Merge branch 'gawk-5.1-stable'

8 files changed, 744 insertions, 618 deletions

diff --git a/ChangeLog b/ChangeLog
index 5dfc2bf1..d67d139a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2021-10-20         Andrew J. Schorr      <aschorr@telemetry-investments.com>
+
+	* msg.c (err): Need to check that FNR_node->var_value is non-NULL
+	before testing with is_mpg_number.
+
 2021-10-18         Arnold D. Robbins     <arnold@skeeve.com>
 
 	Factor duplicated code out into a separate function.
diff --git a/doc/ChangeLog b/doc/ChangeLog
index cfb55b4c..e30a2afa 100644
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -1,3 +1,8 @@
+2021-10-20         Arnold D. Robbins     <arnold@skeeve.com>
+
+	* gawktexi.in (FS versus FPAT): New subsection.
+	(String Functions): Reference it.
+
 2021-10-13         Arnold D. Robbins     <arnold@skeeve.com>
 
 	* awkcard.in, gawkinet.texi, gawkworkflow.texi: Update copyright year.
diff --git a/doc/gawk.info b/doc/gawk.info
index 29209ebf..de53a993 100644
--- a/doc/gawk.info
+++ b/doc/gawk.info
@@ -205,6 +205,7 @@ in (a) below.  A copy of the license is included in the section entitled
 * Fields with fixed data::              Field values with fixed-width data.
 * Splitting By Content::                Defining Fields By Content
 * More CSV::                            More on CSV files.
+* FS versus FPAT::                      A subtle difference.
 * Testing field creation::              Checking how 'gawk' is
                                         splitting records.
 * Multiple Line::                       Reading multiline records.
@@ -5593,6 +5594,7 @@ File: gawk.info,  Node: Splitting By Content,  Next: Testing field creation,  Pr
 * Menu:
 
 * More CSV::                    More on CSV files.
+* FS versus FPAT::              A subtle difference.
 
 This minor node discusses an advanced feature of 'gawk'.  If you are a
 novice 'awk' user, you might want to skip it on the first reading.
@@ -5691,7 +5693,7 @@ years.  RFC 4180 (http://www.ietf.org/rfc/rfc4180.txt) standardizes the
 most common practices.
 
 
-File: gawk.info,  Node: More CSV,  Up: Splitting By Content
+File: gawk.info,  Node: More CSV,  Next: FS versus FPAT,  Up: Splitting By Content
 
 4.7.1 More on CSV Files
 -----------------------
@@ -5748,6 +5750,33 @@ that isn't covered.  We recommend, instead, that you use Manuel
 Collado's 'CSVMODE' library for 'gawk' (http://mcollado.z15.es/xgawk/).
 
 
+File: gawk.info,  Node: FS versus FPAT,  Prev: More CSV,  Up: Splitting By Content
+
+4.7.2 'FS' Versus 'FPAT': A Subtle Difference
+---------------------------------------------
+
+As we discussed earlier, 'FS' describes the data between fields ("what
+fields are not") and 'FPAT' describes the fields themselves ("what
+fields are").  This leads to a subtle difference in how fields are found
+when using regexps as the value for 'FS' or 'FPAT'.
+
+   In order to distinguish one field from another, there must be a
+non-empty separator between each field.  This makes intuitive
+sense--otherwise one could not distinguish fields from separators.
+
+   Thus, regular expression matching as done when splitting fields with
+'FS' is not allowed to match the null string; it must always match at
+least one character, in order to be able to proceed through the entire
+record.
+
+   On the other hand, regular expression matching with 'FPAT' can match
+the null string, and the non-matching intervening characters function as
+the separators.
+
+   This same difference is reflected in how matching is done with the
+'split()' and 'patsplit()' functions (*note String Functions::).
+
+
 File: gawk.info,  Node: Testing field creation,  Next: Multiple Line,  Prev: Splitting By Content,  Up: Reading Files
 
 4.8 Checking How 'gawk' Is Splitting Records
@@ -13579,6 +13608,15 @@ Options::):
      Nonalphabetic characters are left unchanged.  For example,
      'toupper("MiXeD cAsE 123")' returns '"MIXED CASE 123"'.
 
