Mac-Source 1994 July/Mac-Source_July_1994.iso/C and C++/Science⁄Math/VideoToolbox/VideoToolboxSources/Assign.note

Text File | 1993-09-20 | 29.8 KB | 584 lines | [TEXT/KAHL]

Assign.note
This file documents Assign.c.

Assign is a portable runtime C interpreter that reads and executes any text
"assignment" file that contains only C assignments and comments, e.g.
    viewingDistance=57.0;    /* inches */
It uses an array of "Description" structures to associate each name in the
assignment file with a corresponding runtime C variable. It can also write an
assignment file, making the assignment file a portable way of writing and
reading calibrations and experimental parameters. Arrays can be allocated
dynamically, automatically dimensioned so as to hold all the assignments in any
part of the assignment file.

Programs written before 1993, using the old ReadAssignments.c (now superceded by
Assign.c), may #include the header file Assign92.h to minimize the effort of
updating.

PORTABILITY: Standard C

SYNTAX OF THE ASSIGNMENT FILE

The entire "assignment" file may contain only simple assignments--which are
interpreted--and whitespace and comments--which are ignored. Both C style /* */
and C++ style // comments are allowed. Ignoring any intervening whitespace and
comments, each assignment consists of a variable name of any length, possibly
with a constant subscript (of up to ASSIGN_DIMS dimensions), an equal sign,
either a constant (decimal 13, 1.3, 1.3e9, hex 0x1a, char 'a', transcendental
Inf, or not-a-number NaN) or a string literal (e.g. "hello"), and a semicolon.
    i=3; a=1.0; b[1][2]=INF; msg="hello world"; /* examples */
NaN (or NAN or nan), Inf (or INF or inf), and -Inf are accepted as floating
values. (The NAN specification may include a NAN-code, a la THINK C, e.g. NANFF,
or ala MPW C, e.g. NAN[255].) Backslash escapes, e.g. \007 and \n, within single
or double quotes are fully supported (except that there is no support for the
new international "wide" characters and trigraph sequences). A simple variable
name, as in C, consists of an alphabetic character (or _) followed by any number
of alphanumeric characters (or _). To simulate structure references, composite
names are allowed, made up of simple names joined by "." or "->", e.g.
    a->b.c=0.0; myStruct->b=INF;
Unlike C, each line must be less than BUFFER_SIZE characters long (currently
512). Lines that end with the backslash character \ are spliced to the next by
deleting the backslash and the following newline character. The splice may be
between tokens or inside a comment or string literal, but, unlike C, the splice
may not break a token. Adjacent string literals "a" "b" are concatenated "ab",
as specified by Standard C. (The line splicing and adjacent concatenation allow
creation of strings of arbitrary length, not limited by BUFFER_SIZE.) No further
expression evaluation is supported. No other operators, e.g. +-&*, enums, casts,
e.g. (unsigned long), or number suffixes, e.g. UL, are allowed. No macros or
preprocessor directives are supported (e.g. #if, #define). The syntax rules are
strictly enforced. If ReadAssignmentLine can't interpret your file it will, at
the caller's option, either return an error code or call PrintfExit with an
error message pinpointing the error.

