Table 8-1 lists and describes all the memory allocation functions found in the Compaq C Run-Time Library (RTL). For a more detailed description of each function, see the Reference Section.

Table 8-1 Memory Allocation Functions

Function	Description
brk, sbrk	Determine the lowest virtual address that is not used with the program.
calloc, malloc	Allocate an area of memory.
cfree, free	Make available for reallocation the area allocated by a previous calloc , malloc , or realloc call.
realloc	Changes the size of the area pointed to by the first argument to the number of bytes given by the second argument.
strdup	Finds and points to a duplicate string.

All Compaq C RTL functions requiring additional storage from the heap get that storage using the Compaq C RTL memory allocation functions malloc , calloc , realloc , free , and cfree . Memory allocated by these functions is quadword-aligned.

The ANSI C standard does not include cfree . For this reason, it is preferable to free memory using the functionally equivalent free function.

The brk and sbrk functions assume that memory can be allocated contiguously from the top of your address space. However, the malloc function and RMS may allocate space from this same address space. You should not use the brk and sbrk functions in conjunction with RMS and Compaq C RTL routines that use malloc .

Previous versions of the VAX C RTL documentation indicated that the memory allocation routines used the OpenVMS RTL functions LIB$GET_VM and LIB$FREE_VM to acquire and return dynamic memory. This is no longer the case; interaction between these routines and the Compaq C RTL memory allocation routines is no longer problematic (although LIB$SHOW_VM can no longer be used to track Compaq C RTL malloc and free usage).

The Compaq C RTL memory allocation functions calloc , malloc , realloc , and free are based on the LIB$ routines LIB$VM_CALLOC, LIB$VM_MALLOC, LIB$VM_REALLOC and LIB$VM_FREE, respectively.

The routines vaxc$calloc_opt , vaxc$cfree_opt , vaxc$free_opt , vaxc$malloc_opt , and vaxc$realloc_opt are now obsolete and should not be used in new development. However, versions of these routines that are equivalent to the standard C memory allocation routines are provided for backward compatibility.

8.1 Program Example

Example 8-1 shows the use of the malloc , calloc , and free functions.

Example 8-1 Allocating and Deallocating Memory for Structures

/* CHAP_8_MEM_MANAGEMENT.C */ /* This example takes lines of input from the terminal until */ /* it encounters a Ctrl/Z, places the strings into an */ /* allocated buffer, copies the strings to memory allocated for */ /* structures, prints the lines back to the screen, and then */ /* deallocates all the memory used for the structures. */ #include <stdlib.h> #include <stdio.h> #define MAX_LINE_LENGTH 80 struct line_rec { /* Declare the structure. */ struct line_rec *next; /* Pointer to next line. */ char *data; /* A line from terminal. */ }; int main(void) { char *buffer; /* Define pointers to */ /* structure (input lines). */ struct line_rec *first_line = NULL, *next_line, *last_line = NULL; /* Buffer points to memory. */ buffer = malloc(MAX_LINE_LENGTH); if (buffer == NULL) { /* If error ... */ perror("malloc"); exit(EXIT_FAILURE); } while (gets(buffer) != NULL) { /* While not Ctrl/Z ... */ /* Allocate for input line. */ next_line = calloc(1, sizeof (struct line_rec)); if (next_line == NULL) { perror("calloc"); exit(EXIT_FAILURE); } /* Put line in data area. */ next_line->data = buffer; if (last_line == NULL) /* Reset pointers. */ first_line = next_line; else last_line->next = next_line; last_line = next_line; /* Allocate space for the */ /* next input line. */ buffer = malloc(MAX_LINE_LENGTH); if (buffer == NULL) { perror("malloc"); exit(EXIT_FAILURE); } } free(buffer); /* Last buffer always unused. */ next_line = first_line; /* Pointer to beginning. */ while (next_line != NULL) { puts(next_line->data); /* Write line to screen. */ free(next_line->data); /* Deallocate a line. */ last_line = next_line; next_line = next_line->next; free(last_line); } exit(EXIT_SUCCESS); }

The sample input and output for Example 8-1 are as follows:

$ RUN EXAMPLE line one line two [Ctrl/Z] EXIT line one line two $

The C programming language is a good choice if you wish to write operating systems. For example, much of the UNIX operating system is written in C. When writing system programs, it is sometimes necessary to retrieve or modify the environment in which the program is running. This chapter describes the Compaq C Run-Time Library (RTL) functions that accomplish this and other system tasks.

Table 9-1 lists and describes all the system functions found in the Compaq C RTL. For a more detailed description of each function, see the Reference Section.