+   At first glance, the 'split()' and 'patsplit()' functions appear to
+be mirror images of each other.  But there are differences:
+
+   * 'split()' treats its third argument like 'FS', with all the special
+     rules involved for 'FS'.
+
+   * Matching of null strings differs.  This is discussed in *note FS
+     versus FPAT::.
+
                        Matching the Null String
 
    In 'awk', the '*' operator can match the null string.  This is
@@ -35189,7 +35227,7 @@ Index
 * * (asterisk), *= operator <1>:         Precedence.          (line  94)
 * * (asterisk), **= operator <1>:        Precedence.          (line  94)
 * * (asterisk), * operator, null strings, matching: String Functions.
-                                                              (line 556)
+                                                              (line 565)
 * + (plus sign), regexp operator:        Regexp Operator Details.
                                                               (line 107)
 * + (plus sign), += operator:            Assignment Ops.      (line  81)
@@ -35478,7 +35516,7 @@ Index
 * adding, features to gawk:              Adding Code.         (line   6)
 * advanced features, fixed-width data:   Constant Size.       (line   6)
 * advanced features, specifying field content: Splitting By Content.
-                                                              (line  13)
+                                                              (line  14)
 * advanced features, gawk:               Advanced Features.   (line   6)
 * advanced features, nondecimal input data: Nondecimal Data.  (line   6)
 * advanced features, processes, communicating with: Two-way I/O.
@@ -35630,7 +35668,7 @@ Index
 * asterisk (*), *= operator <1>:         Precedence.          (line  94)
 * asterisk (*), **= operator <1>:        Precedence.          (line  94)
 * asterisk (*), * operator, null strings, matching: String Functions.
-                                                              (line 556)
+                                                              (line 565)
 * at-sign (@), @include directive:       Include Files.       (line   8)
 * at-sign (@), @load directive:          Loading Shared Libraries.
                                                               (line   8)
@@ -35987,7 +36025,7 @@ Index
 * columns, cutting:                      Cut Program.         (line   6)
 * comma (,), in range patterns:          Ranges.              (line   6)
 * Comma separated values (CSV) data, parsing with FPAT: Splitting By Content.
-                                                              (line  19)
+                                                              (line  20)
 * Comma separated values (CSV) data, parsing with FPAT library: More CSV.
                                                               (line  52)
 * command completion, in debugger:       Readline Support.    (line   6)
@@ -36106,7 +36144,7 @@ Index
 * csh utility, POSIXLY_CORRECT environment variable: Options. (line 406)
 * csh utility, |& operator, comparison with: Two-way I/O.     (line  27)
 * CSV (comma separated values) data, parsing with FPAT: Splitting By Content.
-                                                              (line  19)
+                                                              (line  20)
 * CSV (comma separated values) data, parsing with CSVMODE library: More CSV.
                                                               (line  52)
 * CSVMODE library for gawk:              More CSV.            (line  52)
@@ -36816,7 +36854,7 @@ Index
 * forward slash (/), /= operator <1>:    Precedence.          (line  94)
 * forward slash (/), patterns and:       Expression Patterns. (line  24)
 * FPAT variable:                         Splitting By Content.
-                                                              (line  29)
+                                                              (line  30)
 * FPAT variable <1>:                     User-modified.       (line  46)
 * frame debugger command:                Execution Stack.     (line  27)
 * frawk:                                 Other Versions.      (line 117)
@@ -36927,7 +36965,7 @@ Index
 * gawk, RT variable in <1>:              gawk split records.  (line  66)
 * gawk, FIELDWIDTHS variable in:         Fixed width data.    (line  17)
 * gawk, FPAT variable in:                Splitting By Content.
-                                                              (line  29)
+                                                              (line  30)
 * gawk, splitting fields and:            Testing field creation.
                                                               (line   6)
 * gawk, RT variable in <2>:              Multiple Line.       (line 139)
@@ -37399,7 +37437,7 @@ Index
 * matching, leftmost longest:            Multiple Line.       (line  26)
 * matching, expressions:                 Typing and Comparison.
                                                               (line   9)
-* matching, null strings:                String Functions.    (line 556)
+* matching, null strings:                String Functions.    (line 565)
 * mawk utility:                          Escape Sequences.    (line 121)
 * mawk utility <1>:                      Getline/Pipe.        (line  62)
 * mawk utility <2>:                      Concatenation.       (line  36)
@@ -37529,7 +37567,7 @@ Index
 * null strings, as array subscripts:     Uninitialized Subscripts.
                                                               (line  43)
 * null strings, deleting array elements and: Delete.          (line  27)
-* null strings, matching:                String Functions.    (line 556)
+* null strings, matching:                String Functions.    (line 565)
 * null strings <3>:                      Basic Data Typing.   (line  26)
 * number of array elements:              String Functions.    (line 215)
 * number sign (#), #! (executable scripts): Executable Scripts.
@@ -38200,7 +38238,7 @@ Index
                                                               (line 147)
 * sidebar, Operator Evaluation Order:    Increment Ops.       (line  58)
 * sidebar, Changing NR and FNR:          Auto-set.            (line 407)
-* sidebar, Matching the Null String:     String Functions.    (line 554)
+* sidebar, Matching the Null String:     String Functions.    (line 563)
 * sidebar, Interactive Versus Noninteractive Buffering: I/O Functions.
                                                               (line  74)
 * sidebar, Controlling Output Buffering with system(): I/O Functions.
@@ -38649,612 +38687,613 @@ Index
 
