HP OpenVMS Systems Documentation

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Run-Time Library Reference Manual for OpenVMS Systems

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1.7 32-Bit UIDs/GIDs and POSIX Style Identifiers

Where supported in versions of the OpenVMS operating system, POSIX style identifiers refers to the User Identifier (UID), Group Identifier (GID), and Process Group. The scope includes real and effective identifiers.

The support for POSIX style identifiers in the HP C RTL requires 32-bit user and group ID support and also depends on features in the base version of OpenVMS. POSIX style IDs are supported by OpenVMS Version 7.3-2 and higher.

To use POSIX style identifiers on OpenVMS versions that support them requires applications to be compiled for 32-bit UID/GID. On OpenVMS versions where 32-bit UID/GID is the default, the user or application must still enable POSIX style IDs by defining the DECC$POSIX_STYLE_UID feature logical name:


With POSIX style IDs enabled, at compile time you can selectively invoke the traditional (UIC-based) definition for an individual function by explicitly calling it by its decc$ -prefixed entry point (as opposed to the decc$__long_gid_ -prefixed entry point, which provides the POSIX style behavior).

To disable POSIX style IDs:


OpenVMS Version 7.3-2 and higher supports POSIX style IDs as well as 32-bit UID/GIDs. When an application is compiled to use 32-bit UID/GIDs, the UID and GID are derived from the UIC as in previous versions of the operating system. In some cases, such as with the getgroups function, more information may be returned when the application supports 32-bit GIDs.

To compile an application for 16-bit UID/GID support on systems that by default use 32-bit UIDs/GIDs, define the _DECC_SHORT_GID_T macro to 1.

1.8 Input and Output on OpenVMS Systems

After you learn how to link with the HP C RTL and call HP C functions and macros, you can use the HP C RTL for its primary purpose: input/output (I/O).

Since every system has different methods of I/O, familiarize yourself with the OpenVMS specific methods of file access. In this way, you will be equipped to predict functional differences when porting your source program from one operating system to another.

Figure 1-2 shows the I/O methods available with the HP C RTL. The OpenVMS system services communicate directly with the OpenVMS operating system, so they are closest to the operating system. The OpenVMS Record Management Services (RMS) functions use the system services, which manipulate the operating system. The HP C Standard I/O and UNIX I/O functions and macros use the RMS functions. Since the HP C RTL Standard I/O and UNIX I/O functions and macros must go through several layers of function calls before the system is manipulated, they are furthest from the operating system.

Figure 1-2 I/O Interface from C Programs

The C programming language was developed on the UNIX operating system, and the Standard I/O functions were designed to provide a convenient method of I/O that would be powerful enough to be efficient for most applications, and also be portable so that the functions could be used on any system running C language compilers.

The HP C RTL adds functionality to this original specification. Since, as implemented in the HP C RTL, the Standard I/O functions recognize line terminators, the HP C RTL Standard I/O functions are particularly useful for text manipulation. The HP C RTL also implements some of the Standard I/O functions as preprocessor-defined macros.

In a similar manner, the UNIX I/O functions originally were designed to provide a more direct access to the UNIX operating systems. These functions were meant to use a numeric file descriptor to represent a file. A UNIX system represents all peripheral devices as files to provide a uniform method of access.

The HP C RTL adds functionality to the original specification. The UNIX I/O functions, as implemented in HP C, are particularly useful for manipulating binary data. The HP C RTL also implements some of the UNIX I/O functions as preprocessor-defined macros.

The HP C RTL includes the Standard I/O functions that should exist on all C compilers, and also the UNIX I/O functions to maintain compatibility with as many other implementations of C as possible. However, both Standard I/O and UNIX I/O use RMS to access files. To understand how the Standard I/O and UNIX I/O functions manipulate RMS formatted files, learn the fundamentals of RMS. See Section 1.8.1 for more information about Standard I/O and UNIX I/O in relationship to RMS files. For an introduction to RMS, see the Guide to OpenVMS File Applications.

Before deciding which method is appropriate for you, first ask this question: Are you concerned with UNIX compatibility or with developing code that will run solely under the OpenVMS operating system?