Assign.c supports one optional extension beyond C syntax that makes it practical
to save images within assignment files. If a non-string variable is assigned a
string,
    a="1111aaaa1111";
then ReadAssignmentLine assumes that the string is an encoding of the desired
binary object as a hexadecimal number, and assigns the decoded binary object to
the variable, one byte for every two hex digits. This is supported for both
scalar and array variables--treating either an element or a row (last subscript,
fastest changing) of the array as a single binary object--so you can load or
save a huge row by a single statement that assigns a humungous string of hex
data to the array. E.g. if m[][] is of type char, then the following three lines
are equivalent.
    m[3][0]=155;m[3][1]=95;m[3][2]=255;m[3][3]=127;
    m[3][0]="9b";m[3][1]="5f";m[3][2]="ff";m[3][3]="7f";
    m[3]="9b5fff7f";
This format is supported by both ReadAssignmentLine and PrintAnAssignment. When
reading an assignment file, a binary object containing only one element, as in
m[3][0]="9b", is interpreted as a scalar, rather than as a row of length 1. When
writing an assignment file a whole row of each array is assigned at once
m[3]="9b5fff7f" unless the row is only one element long, in which case each
element is assigned separately. The default behavior of ReadAssignmentLine and
PrintAnAssignment is to use the binary object assignments only for integers
(including char) and to write it only when it would greatly reduce the file
size. The motivation for this extension is to provide a compact way to specify
the contents of an image, typically an array of small integers. It may also be
useful in situations where one wants to save and restore the exact value of a
double, since typical implementations of the C stdio library lose a few bits of
precision translating to or from decimal representations of floating numbers.
You grant or deny permission to read and write hex-encoded binary objects by
setting or clearing two flag bits, assignNoHexInts and assignHexFloats, which
are explained below.

DESCRIBING YOUR VARIABLES TO THE ASSIGN ROUTINES

The Assign routines use your pre-existing C variables. Each C variable that
you want to be able to load from (or print to) an assignment file must be
described by a "Description" structure, which has fields for the type, name,
address ("ptr"), and other characteristics of your C variable.
    typedef struct {
        short type;
        unsigned sizedOnce:1;    /* Is dim[] meaningful? */
        unsigned sized:1;        /* Is dim[] final? */
        unsigned malloced:1;    /* Allocated by malloc? */
        char *name;
        void *ptr;                /* for array, address of element zero */
        long dim[ASSIGN_DIMS];    /* zero indicates a scalar */
        long firstElement;/* subscript of first element of last dimension. Usually 0 */
        const char *comment;    /* text string, or NULL */
    } Description;
The routines Describe, DescribeArray, and DescribeFirstLast are convenient ways
to set all the fields. Use these routines (instead of explicitly loading the
fields yourself) to enhance the compatibility of your programs with future
enhancements to Assign.c, which might redefine the fields of the Description
structure.
    Description Describe(short type,void *ptr,char *name,const char *comment);
    short n;
    d[i++]=Describe(shortType,&n,"n","hello");
The available types fall into three groups. The basic types correspond to the
basic types in C: charType, unsignedCharType, shortType, unsignedShortType,
longType, unsignedLongType, floatType, shortDoubleType, and doubleType. A
second, corresponding, group of types, adding the word "Ptr", requests dynamic
array allocation: charPtrType, unsignedCharPtrType, shortPtrType,
unsignedShortPtrType, longPtrType, unsignedLongPtrType, floatPtrType,
shortDoublePtrType, and doublePtrType. The third group consists solely of the
"stringType", which is handled somewhat differently from the charPtrType. (Note:
since Standard C doesn't allow the "short double" type, the shortDoubleType and
shortDoublePtrType are conditionally compiled, controlled by SHORT_DOUBLE_OK,
which is defined in VideoToolbox.h.) The name argument string is used in reading the
assignment file. The ptr argument should be the address of a C variable of the
type specified by the type argument (e.g. float). When the type is one of the
Ptr types, or stringType, then the C variable should be a pointer of the
appropriate kind, so the "ptr" argument you supply will be the address of that
pointer. The last argument to Describe() is comment, which must be a string or
NULL. Any call to PrintAnAssignment() will print the comment (if not NULL) after
the assignment statement, surrounding it by /* */.
    Description DescribeArray(short type,void *ptr,char *name,const char *comment,...);
    double b[3][4];
    d[i++]=DescribeArray(doubleType,&b,"b","coefficients",3L,4L,0L);
For the basic and string types, the supplied C variable can be an array, whose
dimensions are specified as additional arguments to DescribeArray. There may
be up to ASSIGN_DIMS dimensions. Each dimension must be a long, and the argument
list must be terminated by 0L, i.e. a zero of type long.

Arrays of stringType are allowed, e.g.
    char *nicknames[10];
    DescribeArray(stringType,&nicknames,"nicknames",NULL,10L,0L);
But, in that case, malloced space for a string is not freed when a new string
is assigned.