 Tag Table:
 Node: Top1200
-Node: Foreword345523
-Node: Foreword449965
-Node: Preface51497
-Ref: Preface-Footnote-154356
-Ref: Preface-Footnote-254465
-Ref: Preface-Footnote-354699
-Node: History54841
-Node: Names57193
-Ref: Names-Footnote-158297
-Node: This Manual58444
-Ref: This Manual-Footnote-165083
-Node: Conventions65183
-Node: Manual History67552
-Ref: Manual History-Footnote-170549
-Ref: Manual History-Footnote-270590
-Node: How To Contribute70664
-Node: Acknowledgments71590
-Node: Getting Started76527
-Node: Running gawk78966
-Node: One-shot80156
-Node: Read Terminal81419
-Node: Long83412
-Node: Executable Scripts84925
-Ref: Executable Scripts-Footnote-187558
-Node: Comments87661
-Node: Quoting90145
-Node: DOS Quoting95671
-Node: Sample Data Files97727
-Node: Very Simple100322
-Node: Two Rules106424
-Node: More Complex108309
-Node: Statements/Lines110641
-Ref: Statements/Lines-Footnote-1115125
-Node: Other Features115390
-Node: When116326
-Ref: When-Footnote-1118080
-Node: Intro Summary118145
-Node: Invoking Gawk119029
-Node: Command Line120543
-Node: Options121341
-Ref: Options-Footnote-1139255
-Ref: Options-Footnote-2139486
-Node: Other Arguments139511
-Node: Naming Standard Input143522
-Node: Environment Variables144732
-Node: AWKPATH Variable145290
-Ref: AWKPATH Variable-Footnote-1148702
-Ref: AWKPATH Variable-Footnote-2148736
-Node: AWKLIBPATH Variable149107
-Ref: AWKLIBPATH Variable-Footnote-1150804
-Node: Other Environment Variables151179
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-Node: Include Files155808
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-Node: Regexp164756
-Node: Regexp Usage166210
-Node: Escape Sequences168247
-Node: Regexp Operators174488
-Node: Regexp Operator Details174973
-Ref: Regexp Operator Details-Footnote-1182337
-Node: Interval Expressions182484
-Ref: Interval Expressions-Footnote-1184684
-Node: Bracket Expressions184782
-Ref: table-char-classes187258
-Node: Leftmost Longest190584
-Node: Computed Regexps191887
-Node: GNU Regexp Operators195314
-Node: Case-sensitivity199051
-Ref: Case-sensitivity-Footnote-1201917
-Ref: Case-sensitivity-Footnote-2202152
-Node: Regexp Summary202260
-Node: Reading Files203726
-Node: Records205995
-Node: awk split records207070
-Node: gawk split records211770
-Ref: gawk split records-Footnote-1216844
-Node: Fields216881
-Node: Nonconstant Fields219622
-Ref: Nonconstant Fields-Footnote-1221858
-Node: Changing Fields222062
-Node: Field Separators228093
-Node: Default Field Splitting230791
-Node: Regexp Field Splitting231909
-Node: Single Character Fields235586
-Node: Command Line Field Separator236646
-Node: Full Line Fields239864
-Ref: Full Line Fields-Footnote-1241386
-Ref: Full Line Fields-Footnote-2241432
-Node: Field Splitting Summary241533
-Node: Constant Size243607
-Node: Fixed width data244339
-Node: Skipping intervening247806
-Node: Allowing trailing data248604
-Node: Fields with fixed data249641
-Node: Splitting By Content251159
-Ref: Splitting By Content-Footnote-1254942
-Node: More CSV255105
-Node: Testing field creation256697
-Node: Multiple Line258322
-Node: Getline264599
-Node: Plain Getline267068
-Node: Getline/Variable269641
-Node: Getline/File270792
-Node: Getline/Variable/File272180