Table 9-1 System Functions

Function	Description
System Functions---Searching and Sorting Utilities
bsearch	Performs a binary search on an array of sorted objects for a specified object.
qsort	Sorts an array of objects in place by implementing the quick-sort algorithm.
System Functions---Retrieving Process Information
ctermid	Returns a character string giving the equivalence string of SYS$COMMAND, which is the name of the controlling terminal.
cuserid	Returns a pointer to a character string containing the name of the user who initiated the current process.
getcwd	Returns a pointer to the file specification for the current working directory.
getegid , geteuid , getgid , getuid	Returns, in OpenVMS terms, group and member numbers from the user identification code (UIC).
getenv	Searches the environment array for the current process and returns the value associated with a specified environment.
getlogin	Gets the login name of the user associated with the current session.
getpid	Returns the process ID of the current process.
getppid	Returns the parent process ID of the calling process.
getpwnam	Accesses user-name information in the user database.
getpwuid	Accesses user-ID information in the user database.
System Functions---Changing Process Information
chdir	Changes the default directory.
chmod	Changes the file protection of a file.
chown	Changes the owner user identification code (UIC) of a file.
mkdir	Creates a directory.
nice	Increases or decreases the process priority to the process base priority by the amount of the argument.
putenv	Sets an environmental variable.
setenv	Inserts or resets the environment variable name in the current environment list.
setgid, setuid	Implemented for program portability and have no functionality.
sleep , usleep	Suspends the execution of the current process for at least the number of seconds indicated by its argument.
umask	Creates a file protection mask that is used whenever a new file is created. It returns the old mask value.
System Functions---Retrieving and Converting Date/Time Information
asctime	Converts a broken-down time into a 26-character string.
clock	Determines the CPU time (in microseconds) used since the beginning of the program execution.
ctime	Converts a time, in seconds, to an ASCII string in the form generated by the asctime function.
decc$fix_time	Converts OpenVMS binary system times to UNIX binary times.
difftime	Computes the difference, in seconds, between the two times specified by its arguments.
ftime	Returns the elapsed time since 00:00:00, January 1, 1970, in the structure timeb .
getclock	Gets the current value of the system-wide clock.
getdate	Converts a formatted string to a time/date structure.
getitimer	Returns the value of interval timers.
gettimeofday	Gets date and time.
gmtime	Converts time units to broken-down UTC time.
localtime	Converts a time (expressed as the number of seconds elapsed since 00:00:00, January 1, 1970) into hours, minutes, seconds, and so on.
mktime	Converts a local-time structure into time since the Epoch.
setitimer	Sets the value of interval timers.
strftime, wcsftime	Places characters into an array, as controlled by a specified format string.
strptime	Converts a character string into date and time values.
time	Returns the time elapsed since 00:00:00, January 1, 1970, in seconds.
times	Returns the accumulated times of the current process and of its terminated child processes.
tzset	Sets and accesses time-zone conversion.
ualarm	Sets or changes the timeout of interval timers.
wcsftime	Uses date and time information stored in a tm structure to create a wide-character output string.
System Functions---Miscellaneous
vaxc$crtl_init	Initializes the run-time environment and establishes an exit and condition handler, which makes it possible for Compaq C RTL functions to be called from other languages.

Example 9-1 shows how the cuserid function is used.

Example 9-1 Accessing the User Name

/* CHAP_9_GET_USER.C */ /* Using cuserid, this program returns the user name. */ #include <stdio.h> main() { static char string[L_cuserid]; cuserid(string); printf("Initiating user: %s\n", string); }

If a user named TOLLIVER runs the program, the following is displayed on stdout:

$ RUN EXAMPLE1 Initiating user: TOLLIVER

Example 9-2 shows how the getenv function is used.

Example 9-2 Accessing Terminal Information

/* CHAP_9_GETTERM.C */ /* Using getenv, this program returns the terminal. */ #include <stdio.h> #include <stdlib.h> main() { printf("Terminal type: %s\n", getenv("TERM")); }

Running Example 9-2 on a Compaq VT100 terminal in 132-column mode displays the following:

$ RUN EXAMPLE3 Terminal type: vt100-132

Example 9-3 shows how to use getenv to find the user's default login directory and how to use chdir to change to that directory.

Example 9-3 Manipulating the Default Directory

/* CHAP_9_CHANGE_DIR.C */ /* This program performs the equivalent of the DCL command */ /* SET DEFAULT SYS$LOGIN. However, once the program exits, the */ /* directory is reset to the directory from which the program */ /* was run. */ #include <stdio.h> #include <unistd.h> #include <stdlib.h> main() { char *dir; int i; dir = getenv("HOME"); if ((i = chdir(dir)) != 0) { perror("Cannot set directory"); exit(0); } printf("Current directory: %s\n", dir); }

Running Example 9-3 displays the following:

$ RUN EXAMPLE4 Current directory: dba0:[tolliver] $

Example 9-4 shows how to use the time , localtime , and strftime functions to print the correct date and time at the terminal.

Example 9-4 Printing the Date and Time

/* CHAP_9_DATE_TIME.C */ /* The time function returns the time in seconds; the localtime */ /* function converts the time to hours, minutes, and so on; */ /* the strftime function uses these values to obtain a string */ /* in the desired format. */ #include <time.h> #include <stdio.h> #define MAX_STRING 80 main() { struct tm *time_structure; time_t time_val; char output_str[MAX_STRING]; time(&time_val); time_structure = localtime(&time_val); /* Print the date */ strftime(output_str, MAX_STRING, "Today is %A, %B %d, %Y", time_structure); printf("%s\n", output_str); /* Print the time using a 12-hour clock format. */ strftime(output_str, MAX_STRING, "The time is %I:%M %p", time_structure); printf("%s\n", output_str); }

Running Example 9-4 displays the following:

$ RUN EXAMPLE5 Today is Thursday, May 20, 1993 The time is 10:18 AM $

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