READING AN ASSIGNMENT FILE

ReadAssignmentLine does all the reading. ReadAssignmentBlock and
ReadAssignmentFile just call ReadAssignmentLine repeatedly. One by one,
ReadAssignmentLine reads assignments from the assignment file (until it reaches
the end of the line, or detects an error). Syntax errors are reported.
ReadAssignmentLine gets a variable name from the assignment file and looks for a
matching name in the user-supplied Description array. Any inconsistency of the
assignment statement with the variable's declared type and dimensions is
reported. Then the assignment is made.

Assignment to basic variables is straightforward, equivalent to assignment in
a C program, e.g.
    a[2][1]=3.0;
When ReadAssignmentLine assigns a string to a stringType variable,
    s="hello world";
the string is allocated by malloc() and the string's address is poked into your
char * variable. Prior to assignment of a string, FreeADescribedVar will be
called, which, if any pre-existing string was allocated by malloc--as indicated
by the d->malloced flag--will free it. You can free all your strings (and
PtrType variables) by calling FreeDescribedVars.

The PtrType variables initially have no space allocation for storage of data. Each
unallocated Ptr variable is automatically allocated space for a scalar or array
dimensioned just big enough to contain all the assignments in the part (or
"gulp") of the assignment file that the user has just asked to read: a line, a
block, or the whole file. (If the Ptr variable is dimensioned, from a call to
DescribeArray, but not allocated, then it will be allocated at the declared
size.) Once allocated, the array dimensions are fixed, unless the user frees the
allocations by calling FreeDescribedPtrVars() or FreeDescribedVars(), which also
clear the array's dimensions (whether set dynamically or by an initial call to
DescribeArray). Restricting the scan to the current gulp makes it possible to
read assignment files in which the array sizes change from gulp to gulp,
provided only that between gulps the user calls FreeDescribedPtrVars() or
FreeDescribedVars(). An inconsistent number of array dimensions
    a[0]=1; a[0][0]=1;
within a gulp is flagged as an error.

ROUTINES

    stream=OpenCalFileWrite(filename);
Open up a calibration file for appending. If the file exists in the current
directory, it is used. If not, then if it exists in the Preferences folder, it
used. If not, the file is created in the Preferences folder.

    stream=OpenCalFileRead(filename);
Open up a calibration file for reading. First looks in the current directory,
then in the preferences folder. On success returns the opened file's stream. On
failure returns NULL.

    stream=OpenCalFileReadAndCheck(filename);
Open up a calibration file for reading. If the file exists in the current
directory, it is used. If not, then if it exists in the preferences folder, it
used. If no file is found then prints an error message and exits.

    AppendDescriptions(&d,s);
Appends the second descriptions array onto the end of the first, which is
reallocated with more space. The source array is not freed; the caller should do
that. Both arrays must be null-terminated.

    CopyDescriptions(d,s);
Copy one null-terminated array of descriptions to another, which is assumed to
be big enough.

    d=AllocateDescriptions(n);
Allocate space for variable descriptions. Adds one to the passed size to hold
the null descriptor. Nulls the first element.

    FreeDescriptions(d);
Free the descriptions array. Doesn't affect the described variables.

    n=NumberOfDescriptions(d);
Find the size of an array of descriptions. Does not count the trailing null
description.

    d[i]=NullDescription();
Returns a null description, to terminate a descriptions array.

    if( IsNullDescription(d[i]) )...;
Determine whether we are looking at a null description. Implemented as a macro.