-Ref: Getline/Variable/File-Footnote-1273785
-Node: Getline/Pipe273873
-Node: Getline/Variable/Pipe276577
-Node: Getline/Coprocess277712
-Node: Getline/Variable/Coprocess278979
-Node: Getline Notes279721
-Node: Getline Summary282518
-Ref: table-getline-variants282942
-Node: Read Timeout283690
-Ref: Read Timeout-Footnote-1287596
-Node: Retrying Input287654
-Node: Command-line directories288853
-Node: Input Summary289759
-Node: Input Exercises292931
-Node: Printing293365
-Node: Print295199
-Node: Print Examples296656
-Node: Output Separators299436
-Node: OFMT301453
-Node: Printf302809
-Node: Basic Printf303594
-Node: Control Letters305168
-Node: Format Modifiers310330
-Node: Printf Examples316345
-Node: Redirection318831
-Node: Special FD325672
-Ref: Special FD-Footnote-1328840
-Node: Special Files328914
-Node: Other Inherited Files329531
-Node: Special Network330532
-Node: Special Caveats331392
-Node: Close Files And Pipes332341
-Ref: table-close-pipe-return-values339248
-Ref: Close Files And Pipes-Footnote-1340061
-Ref: Close Files And Pipes-Footnote-2340209
-Node: Nonfatal340361
-Node: Output Summary342699
-Node: Output Exercises343921
-Node: Expressions344600
-Node: Values345788
-Node: Constants346466
-Node: Scalar Constants347157
-Ref: Scalar Constants-Footnote-1349667
-Node: Nondecimal-numbers349917
-Node: Regexp Constants352918
-Node: Using Constant Regexps353444
-Node: Standard Regexp Constants354066
-Node: Strong Regexp Constants357254
-Node: Variables360967
-Node: Using Variables361624
-Node: Assignment Options363534
-Node: Conversion366005
-Node: Strings And Numbers366529
-Ref: Strings And Numbers-Footnote-1369592
-Node: Locale influences conversions369701
-Ref: table-locale-affects372459
-Node: All Operators373077
-Node: Arithmetic Ops373706
-Node: Concatenation376422
-Ref: Concatenation-Footnote-1379269
-Node: Assignment Ops379376
-Ref: table-assign-ops384367
-Node: Increment Ops385680
-Node: Truth Values and Conditions389140
-Node: Truth Values390214
-Node: Typing and Comparison391262
-Node: Variable Typing392082
-Ref: Variable Typing-Footnote-1398545
-Ref: Variable Typing-Footnote-2398617
-Node: Comparison Operators398694
-Ref: table-relational-ops399113
-Node: POSIX String Comparison402608
-Ref: POSIX String Comparison-Footnote-1404303
-Ref: POSIX String Comparison-Footnote-2404442
-Node: Boolean Ops404526
-Ref: Boolean Ops-Footnote-1409008
-Node: Conditional Exp409100
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-Node: Locales418372
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-Node: Patterns and Actions422577
-Node: Pattern Overview423697
-Node: Regexp Patterns425374
-Node: Expression Patterns425916
-Node: Ranges429697
-Node: BEGIN/END432805
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-Node: ARGC and ARGV496464
-Node: Pattern Action Summary500677
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-Node: Array Intro505280
-Ref: figure-array-elements507255
-Ref: Array Intro-Footnote-1509959
-Node: Reference to Elements510087
-Node: Assigning Elements512551
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-Node: Boolean Functions546941
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-Ref: Numeric Functions-Footnote-2552170
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-Ref: String Functions-Footnote-2577133