  • If UNIX compatibility is important, you probably want to use the highest levels of I/O---Standard I/O and UNIX I/O---because that level is largely independent of the operating system. Also, the highest level is easier to learn quickly, an important consideration if you are a new programmer.
  • If UNIX compatibility is not important to you or if you require the sophisticated file processing that the Standard I/O and UNIX I/O methods do not provide, you might find RMS desirable.

If you are writing system-level software, you may need to access the OpenVMS operating system directly through calls to system services. For example, you may need to access a user-written device driver directly through the Queue I/O Request System Service ($QIO). To do this, use the OpenVMS level of I/O; this level is recommended if you are an experienced OpenVMS programmer. For examples of programs that call OpenVMS system services, see the HP C User's Guide for OpenVMS Systems.

You may never use the RMS or the OpenVMS system services. The Standard I/O and UNIX I/O functions are efficient enough for a large number of applications. Figure 1-3 shows the dependency of the Standard I/O and the UNIX I/O functions on RMS, and the various methods of I/O available to you.

Figure 1-3 Mapping Standard I/O and UNIX I/O to RMS

1.8.1 RMS Record and File Formats

To understand the capabilities and the restrictions of the Standard I/O and UNIX I/O functions and macros, you need to understand OpenVMS Record Management Services (RMS).

RMS supports the following file organizations:

  • Sequential
  • Relative
  • Indexed

Sequential files have consecutive records with no empty records in between; relative files have fixed-length cells that may or may not contain a record; and indexed files have records that contain data, carriage-control information, and keys that permit various orders of access.

The HP C RTL functions can access only sequential files. If you wish to use the other file organizations, you must use the RMS functions. For more information about the RMS functions, see the HP C User's Guide for OpenVMS Systems.

RMS is not concerned with the contents of records, but it is concerned about the record format, which is the way a record physically appears on the recording surface of the storage medium.

RMS supports the following record formats:

  • Fixed-length
  • Variable-length
  • Variable with fixed-length control (VFC)
  • Stream

You can specify a fixed-length record format at the time of file creation. This means that all records occupy the same amount of space in the file. You cannot change the record format once you create the file.

The length of records in variable-length, VFC, and stream file formats can vary up to a maximum size that must be specified when you create the file. With variable-length record or VFC format files, the size of the record is held in a header section at the beginning of the data record. With stream files, RMS terminates the records when it encounters a specific character, such as a carriage-control or line-feed character. Stream files are useful for storing text.

RMS allows you to specify carriage-control attributes for records in a file. Such attributes include the implied carriage-return or the Fortran formatted records. RMS interprets these carriage controls when the file is output to a terminal, a line printer, or other device. The carriage-control information is not stored in the data records.

By default, files inherit the RMS record format, maximum record size and record attributes, from the previous version of the file, if one exists; to an OpenVMS system programmer, the inherited attributes are known as FAB$B_RFM, FAB$W_MRS and FAB$B_RAT. If no previous versions exist, the newly created file defaults to stream format with line-feed record separator and implied carriage-return attributes. (This manual refers to this type of file as a stream file.) You can manipulate stream files using the Standard I/O and the UNIX I/O functions of the HP C RTL. When using these files and fixed-record files with no carriage control, there is no restriction on the ability to seek to any random byte of the file using the fseek or the lseek functions. However, if the file has one of the other RMS record formats, such as variable-length record format, then these functions, due to RMS restrictions, can seek only to record boundaries. Use the default VAX stream format unless you need to create or access files to be used with other VAX languages or utilities.

1.8.2 Access to RMS Files

RMS sequential files can be opened in record mode or stream mode. By default, STREAM_LF files are opened in stream mode; all other file types are opened in record mode. When opening a file, you can override these defaults by specifying the optional argument "ctx=rec" to force record mode, or "ctx=stm" to force stream mode. RMS relative and indexed files are always opened in record mode. The access mode determines the behavior of various I/O functions in the HP C RTL.

One of the file types defined by RMS is an RMS-11 stream format file, corresponding to a value of FAB$C_STM for the record format. The definition of this format is such that the RMS record operation SYS$GET removes leading null bytes from each record. Because this file type is processed in record mode by the HP C RTL, it is unsuitable as a file format for binary data unless it is explicitly opened with "ctx=stm", in which case the raw bytes of data from the file are returned.


