                OPEN_ACTION_CREATE_IF_NEW |
                OPEN_ACTION_OPEN_IF_EXISTS,     /* Open function type */
                OPEN_FLAGS_NOINHERIT |
                OPEN_SHARE_DENYNONE  |
                OPEN_ACCESS_READWRITE,          /* Open mode of the file */
                0L);                            /* No extended attribute */

   if (rc != NO_ERROR) {
      printf("DosOpen error: return code = %u\n", rc);
      return 1;
   } else {
     printf ("DosOpen: Action taken = %ld\n", ulAction);
   } /* endif */
   /* Write a string to the file */
   strcpy (uchFileData, "testing...\n1...\n2...\n3\n");

   rc = DosWrite (hfFileHandle,                /* File handle */
                  (PVOID) uchFileData,         /* String to be written */
                  sizeof (uchFileData),        /* Size of string to be written */
                  &ulWrote);                   /* Bytes actually written */

   if (rc != NO_ERROR) {
      printf("DosWrite error: return code = %u\n", rc);
      return 1;
   } else {
      printf ("DosWrite: Bytes written = %u\n", ulWrote);
   } /* endif */

   /* Move the file pointer back to the beginning of the file */
   rc = DosSetFilePtr (hfFileHandle,           /* File Handle */
                       0L,                     /* Offset */
                       FILE_BEGIN,             /* Move from BOF */
                       &ulLocal);              /* New location address */
   if (rc != NO_ERROR) {
      printf("DosSetFilePtr error: return code = %u\n", rc);
      return 1;
   }

   /* Read the first 100 bytes of the file */
   rc = DosRead (hfFileHandle,                /* File Handle */
                 uchFileData,                 /* String to be read */
                 100L,                        /* Length of string to be read */
                 &ulBytesRead);               /* Bytes actually read */

   if (rc != NO_ERROR) {
      printf("DosRead error: return code = %u\n", rc);
      return 1;
   } else {
      printf ("DosRead: Bytes read = %u\n%s\n", ulBytesRead, uchFileData);
   } /* endif */

   rc = DosClose(hfFileHandle);                /* Close the file */

   if (rc != NO_ERROR) {
      printf("DosClose error: return code = %u\n", rc);
      return 1;
   }
   return NO_ERROR;
}

The following example shows how to open a communications port:

#define INCL_DOSFILEMGR          /* File Manager values      */
#define INCL_DOSERRORS           /* DOS Error values         */
#define INCL_DOSPROCESS          /* DOS Process values       */
#define INCL_DOSMISC             /* DOS Miscellanous values  */
#include <os2.h>
#include <stdio.h>
#include <string.h>

int main(void) {

PSZ      pszCommPort    = "COM1";     /* Port name, could use "\\DEV\COM1"  */
HFILE    hPort          = NULLHANDLE; /* Handle for accessing port          */
ULONG    ulAction       = 0L;         /* DosOpen action                     */
ULONG    ulWrote        = 0;          /* Number of bytes written to port    */
UCHAR    uchPortData[100] = " ";      /* Data to write to port              */
APIRET   rc             = NO_ERROR;   /* Return code                        */
DosError( FERR_DISABLEHARDERR);       /* Disable hard error pop-up messages */
rc = DosOpen( pszCommPort,            /* Communications port to open        */
              &hPort,
              &ulAction,              /* Returns action taken by DosOpen    */
              0L,                     /* Not needed for byte stream devices */
              FILE_NORMAL,
              OPEN_ACTION_OPEN_IF_EXISTS,
              OPEN_ACCESS_READWRITE |
              OPEN_FLAGS_NOINHERIT  |
              OPEN_SHARE_DENYREADWRITE ,  /* Prevents us from opening port      */
                                          /* if another application is using it */
                                          /* and prevents other applications    */
                                          /* from using port while we have it   */
              0L);                        /* No extended attributes             */
DosError( FERR_ENABLEHARDERR);            /* Re-enable hard error pop-ups       */


if (rc != NO_ERROR) {
  printf("DosOpen error: return code = %u\n", rc);
  return 1;
} else {
  printf ("DosOpen: Action taken = %ld\n", ulAction);
} /* endif */