-Ref: String Functions-Footnote-3577381
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-Node: I/O Functions584659
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-Ref: Time Functions-Footnote-4604469
-Ref: Time Functions-Footnote-5604581
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-Node: Round Function668462
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-Ref: Ordinal Functions-Footnote-1674081
-Ref: Ordinal Functions-Footnote-2674333
-Node: Join Function674543
-Ref: Join Function-Footnote-1676313
-Node: Getlocaltime Function676513
-Node: Readfile Function680255
-Node: Shell Quoting682232
-Node: Isnumeric Function683660
-Node: Data File Management685048
-Node: Filetrans Function685680
-Node: Rewind Function689776
-Node: File Checking691685
-Ref: File Checking-Footnote-1693019
-Node: Empty Files693220
-Node: Ignoring Assigns695199
-Node: Getopt Function696749
-Ref: Getopt Function-Footnote-1711972
-Node: Passwd Functions712172
-Ref: Passwd Functions-Footnote-1721011
-Node: Group Functions721099
-Ref: Group Functions-Footnote-1728997
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-Ref: Split Program-Footnote-1775774
-Node: Tee Program775947
-Node: Uniq Program778737
-Node: Wc Program786325
-Node: Bytes vs. Characters786712
-Node: Using extensions788260
-Node: wc program789014
-Node: Miscellaneous Programs793879
-Node: Dupword Program795092
-Node: Alarm Program797122
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-Ref: Labels Program-Footnote-1810163
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-Ref: Igawk Program-Footnote-2842205
-Ref: Igawk Program-Footnote-3842327
-Node: Anagram Program842442
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-Node: Extension Philosophy892974
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-Node: Debugging923897
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-Node: Finding The Bug931719
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-Ref: table-ieee-formats989286
-Node: MPFR features989853
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-Node: Errors accumulate996644
-Node: Strange values998100
-Ref: Strange values-Footnote-11000688
-Node: Getting Accuracy1000793
-Node: Try To Round1003503
-Node: Setting precision1004402
-Ref: table-predefined-precision-strings1005099
-Node: Setting the rounding mode1006929
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-Node: Extracting1234932
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-Node: Shell Startup Files1247683
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-Node: Building the Documentation1254038
-Node: Non-Unix Installation1255422
-Node: PC Installation1255882
-Node: PC Binary Installation1256720
-Node: PC Compiling1257155
-Node: PC Using1258272
-Node: Cygwin1261825
-Node: MSYS1263049
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-Node: VMS Installation Details1267342
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-Node: VMS GNV1273883
-Node: Bugs1274597
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-Node: Bug address1278445
-Node: Usenet1281833
-Node: Performance bugs1283022
-Node: Asking for help1285943
-Node: Maintainers1287910
-Node: Other Versions1289104