In OpenVMS Version 7.0 the default LRL value on stream files was changed from 0 to 32767. This change caused significant performance degradation on certain file operations such as sort.

This is no longer a problem. The HP C RTL now lets you define the logical DECC$DEFAULT_LRL to change the default record-length value on stream files.

The HP C RTL first looks for this logical. If it is found and it translates to a numeric value between 0 and 32767, that value is used for the default LRL.

To restore the behavior prior to OpenVMS Version 7.0, enter the following command:

$ DEFINE DECC$DEFAULT_LRL 0 Accessing RMS Files in Stream Mode

Stream access to RMS files is done with the block I/O facilities of RMS. Stream input is performed from RMS files by passing each byte of the on-disk representation of the file to your program. Stream output to RMS files is done by passing each byte from your program to the file. The HP C RTL performs no special processing on the data.

When opening a file in stream mode, the HP C RTL allocates a large internal buffer area. Data is read from the file using a single read into the buffer area and then passing the data to your program as needed. Data is written to the file when the internal buffer is full or when the fflush function is called. Accessing RMS Record Files in Record Mode

Record access to record files is done with the record I/O facilities of RMS. The HP C RTL emulates a byte stream by translating carriage-control characters during the process of reading and writing records. Random access is allowed to all record files, but positioning (with fseek and lseek ) must be on a record boundary for VFC files, variable record files, or files with non-null carriage control. Positioning a record file causes all buffered input to be discarded and buffered output to be written to the file.

Record input from RMS record files is emulated by the HP C RTL in two steps:

  1. The HP C RTL reads a logical record from the file.
    If the record format is variable length with fixed control (RFM = VFC), and the record attributes are not print carriage control (RAT is not PRN), then the HP C RTL concatenates the fixed-control area to the beginning of the record.
  2. The HP C RTL expands the record to simulate a stream of bytes by translating the record's carriage-control information (if any).

In RMS terms, the HP C RTL translates the record's carriage-control information using one of the following methods:

  • If the record attribute is implied carriage control (RAT = CR), then the HP C RTL appends a new-line character to the record.
    This new-line character is considered an integral part of the record, which means for example, that it can be obtained by the fgetc function and is considered a line terminator by the fgets function. Since fgets reads the file up to the new-line character, for RAT=CR files this function cannot retrieve a string that crosses the record boundaries.
  • If the record attributes are print carriage control (RAT = PRN), then the HP C RTL expands and concatenates the prefix and postfix carriage controls before and after the record.
    This translation is done according to rules specified by RMS, with one exception: if the prefix character is x01 and the postfix character is x8D, then nothing is attached to the beginning of the record and a single new-line character is attached to the end of it. This is done because this prefix/postfix combination is normally used to represent a line.
  • If the record attributes are Fortran carriage control (RAT = FTN), then the HP C RTL removes the initial control byte and attaches the appropriate carriage-control characters before and after the data as defined by RMS, with the exception of the space and default carriage-control characters. In these cases, which are used to represent a line, the HP C RTL appends a single new-line character to the data.
    The mapping of Fortran carriage-control can be disabled by using "ctx=nocvt".
  • If the record attributes are null (RAT = NONE) and the input is coming from a terminal, then the HP C RTL appends the terminating character to the record. If the terminator is a carriage return or Ctrl/Z, then HP C translates the character to a new-line character (\n).
    If the input is coming from a nonterminal file, then the HP C RTL passes the record unchanged to your program with no additional prefix or postfix characters.

As you read from the file, the HP C RTL delivers a stream of bytes resulting from the translations. Information that is not read from an expanded record by one function call is delivered on the next input function call.

The HP C RTL performs record output to RMS record files in two steps.

The first part of the record output emulation is the formation of a logical record. As you write bytes to a record file, the emulator examines the information being written for record boundaries. The handling of information in the byte stream depends on the attributes of the destination file or device, as follows:

  • For all files, if the number of output bytes is greater than the internal buffer allocated by the HP C RTL, a record is output.
  • For files with fixed record length (RFM = FIX) or for files opened with "ctx=bin" or "ctx=xplct", a record is output only when the internal buffer is filled or when the flush function is called.
  • For files with STREAM_CR record format (RFM = STMCR), the HP C RTL outputs a record when it encounters a carriage-return character (\r).
  • For files with STREAM record format (RFM = STM) the HP C RTL outputs a record when it encounters a new-line (\n), form feed (\f), or vertical tab (\v) character.
  • For all other file types, the HP C RTL outputs a record when it encounters a new-line (\n) character.

