|
HP OpenVMS Systems Documentation |
|
HP C
|
| Previous | Contents | Index |
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:
|
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.
1.8.2.2 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:
In RMS terms, the HP C RTL translates the record's carriage-control information using one of the following methods:
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:
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:
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$record_write are equivalent, respectively, to read and write , except that they work with file pointers rather than file descriptors.
Unlike the read function, which reads at most one record, the fread function can span records. Rather than read number_items records (where number_items is the third parameter to fread ), fread tries to read the number of bytes equal to number_items x size_of_item (where size_of_item is the second parameter to fread ). The value returned by fread is equal to the number of bytes read divided by size_of_item.
However, the fwrite function always generates at least number_items records.
The fgets and gets functions read to either a new-line character or a record boundary.
The fflush function always generates a record if there is unwritten data in the buffer. The same is true of close , fclose , fseek , lseek , rewind , and fsetpos , all of which perform implicit fflush functions.
A record is also generated whenever an attempt is made to write more characters than allowed by the maximum record size.
For more information on these functions, see the Reference Section.
1.8.2.2.2 Accessing Fixed-Length Record Files in Record Mode
When accessing a fixed-length record file in record mode, the I/O functions generally behave as described in Section 1.8.2.2.1.
The write , fwrite , and decc$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. |
Example 1-1 demonstrates the difference between stream mode and record mode access.
| Example 1-1 Differences Between Stream Mode and Record Mode Access |
|---|
/* CHAP_1_STREAM_RECORD.C */
/* This program demonstrates the difference between */
/* record mode and stream mode input/output. */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void process_records(const char *fspec, FILE * fp);
main()
{
FILE *fp;
fp = fopen("example-fixed.dat", "w", "rfm=fix", "mrs=40", "rat=none");
if (fp == NULL) {
perror("example-fixed");
exit(EXIT_FAILURE);
}
printf("Record mode\n");
process_records("example-fixed.dat", fp);
fclose(fp);
printf("\nStream mode\n");
fp = fopen("example-streamlf.dat", "w");
if (fp == NULL) {
perror("example-streamlf");
exit(EXIT_FAILURE);
}
process_records("example-streamlf.dat", fp);
fclose(fp);
}
void process_records(const char *fspec, FILE * fp)
{
int i,
sts;
char buffer[40];
/* Write records of all 1's, all 2's and all 3's */
for (i = 0; i < 3; i++) {
memset(buffer, '1' + i, 40);
sts = fwrite(buffer, 40, 1, fp);
if (sts != 1) {
perror("fwrite");
exit(EXIT_FAILURE);
}
}
/* Rewind the file and write 10 characters of A's, then 10 B's, */
/* then 10 C's. */
/* */
/* For stream mode, each fwrite call outputs 10 characters */
/* and advances the file position 10 characters */
/* characters. */
/* */
/* For record mode, each fwrite merges the 10 characters into */
/* the existing 40-character record, updates the record and */
/* advances the file position 40 characters to the next record. */
rewind(fp);
for (i = 0; i < 3; i++) {
memset(buffer, 'A' + i, 10);
sts = fwrite(buffer, 10, 1, fp);
if (sts != 1) {
perror("fwrite2");
exit(EXIT_FAILURE);
}
}
/* Now reopen the file and output the records. */
fclose(fp);
fp = fopen(fspec, "r");
for (i = 0; i < 3; i++) {
sts = fread(buffer, 40, 1, fp);
if (sts != 1)
perror("fread");
printf("%.40s\n", buffer);
}
return;
}
|
Running this program produces the following output:
Record Mode AAAAAAAAAA111111111111111111111111111111 BBBBBBBBBB222222222222222222222222222222 CCCCCCCCCC333333333333333333333333333333 Stream mode AAAAAAAAAABBBBBBBBBBCCCCCCCCCC1111111111 2222222222222222222222222222222222222222 3333333333333333333333333333333333333333 |
One of the last tasks in preparing to use the HP C RTL, if you are going to port your source programs across systems, is to be aware of specific differences between the HP C RTL and the run-time libraries of other implementations of the C language. This section describes some of the problems that you might encounter when porting programs to and from an OpenVMS system. Although portability is closely tied to the implementation of the HP C RTL, this section also contains information on the portability of other HP C for OpenVMS constructs.
The HP C RTL provides ANSI C defined library functions as well as many commonly available APIs and a few OpenVMS extensions. See Section 1.5 for specific standards, portions of which are implemented by the HP C RTL. Attempts have been made to maintain complete portability in functionality whenever possible. Many of the Standard I/O and UNIX I/O functions and macros contained in the HP C RTL are functionally equivalent to those of other implementations.
The RTL function and macro descriptions elaborate on issues presented in this section and describe concerns not documented here.
The following list documents issues of concern if you wish to port C programs to the OpenVMS environment:
int foo 123; |
int a, b, c; |
a[i] = i++; |
x = func_y() + func_z(); |
f(p++, p++) |
| Previous | Next | Contents | Index |