Description Describe(short type,void *ptr,char *name,const char *comment);
Description DescribeArray(short type,void *ptr,char *name,const char *comment,...);
    double a,*p,x[3];
    d[i++]=Describe(doubleType,&a,"a","hello");
    d[i++]=Describe(doublePtrType,&p,"p",NULL);
    d[i++]=DescribeArray(doubleType,&x,"x",NULL,3L,0L);
Describe and DescribeArray merely return a Description with the field values
that you supply. Use them to make your programs more readable, and to enhance
the compatibility of your programs with future enhancements to Assign.c, which
might redefine the fields of the Description structure. DescribeArray() accepts
a variable number of arguments specifying the array dimensions. You may supply
any number of array dimensions (up to ASSIGN_DIMS), but they MUST be long, and
the argument list MUST be terminated by a 0L following the list of dimensions.
Any additional arguments following the 0L are ignored. A PtrType variable may be
described by DescribeArray(), specifying the dimensions of the storage area to
be allocated, or may be described by Describe() in which case the dimensions
(none up to ASSIGN_DIMS) will be determined from the first gulp of the
assignment file. In either case, storage for the PtrType variable is allocated
only when it is encountered in the assignment file. The array dimensions of a
PtrType variable specified when calling DescribeArray() are wiped out if you
later call FreePtrVariables(), with the same result as just calling Describe().

TYPES: charType, unsignedCharType, shortType, unsignedShortType, longType,
unsignedLongType, floatType, shortDoubleType, doubleType, charPtrType,
unsignedCharPtrType, shortPtrType, unsignedShortPtrType, longPtrType,
unsignedLongPtrType, floatPtrType, shortDoublePtrType, doublePtrType,
stringType.

Description DescribeFirstLast(short type,void *ptr,char *name
    ,const char *comment,long firstElement,long lastElement);
    float *a;
    a=vector(1,10);    /* a Numerical Recipes in C allocation routine */
    d[i++]=DescribeFirstLast(floatType,a,"a",NULL,1L,10L,0L);
This allows you to specify a 1-dimensional array with a nonstandard subscript
range, e.g. starting with subscript 1, as in Numerical Recipes in C. The "ptr"
argument should always point to element zero, even if that element is not part of
the array's allocated storage.

    error=UnequalDescribedVars(descriptions1,descriptions2,flags);
    error=UnequalDescribedVarPair(d1,d2,flags);
These routines compare the data pointed to by the two Description arrays or
structs. They return 0 if the Descriptions are legal and the data are
equal, and a nonzero error code (or PrintfExit, depending on flags) otherwise.
(Comparison of floating numbers allows a tolerance of +/- one part in a million,
because converting to and from decimal may lose some precision, and considers
all NANs equal because NAN indices (NAN04 vs NANFF) are not preserved by most
operations.) This makes it easy to verify what was written to an assignment
file. Just create a new data structure and a Description array describing it,
call ReadAssignmentFile, and use UnequalDescribedVars to verify that you read
back exactly what you wrote.

    n=InitializeDescribedVars(descriptions,flags);
    n=InitializeADescribedVar(d,flags);
Initialize the described variables. All integer variables
are set to zero, all floating variables are set to NAN, and all the strings are
set to point to a particular empty string "". Any PtrType scalars and arrays
that have already been allocated will be initialized; unallocated PtrType
variables are NULLed.

    n=ReadAssignmentLine(stream,descriptions,flags);
Interprets one line from the stream (splicing continuation lines), and continues
to read further lines, if necessary, to reach the end of any incomplete comment
or assignment. ReadAssignmentBlock() and ReadAssignmentFile(), below, do all
their reading by repeatedly calling ReadAssignmentLine().

    blank=AssignmentLineWasBlank();
Returns true (i.e. 1) unless the most recent call to ReadAssignmentLine() read
some non-blank text, in which case it returns false (i.e. 0).

    n=ReadAssignmentBlock(stream,descriptions,flags);
Interprets lines from the stream until it reads a blank line. (Blank lines within
an assignment or comment /* */ are ignored.) Implemented by repeatedly calling
ReadAssignmentLine() until AssignmentLineWasBlank() is true.

    n=ReadAssignmentFile(filename,descriptions,flags);
Interpret a whole file. Implemented by repeatedly calling ReadAssignmentLine().

    FreeDescribedPtrVars(descriptions,flags);
Frees any allocated storage (and wipes out the array dimensions) for any
described PtrType variables; they will be appropriately dimensioned and
allocated when they are next read from an assignment file. This allows reading
of assignment files in which array dimensions may change from gulp to gulp.

