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.

Example 1-1 demonstrates the difference between stream mode and record mode access.

Example 1-1 Differences Between Stream Mode and Record Mode

/* 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

1.9 Specific Portability Concerns

One of the last tasks in preparing to use the Compaq C RTL, if you are going to port your source programs across systems, is to be aware of specific differences between the Compaq 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 Compaq C RTL, this section also contains information on the portability of other Compaq C for OpenVMS constructs.

The Compaq 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 Compaq 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 Compaq 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:

• Compaq C for OpenVMS Systems does not implement the global symbols end , edata , and etext .
• There are differences in how OpenVMS and UNIX systems lay out virtual memory. In some UNIX systems, the address space between 0 and the break address is accessible to your program. In OpenVMS systems, the first page of memory is not accessible.
  For example, if a program tries to reference location 0 on an OpenVMS system, a hardware error (ACCVIO) is returned and the program terminates abnormally. OpenVMS systems reserve the first page of address space to catch incorrect pointer references, such as a reference to a location pointed to by a null pointer. For this reason, some existing programs that run on some UNIX systems may fail and you should modify them, as necessary. (Tru64 UNIX and OpenVMS, however, are compatible in this regard.)
• Some C programmers code all external declarations in #include files. Then, specific declarations that require initialization are redeclared in the relevant module. This practice causes the Compaq C compiler to issue a warning message about multiply declared variables in the same compilation. One way to avoid this warning is to make the redeclared symbols extern variables in the #include files.
• Compaq C does not support asm calls on OpenVMS VAX systems. They are supported on OpenVMS Alpha systems. See the Compaq C User's Guide for OpenVMS Systems for more information on intrinsic functions.
• Some C programs call the counted string functions strcmpn and strcpyn . These names are not used by Compaq C for OpenVMS Systems. Instead, you can define macros that expand the strcmpn and strcpyn names into the equivalent, ANSI-compliant names strncmp and strncpy .
• The Compaq C for OpenVMS compiler does not support the following initialization form:

  int foo 123;

  
  Programs using this form of initialization must be changed.

• Compaq C for OpenVMS Systems predefines several compile-time macros such as __vax , __alpha , __32BITS , __vms , __vaxc , __VMS_VER , __DECC_VER , __D_FLOAT , __G_FLOAT , __IEEE_FLOAT , __X_FLOAT , and others. These predefined macros are useful for programs that must be compatible on other machines and operating systems. For more information, see the predefined macro chapter of the Compaq C User's Guide for OpenVMS Systems.
• The ANSI C language does not guarantee any memory order for the variables in a declaration. For example:

  int a, b, c;

• Depending on the type of external linkage requested, extern variables in a program may be treated differently using Compaq C on OpenVMS systems than they would on UNIX systems. See the Compaq C User's Guide for OpenVMS Systems for more information.
• The dollar sign ($) is a legal character in Compaq C for OpenVMS identifiers, and can be used as the first character.
• The ANSI C language does not define any order for evaluating expressions in function parameter lists or for many kinds of expressions. The way in which different C compilers evaluate an expression is only important when the expression has side effects. Consider the following examples:

  a[i] = i++;

  x = func_y() + func_z();

  f(p++, p++)

  
  Neither Compaq C nor any other C compiler can guarantee that such expressions evaluate in the same order on all C compilers.

• The size of a Compaq C variable of type int is 32 bits on OpenVMS systems. You will have to modify programs that are written for other machines and that assume a different size for a variable of type int . A variable of type long is the same size (32 bits) as a variable of type int .
• The C language defines structure alignment to be dependent on the machine for which the compiler is designed. On OpenVMS VAX systems, Compaq C aligns structure members on byte boundaries, unless #pragma member_alignment is specified. On OpenVMS Alpha systems, Compaq C aligns structure members on natural boundaries, unless #pragma nomember_alignment is specified. Other implementations may align structure members differently.
• References to structure members in Compaq C cannot be vague. For more information, see the Compaq C Language Reference Manual.
• Registers are allocated based upon how often a variable is used, but the register keyword gives the compiler a strong hint that you want to place a particular variable into a register. Whenever possible, the variable is placed into a register. Any scalar variable with the storage class auto or register can be allocated to a register as long as the variable's address is not taken with the ampersand operator (&) and it is not a member of a structure or union.

The Compaq C RTL supports an improved and enhanced reentrancy. The following types of reentrancy are supported:

• AST reentrancy uses the _BBSSI built-in function to perform simple locking around critical sections of RTL code, but it may also disable asynchronous system traps (AST)s in locked regions of code. This type of locking should be used when AST code contains calls to Compaq C RTL I/O routines.
  Failure to specify AST reentrancy might cause I/O routines to fail, setting errno to EALREADY.
• MULTITHREAD reentrancy is designed to be used in threaded programs such as those that use the DECthreads library. It performs DECthreads locking and never disables ASTs. DECthreads must be available on your system to use this form of reentrancy.
• TOLERANT reentrancy uses the _BBSSI built-in function to perform simple locking around critical sections of RTL code, but ASTs are not disabled. This type of locking should be used when ASTs are used and must be delivered immediately.
• NONE gives optimal performance in the Compaq C RTL, but does absolutely no locking around critical sections of RTL code. It should only be used in a single-threaded environment when there is no chance that the thread of execution will be interrupted by an AST that would call the Compaq C RTL.