-Node: Installation summary1296956
-Node: Notes1298320
-Node: Compatibility Mode1299114
-Node: Additions1299896
-Node: Accessing The Source1300821
-Node: Adding Code1302258
-Node: New Ports1308477
-Node: Derived Files1312852
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-Node: Implementation Limitations1319917
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-Node: Basic Data Typing1336410
-Node: Glossary1339738
-Node: Copying1371625
-Node: GNU Free Documentation License1409168
-Node: Index1434288
+Node: Foreword345584
+Node: Foreword450026
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+Node: Extension Sample API Tests1181745
+Node: gawkextlib1182237
+Node: Extension summary1185155
+Node: Extension Exercises1188857
+Node: Language History1190099
+Node: V7/SVR3.11191755
+Node: SVR41193907
+Node: POSIX1195341
+Node: BTL1196722
+Node: POSIX/GNU1197451
+Node: Feature History1203229
+Node: Common Extensions1220404
+Node: Ranges and Locales1221687
+Ref: Ranges and Locales-Footnote-11226303
+Ref: Ranges and Locales-Footnote-21226330
+Ref: Ranges and Locales-Footnote-31226565
+Node: Contributors1226788
+Node: History summary1232785
+Node: Installation1234165
+Node: Gawk Distribution1235109
+Node: Getting1235593
+Node: Extracting1236556
+Node: Distribution contents1238194
+Node: Unix Installation1245337
+Node: Quick Installation1246141
+Node: Compiling with MPFR1248617
+Node: Shell Startup Files1249307
+Node: Additional Configuration Options1250396
+Node: Configuration Philosophy1252711
+Node: Compiling from Git1255107
+Node: Building the Documentation1255662
+Node: Non-Unix Installation1257046
+Node: PC Installation1257506
+Node: PC Binary Installation1258344
+Node: PC Compiling1258779
+Node: PC Using1259896
+Node: Cygwin1263449
+Node: MSYS1264673
+Node: VMS Installation1265275
+Node: VMS Compilation1265994
+Ref: VMS Compilation-Footnote-11267223
+Node: VMS Dynamic Extensions1267281
+Node: VMS Installation Details1268966
+Node: VMS Running1271228
+Node: VMS GNV1275507
+Node: Bugs1276221
+Node: Bug definition1277133
+Node: Bug address1280069
+Node: Usenet1283457
+Node: Performance bugs1284646
+Node: Asking for help1287567
+Node: Maintainers1289534
+Node: Other Versions1290728
+Node: Installation summary1298580
+Node: Notes1299944
+Node: Compatibility Mode1300738
+Node: Additions1301520
+Node: Accessing The Source1302445
+Node: Adding Code1303882
+Node: New Ports1310101
+Node: Derived Files1314476
+Ref: Derived Files-Footnote-11320136
+Ref: Derived Files-Footnote-21320171
+Ref: Derived Files-Footnote-31320769
+Node: Future Extensions1320883
+Node: Implementation Limitations1321541
+Node: Extension Design1322751
+Node: Old Extension Problems1323895
+Ref: Old Extension Problems-Footnote-11325413
+Node: Extension New Mechanism Goals1325470
+Ref: Extension New Mechanism Goals-Footnote-11328834
+Node: Extension Other Design Decisions1329023
+Node: Extension Future Growth1331136
+Node: Notes summary1331742
+Node: Basic Concepts1332900
+Node: Basic High Level1333581
+Ref: figure-general-flow1333863
+Ref: figure-process-flow1334548
+Ref: Basic High Level-Footnote-11337849
+Node: Basic Data Typing1338034
+Node: Glossary1341362
+Node: Copying1373249
+Node: GNU Free Documentation License1410792
+Node: Index1435912
 