    FreeDescribedVars(descriptions,flags);
Frees any allocated storage (and wipes out the array dimensions) for any
described PtrType and stringType variables.

    FreeADescribedVar(description,flags);
Frees any allocated storage (and wipes out the array dimensions) for a described
variable. This only affects PtrType and stringType variables.

    KeepDescribedVars(descriptions,flags);
Prevents the freeing of all the allocated storage for all the described PtrType
and stringType variables. Call this when you want to copy the pointer elsewhere
and you don't want Assign to dispose of the storage pointed to. (This routine
simply clears all the "malloced" fields in the descriptions.)

    i=FindDescription(d,ptr,flags);
Finds your C variable's description in the supplied array. Returns the subscript
(≥0) of the matching Description, or, if the search fails, then either PrintfExits
with an error or returns a negative error code, as determined by the setting of
the assignNoPrintfExit bit in flags. Your C variable's dimensions are in the
d[i].dim[] field, which is dimensioned ASSIGN_DIMS.

    dim=FindDescribedDim(d,ptr,n,flags);
Calls FindDescription to find your C variable's description and returns the
size of your variable's n-th dimension. (All dimensions after the first
ASSIGN_DIMS return zero.) Handling of search failure as in FindDescription().

    n=PrintAnAssignment(stream,d,flags);
Prints out the name and value of the described variable as an assignment
statement "i=2;" suitable for reading by ReadAssignmentLine.

    n=PrintAssignments(stream,descriptions,flags);
Calls PrintAnAssignment and fprintf(stream,"\n"); for each described variable.

    n=PrintAssignmentsToFile(filename,descriptions,flags);
Opens the file, calls PrintAssignments, and closes the file. If a file of that
name already exists it appends to the existing file.

    n=ReadLuminanceRecord(filename,LP,flags)
This uses ReadAssignmentFile() to interpret a LuminanceRecord?.h file. This
routine is in ReadLuminanceRecord.c.

In all cases the returned value, n, is either positive (>=0) indicating the
number of data that were fully processed, or negative, indicating an error code
defined in VideoToolbox.h. Each array element, scalar, or string is considered a
datum. Any assignments that referred to out-of-range array elements or unknown
variables are skipped and are not counted.

The first argument is either a filename (c string) or a stream (FILE *)
returned by fopen(). Open the file in text, not binary, mode, so that
'\r' characters will be translated properly.

The second argument, "descriptions", is an array of Descriptions of
your variables. The number of array elements is not given. Instead you mark the
end by zeroing the "type" field of the last Description. The "Description" data
structure is defined in VideoToolbox.h.

The third argument, "flags", allows you to specify various runtime options. Each
option is controlled by a different bit; flags is the sum of the desired
options. Most users will want the default behavior, i.e. flags==0. The default
behavior of PrintAnAssignment is to never hex-encode floats (float and double)
and to only hex encode integers (i.e. char, short, and long, signed or unsigned)
when this would reduce the number of lines in the assignment file (i.e. when
the last dimension exceeds typeSize+2).

FLAGS

"assignNoPrintfExit" is a flag that asks all the routines to return an error
code if they cannot satisfy the request, otherwise the routines call PrintfExit
with a diagnostic error message that pinpoints the location of the error.

"assignReportUnknown" asks ReadAssignmentLine to report an error if an unknown
variable appears in the assignment file. Array elements with out-of-range
subscripts are considered unknown variables. In the absence of this flag,
attempts to assign to unknown variables are silently skipped (the data are
discarded).

"assignNoHexInts" tells ReadAssignmentLine and PrintAnAssignment to never read
or save char, short, or long as hex strings. This sacrifices the compactness of
hex strings, but enhances human readability of the assignment file. (It also
preserves compatibility with C, making it possible to #include the assignment
file inside a C program, and preserves portability of the assignment file
between computers that use different byte orderings.)