The second part of record output emulation is to write the logical record formed during the first step. The HP C RTL forms the output record as follows:

  • If the record attribute is carriage control (R AT = CR), and if the logical record ends with a new-line character (\n), the HP C RTL drops the new-line character and writes the logical record with implied carriage control.
  • If the record attribute is print carriage control (RAT = PRN), then the HP C RTL writes the record with print carriage control according to the rules specified by RMS. If the logical record ends with a single new-line character (\n), the HP C RTL maps the new-line character to an x01 prefix and x8D postfix character. This is the reverse of the translation for record input files with print carriage-control attributes.
  • If the record attributes are Fortran carriage control (RAT = FTN), then the HP C RTL removes any prefix and/or postfix carriage-control characters and concatenates a single carriage-control byte to the beginning of the record as defined by RMS, with one exception: If the output record ends in a new-line character (\n), the HP C RTL will remove the new-line character and use the space carriage-control byte. This is the reverse of the translation for record input files with Fortran carriage-control attributes.
    The mapping of Fortran carriage-control can be disabled by using "ctx=nocvt".
  • If the logical record is to be written to a terminal device and the last character of the record is a new-line character (\n) the HP C RTL replaces the new-line character with a carriage-return (\r), and attaches a line-feed character (\n) to the front of the record. The HP C RTL then writes out the record with no carriage control.
  • If the output file record format is variable length with fixed control (RFM = VFC), and the record attributes do not include print carriage control (RAT is not PRN), then the HP C RTL takes the beginning of the logical record to be the fixed-control header, and reduces the number of bytes written out by the length of the header. These bytes are then used to construct the fixed-control header. If there are too few bytes in the logical record, an error is signaled. Accessing Variable-Length or VFC Record Files in Record Mode

When you access a variable-length or VFC record file in record mode, many I/O functions behave differently than they would if they were being used with stream mode. This section describes these differences.

In general, the new-line character (\n) is the record separator for all record modes. On output, when a new-line character is encountered, a record is generated unless you specify an optional argument (such as "ctx=bin" or "ctx=xplct") that affects the interpretation of new lines.

The read and decc$record_read functions always read at most one record. The write and decc$record_write functions always generate at least one record.

decc$record_read and decc$record_write are equivalent, respectively, to read and write , except that they work with file pointers rather than file descriptors.

Unlike the read function, which reads at most one record, the fread function can span records. Rather than read number_items records (where number_items is the third parameter to fread ), fread tries to read the number of bytes equal to number_items x size_of_item (where size_of_item is the second parameter to fread ). The value returned by fread is equal to the number of bytes read divided by size_of_item.

However, the fwrite function always generates at least number_items records.

The fgets and gets functions read to either a new-line character or a record boundary.

The fflush function always generates a record if there is unwritten data in the buffer. The same is true of close , fclose , fseek , lseek , rewind , and fsetpos , all of which perform implicit fflush functions.

A record is also generated whenever an attempt is made to write more characters than allowed by the maximum record size.

For more information on these functions, see the Reference Section. Accessing Fixed-Length Record Files in Record Mode

When accessing a fixed-length record file in record mode, the I/O functions generally behave as described in Section

The write , fwrite , and decc$record_write functions will fail if given a record size that is not an integral multiple of the maximum record size, unless the file was opened with the "ctx=xplct" optional argument specified. All other output functions will generate records at every nth byte, where n is the maximum record size.

If a new record is forced by fflush , the data in the buffer is padded to the maximum record size with null characters.


This padding can cause problems for programs that seek to the end-of-file. For example, if a program were to append data to a file, then seek backwards in the file (causing an fflush to occur), and then seek to the end-of-file again, a zero-filled "hole" will have been created between the previous end-of-file and the new end-of-file if the previous end-of-file was not on a record boundary.

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