 End Tag Table
 
diff --git a/doc/gawk.texi b/doc/gawk.texi
index 70832fc1..ef83bb35 100644
--- a/doc/gawk.texi
+++ b/doc/gawk.texi
@@ -579,6 +579,7 @@ particular records in a file and perform operations upon them.
 * Fields with fixed data::              Field values with fixed-width data.
 * Splitting By Content::                Defining Fields By Content
 * More CSV::                            More on CSV files.
+* FS versus FPAT::                      A subtle difference.
 * Testing field creation::              Checking how @command{gawk} is
                                         splitting records.
 * Multiple Line::                       Reading multiline records.
@@ -8452,6 +8453,7 @@ four, and @code{$4} has the value @code{"ddd"}.
 
 @menu
 * More CSV::                    More on CSV files.
+* FS versus FPAT::              A subtle difference.
 @end menu
 
 @c O'Reilly doesn't like it as a note the first thing in the section.
@@ -8661,6 +8663,28 @@ a bed with a blanket that's not quite big enough. There's always a corner
 that isn't covered. We recommend, instead, that you use Manuel Collado's
 @uref{http://mcollado.z15.es/xgawk/, @code{CSVMODE} library for @command{gawk}}.
 
+@node FS versus FPAT
+@subsection @code{FS} Versus @code{FPAT}: A Subtle Difference
+
+As we discussed earlier, @code{FS} describes the data between fields (``what fields are not'')
+and @code{FPAT} describes the fields themselves (``what fields are'').
+This leads to a subtle difference in how fields are found when using regexps as the value
+for @code{FS} or @code{FPAT}.
+
+In order to distinguish one field from another, there must be a non-empty separator between
+each field.  This makes intuitive sense---otherwise one could not distinguish fields from
+separators.
+
+Thus, regular expression matching as done when splitting fields with @code{FS} is not
+allowed to match the null string; it must always match at least one character, in order
+to be able to proceed through the entire record.
+
+On the other hand, regular expression matching with @code{FPAT} can match the null
+string, and the non-matching intervening characters function as the separators.
+
+This same difference is reflected in how matching is done with the @code{split()}
+and @code{patsplit()} functions (@pxref{String Functions}).
+
 @node Testing field creation
 @section Checking How @command{gawk} Is Splitting Records
 
@@ -19392,6 +19416,16 @@ Nonalphabetic characters are left unchanged.  For example,
 @code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}.
 @end table
 
+At first glance, the @code{split()} and @code{patsplit()} functions appear to be
+mirror images of each other. But there are differences:
+
+@itemize @bullet
+@item @code{split()} treats its third argument like @code{FS}, with all the
+special rules involved for @code{FS}.
+
+@item Matching of null strings differs. This is discussed in @ref{FS versus FPAT}.
+@end itemize
+
 @cindex sidebar @subentry Matching the Null String
 @ifdocbook
 @docbook
diff --git a/doc/gawktexi.in b/doc/gawktexi.in
index 3b568e8a..4fd5975b 100644
--- a/doc/gawktexi.in
+++ b/doc/gawktexi.in
@@ -574,6 +574,7 @@ particular records in a file and perform operations upon them.
 * Fields with fixed data::              Field values with fixed-width data.
 * Splitting By Content::                Defining Fields By Content
 * More CSV::                            More on CSV files.
+* FS versus FPAT::                      A subtle difference.
 * Testing field creation::              Checking how @command{gawk} is
                                         splitting records.
 * Multiple Line::                       Reading multiline records.
@@ -7921,6 +7922,7 @@ four, and @code{$4} has the value @code{"ddd"}.
 
 @menu
 * More CSV::                    More on CSV files.
+* FS versus FPAT::              A subtle difference.
 @end menu
 
 @c O'Reilly doesn't like it as a note the first thing in the section.
@@ -8130,6 +8132,28 @@ a bed with a blanket that's not quite big enough. There's always a corner
 that isn't covered. We recommend, instead, that you use Manuel Collado's
 @uref{http://mcollado.z15.es/xgawk/, @code{CSVMODE} library for @command{gawk}}.
 