"assignHexFloats" tells PrintAnAssignment to always save floats (float and
double) as hex strings, and tells ReadAssignmentLine to allow hex strings. This
preserves the exact value (unlike decimal representation, which loses a few
least-significant bits in typical implementations of the C stdio library), but
sacrifices human readability and portability of the assignment file since the
binary representation of floats is highly dependent on the compiler options and
the kind of computer. (It also sacrifices compatibility with C, making it
impossible to #include the assignment file.)

"assignNoComment" tells PrintAnAssignment not to print the comment field.

"assignEchoFile", primarily an aid to debugging, prints each line from the stream
as it is read by ReadAssignmentLine.

"assignEchoAssignments", primarily an aid to debugging, calls PrintAnAssignment
after each assignment is performed by ReadAssignmentLine. Uncommented newline
characters in the assignment file are echoed as well. (If assignReportUnknown is
off then assignment statements containing unknown variables or out-of-bounds
array references are echoed as comments: /*UNKNOWN: x=3; */.)

"assignEchoComments", primarily an aid to debugging, prints both /*..*/ and
//-style comments as they are read by ReadAssignmentLine. Uncommented newline
characters in the assignment file are echoed as well.

ERRORS

An error code is returned only if the assignNoPrintfExit flag is set. Otherwise
PrintfExit() is called with a diagnostic message.

-1 assignMemoryError: couldn't allocate enough memory.
-2 assignTypeError: unknown type, or array of string, which is not allowed.
-3 assignVariableError: couldn't parse the variable name.
-4 assignUnknownVariableError: unknown variable; can only occur if the
    assignReportUnknown flag is set.
-5 assignSubscriptError: couldn't parse the subscript.
-6 assignSubscriptBoundsError: out-of-bounds array element; this error is flagged only
    if the assignReportUnknown flag is set.
-7 assignEqualsError: couldn't find the equals sign.
-8 assignConstantError: couldn't parse the constant.
-9 assignHexError: couldn't parse a hex-encoded binary object constant.
-10 assignSemicolonError: couldn't find the semicolon.
-11 assignFileError: couldn't open the file.
-12 assignInconsistentDescriptionsError: two descriptions are inconsistent.
-13 assignUnequalDataError: two described variables have significantly different data.

IMPORTANT: Do not forget to mark the end of your Description array by a Description
with a type of zero.

USING ASSIGN.C

It is suggested that those who want to use this package create a header file
with a typedef for a data structure, and write a subroutine that allocates and
fills a Description array describing the data structure's fields. For example,
Luminance.h defines the luminanceRecord structure and ReadLuminanceRecord.c
contains the function,
    Description *DescribeLuminanceRecord(luminanceRecord *LP);
that creates a Description array describing the particular luminanceRecord structure.

CONTROLLING EXPERIMENTS

Assign.c was originally written to read parameter files that control
experiments. Typical psychophysical and physiological experiments are very
complicated and require lots of adjustable parameters. One needs an easy way to
change them all that is self documenting and easily reproduced, possibly months
later. The routines provided here come in various flavors allowing you to read a
line at a time (with any number of assignments on the line), a block at a time
(blocks are separated by blank lines), or a whole file at a time. A line may be
continued by ending it with backslash \. In controlling psychophysical
experiments I use a block at the beginning of the file to define the general
experiment followed by a block for each psychophysical "block"; within the block
each line specifies a different experimental condition.

If you decide to read the assignment file a bit at a time, bear in mind that
ReadAssignmentLine will immediately return or PrintfExit if the assignment file
contains even a single error. It is strongly suggested that your experimental
program begin by doing a dry run, reading the whole assignment file through to
check for errors, before initializing your variables, and reading it again a bit
at a time as the experiment progresses. It would obviously be very bad to have
the interpreter quit near the end of a 3 hour experiment.