The default reentrancy type is TOLERANT.

You can set the reentrancy type by compiling with the /REENTRANCY command-line qualifier or by calling the decc$set_reentrancy function. This function must be called exclusively from non-AST level.

When programming an application using multiple threads or ASTs, you should consider three classes of functions:

• Functions with no internal data
• Functions with thread-local internal data
• Functions with process-wide internal data

Most functions have no internal data at all. For these functions, synchronization is necessary only if the parameter is used by the application in multiple threads or in both AST and non-AST context. For example, although the strcat function is ordinarily safe, the following is an example of unsafe usage:

extern char buffer[100]; void routine1(char *data) { strcat( buffer, data ); }

If routine1 executed concurrently in multiple threads, or if routine1 is interrupted by an AST routine that calls it, the results of the strcat call are unpredictable.

The second class of functions are those that have thread-local static data. Typically, these are routines in the library that return a string where the application is not permitted to free the storage for the string. These routines are thread-safe but not AST-reentrant. This means they can safely be called concurrently, and each thread will have its own copy of the data. They cannot be called from AST routines if it is possible that the same routine was executing in non-AST context. The routines in this class are:

asctime stat ctermid strerror ctime strtok cuserid VAXC$ESTABLISH gmtime the errno variable localtime wcstok perror

All the socket functions are also included in this list if the TCP/IP product in use is thread-safe.

The third class of functions are those that affect process-wide data. These functions are neither thread-safe nor AST-reentrant. For example, sigsetmask establishes the process-wide signal mask. Consider a routine like the following:

void update_data base() { int old_mask; old_mask = sigsetmask( 1 << (SIGINT - 1)); /* Do work here that should not be aborted. */ sigsetmask( old_mask ); }

If update_database was called concurrently in multiple threads, thread 1 might unblock SIGINT while thread 2 was still performing work that should not be aborted.

The routines in this class are:

• All the signal routines
• All the exec routines
• The exit , _exit , nice , system , wait , getitimer , setitimer , and setlocale routines.

Mixing the multithread programming model and the OpenVMS AST programming model in the same application is not recommended. The application has no mechanism to control which thread gets interrupted by an AST. This can result in a resource deadlock if the thread holds a resource that is also needed by the AST routine. The following routines use mutexes. To avoid a potential resource deadlock, do not call them from AST routines in a multithreaded application.

• All the I/O routines
• All the socket routines
• All the signal routines
• vfork , exec , wait , system
• catgets
• set_new_handler (C++ only)
• getenv
• rand and srand
• exit and _exit
• clock
• nice
• times
• ctime , localtime , asctime , mktime

This section is for application developers who need to use 64-bit virtual memory addressing on OpenVMS Alpha Version 7.0 or higher.

OpenVMS Alpha 64-bit virtual addressing support makes the 64-bit virtual address space defined by the Alpha architecture available to both the OpenVMS operating system and its users. It also allows per-process virtual addressing for accessing dynamically mapped data beyond traditional 32-bit limits.

The Compaq C Run-Time Library on OpenVMS Alpha Version 7.0 systems and higher includes the following features in support of 64-bit pointers:

• Guaranteed binary and source compatibility of existing programs
• No impact on applications that are not modified to exploit 64-bit support
• Enhanced memory allocation routines that allocate 64-bit memory
• Widened function parameters to accommodate 64-bit pointers
• Dual implementations of functions that need to know the pointer size used by the caller
• New information available to the DEC C Version 5.2 compiler or higher to seamlessly call the correct implementation
• Ability to explicitly call either the 32-bit or 64-bit form of functions for applications that mix pointer sizes
• A single shareable image for use by 32-bit and 64-bit applications

The Compaq C Run-Time library on OpenVMS Alpha Version 7.0 systems and higher can generate and accept 64-bit pointers. Functions that require a second interface to be used with 64-bit pointers reside in the same object libraries and shareable images as their 32-bit counterparts. No new object libraries or shareable images are introduced. Using 64-bit pointers does not require changes to your link command or link options files.

The Compaq C 64-bit environment allows an application to use both 32-bit and 64-bit addresses. For more information about how to manipulate pointer sizes, see the /POINTER_SIZE qualifier and #pragma pointer_size and #pragma required_pointer_size preprocessor directives in the Compaq C User's Guide for OpenVMS Systems.

The /POINTER_SIZE qualifier requires you to specify a value of 32 or 64. This value is used as the default pointer size within the compilation unit. As an application programmer, you can compile one set of modules by using 32-bit pointers and another set by using 64-bit pointers. Care must be taken when these two separate groups of modules call each other.

Use of the /POINTER_SIZE qualifier also influences the processing of Compaq C RTL header files. For those functions that have a 32-bit and 64-bit implementation, specifying /POINTER_SIZE enables function prototypes to access both functions, regardless of the actual value supplied to the qualifier. In addition, the value specified to the qualifier determines the default implementation to call during that compilation unit.

The #pragma pointer_size and #pragma required_pointer_size preprocessor directives can be used to change the pointer size in effect within a compilation unit. You can default pointers to 32-bit pointers and then declare specific pointers within the module as 64-bit pointers. You would also need to specifically call the _malloc64 form of malloc to obtain memory from the 64-bit memory area.