+@node FS versus FPAT
+@subsection @code{FS} Versus @code{FPAT}: A Subtle Difference
+
+As we discussed earlier, @code{FS} describes the data between fields (``what fields are not'')
+and @code{FPAT} describes the fields themselves (``what fields are'').
+This leads to a subtle difference in how fields are found when using regexps as the value
+for @code{FS} or @code{FPAT}.
+
+In order to distinguish one field from another, there must be a non-empty separator between
+each field.  This makes intuitive sense---otherwise one could not distinguish fields from
+separators.
+
+Thus, regular expression matching as done when splitting fields with @code{FS} is not
+allowed to match the null string; it must always match at least one character, in order
+to be able to proceed through the entire record.
+
+On the other hand, regular expression matching with @code{FPAT} can match the null
+string, and the non-matching intervening characters function as the separators.
+
+This same difference is reflected in how matching is done with the @code{split()}
+and @code{patsplit()} functions (@pxref{String Functions}).
+
 @node Testing field creation
 @section Checking How @command{gawk} Is Splitting Records
 
@@ -18533,6 +18557,16 @@ Nonalphabetic characters are left unchanged.  For example,
 @code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}.
 @end table
 
+At first glance, the @code{split()} and @code{patsplit()} functions appear to be
+mirror images of each other. But there are differences:
+
+@itemize @bullet
+@item @code{split()} treats its third argument like @code{FS}, with all the
+special rules involved for @code{FS}.
+
+@item Matching of null strings differs. This is discussed in @ref{FS versus FPAT}.
+@end itemize
+
 @sidebar Matching the Null String
 @cindex matching @subentry null strings
 @cindex null strings @subentry matching
diff --git a/msg.c b/msg.c
index 0dc3f718..2ec3dfa9 100644
--- a/msg.c
+++ b/msg.c
@@ -78,7 +78,7 @@ err(bool isfatal, const char *s, const char *emsg, va_list argp)
 	}
 
 #ifdef HAVE_MPFR
-	if (FNR_node && is_mpg_number(FNR_node->var_value)) {
+	if (FNR_node && FNR_node->var_value && is_mpg_number(FNR_node->var_value)) {
 		NODE *val;
 		val = mpg_update_var(FNR_node);
 		assert((val->flags & MPZN) != 0);
diff --git a/pc/ChangeLog b/pc/ChangeLog
index 62534fdf..8592deba 100644
--- a/pc/ChangeLog
+++ b/pc/ChangeLog
@@ -1,3 +1,7 @@
+2021-10-20         Arnold D. Robbins     <arnold@skeeve.com>
+
+	* Makefile.tst: Regenerated.
+
 2021-10-13         Arnold D. Robbins     <arnold@skeeve.com>
 
 	* Makefile.tst: Regenerated.
diff --git a/pc/Makefile.tst b/pc/Makefile.tst
index c4b98e49..09a061d5 100644
--- a/pc/Makefile.tst
+++ b/pc/Makefile.tst
@@ -3163,6 +3163,11 @@ profile9:
 	@AWKPATH="$(srcdir)" $(AWK) -f $@.awk  --pretty-print=_$@ >_$@ 2>&1 || echo EXIT CODE: $$? >>_$@
 	@-$(CMP) "$(srcdir)"/$@.ok _$@ && rm -f _$@
 
+profile16:
+	@echo $@
+	@AWKPATH="$(srcdir)" $(AWK) -f $@.awk  --pretty-print=_$@ >_$@ 2>&1 || echo EXIT CODE: $$? >>_$@
+	@-$(CMP) "$(srcdir)"/$@.ok _$@ && rm -f _$@
+
 regexsub:
 	@echo $@
 	@AWKPATH="$(srcdir)" $(AWK) -f $@.awk  >_$@ 2>&1 || echo EXIT CODE: $$? >>_$@