READING AND WRITING CALIBRATION DATA

David Brainard and I have been discussing how to save and read calibrations of
video displays in a portable way, so that the vision community could share
calibration programs, and experimental programs could simply read in the data
associated with the particular display. The current idea is that there would be
a text file called "Monitor nn calib" associated with each display (where nn is
the video card's slot number), and that this file would hold all kinds of
calibration data for this display, typically produced by a variety of
calibration programs that would each append their results. On a Macintosh
computer these calibration files would reside in the Preferences folder. (Where
should they go on a DOS machine?) David has developed color calibration software
and I've developed software to calibrate gamma and MTF. We are now rewriting
these to make them more portable. We have begun drafting an open-ended standard
for this calibration file, to allow any number of programs to read and write to
it, extracting or adding the data they need and know about and ignoring the
rest. Tentatively, in order to conform to this calibration standard a new
supplement (e.g. to calibrate the geometry of the display) should include: 1. a
C header file that defines a suitable data structure to hold the measurements
(and a few standard facts, e.g. the date of calibration, the name of the video
card and serial number of the monitor, and the names of the person and program
that did the calibration), 2. a C subroutine that accepts a pointer to such a
data structure and returns a pointer to a "Description" array describing each of
the data structure's fields. 3. a program (in any language) that makes the
measurements and appends them to the monitor's calibration file as C assignments
to the data structure's fields, which is easily accomplished by one call to
PrintAssignmentFile. These C assignments must be readable by ReadAssignmentFile
using the Description array created in 2. above, and it is strongly recommended
that the calibration program itself call ReadAssignmentFile, and use
UnequalDescribedVars() to verify that it read back exactly what it wrote. 4.
finally, it will normally be important to provide some sort of advice on how to
use the calibration data when they are read from the calibration file.

EXAMPLE

An assignment file might contain this:
    /* first block */
    viewingDistance=57.0;
    trials=40;
    message="Monitor calibrated November 21, 1993 by Katie Burns";
    a=3;

    // second block
    logC=-1.0;
    a[0]=1;

    // third block
    logC=-2;
    a[1][1]=1.1;a[0][0]=-INF;

Here is a sample program to read the assignments given above:
#include "VideoToolbox.h"
#include "assert.h"
typedef struct {
    double *a,logC,viewingDistance;
    long trials;
    char *message;
} Psychophysics;
Description *DescribePsychophysics(Psychophysics *p);

Description *DescribePsychophysics(Psychophysics *p)
{
    Description *d;
    int i;

    d=AllocateDescriptions(10);
    i=0;
    d[i++]=Describe(stringType,&p->message,"message",NULL);
    d[i++]=Describe(doubleType,&p->viewingDistance,"viewingDistance",NULL);
    d[i++]=Describe(shortType,&p->trials,"trials",NULL);
    d[i++]=Describe(doubleType,&p->logC,"logC",NULL);
    d[i++]=Describe(doublePtrType,&p->a,"a",NULL);
    assert(i<=10);        /*  make sure array was big enough */
    d[i++]=NullDescription();        /*  mark end of array */
    return d;
}

void main(void)
{
    Description *d;
    Psychophysics psychophysics,*p;
    short i,flags=0;
    FILE *stream;

    p=&psychophysics;
    d=DescribePsychophysics(p);
    InitializeDescribedVars(d,flags);
    stream=fopen("assignments","r");
    do{
        printf("\n/******** New block ********/\n");
        ReadAssignmentBlock(stream,d,flags);
        /*
        a real program would do something useful here with the data in p
        */
        PrintAssignments(stdout,d,flags);
        FreeDescribedPtrVars(d,flags);
    } while(!feof(stream));
    fclose(stream);
    FreeDescriptions(d);
}

NOTES FOR FUTURE ENHANCEMENT
The "firstElement" approach supports Numerical Recipes vectors, i.e. 1-d arrays.
For multi-dimensional Numerical Recipes arrays I would also need a
"vectoredArray" flag. Vectored arrays would be fairly easy to add, only
affecting ElementPtr, and allocation and freeing. In the meantime, note that
Numerical Recipes provides a convert_matrix routine that provides vectored
addressing of a normal 2-d c array.

ACKNOWLEDGEMENTS
Assign.c was inspired by an idea Beau Watson mentioned to me in the 1980's: a
routine to read free-form parameter values at run time. Discussions with David
Brainard motivated the extensions to support arrays, dynamic allocation, and
unknown variables.