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David A's C Software

C Programming

All the code here is provided in C or Python source and is free software. Enjoy!

Printing PG&E Electricity Use (Pacific Gas and Electric) provides a way (from one's account on to download one's electrical usage in 15 minute increments over a date-range you choose.

It's hard to find the download button in the PGE web pages. Go to the pge page with your Usage values. Find the small green button labeled Green Button, following a graph of your usage. It is the download button. You can choose either csv form or xml form.

You can import the csv into a spreadsheet or you can just read it as text. The csv form will show 'Estimated value' on 15 minute periods where pge has not yet gotten your actual usage. Sometimes here we've seen it take several days for pge to clear up the Estimated values (the temporary guesses pge made were absurd).

I think the xml form is neat, but it has no indication of 'Estimated' records and it's unreadable unless one uses code to digest the xml. Here I present python code that does that (assuming you can run command-line applications, there is no graphical interface here).

An example of use:

python3 pge_electric_interval_data_2019-05-25_to_2019-06-25.xml

Here are the first few lines output from a pg&e xml file: Dates are presented in ISO-extended date form.

Reading pge usage data
Option: [-h] print usage per hour default: True
Option: [--day] print usage per day, turn off by hour
Option: [--detail] print usage pge 15 minute interval
Option: [--date=daychoice] where daychoice is yyyy-mm-dd
 as in -date=2019-06-23 which selects a single day to report
file  pge_electric_interval_data_2019-05-25_to_2019-06-25.xml
File opened ok
Hourly  kwh 0.000000
Hourly 2019-06-19 00 kwh 1.589000
Hourly 2019-06-19 01 kwh 1.610000
Hourly 2019-06-19 02 kwh 1.571000

The python3 source code is available and I have marked it as being in the public domain.

Download or read as

Testing Malloc Failure

"Code Testing Through Fault Injection" in ":login;" magazine (December, 2014. by Peter Gutmann offered a simple example by an unnamed friend: instrument malloc() so on call N it returns NULL. Try with N from 0 to some higher number. On any system using dynamically-loaded libc it is easy to do. Here we set it for current Linux libc. Run your chosen executable and see how it fares.

Here is the basic C source. Using a script (example below) link this into the executable you want to test.

/* */
/*  script below modifies this into a temporary fakemalloc.c */
static unsigned count;
extern void * __libc_malloc();

void *malloc(size_t len)
  /*  Perhaps the test should be count >= FAILCOUNT  ??? */
  if (count == FAILCOUNT) {
    return 0;
  return __libc_malloc(len);

Here we build the executable to be tested. The executable to test can be anything. Naturally you have some application in mind. Normally you would modify the link step of your executable creation . Here we assume you use make to build the executable under test. Lets say the executable is named mycode

# This is a version of a link line presumed to be in Makefile
# Fairly typical use of make.
# It is essential that fakemalloc.o is linked in before
# the standard libraries, but its exact location on the link
# line is normally not critical.
   # This line new, for test
   cc -g -c fakemalloc.c
   # This line adds fakemalloc.o
   cc -g $(CFLAGS) mycode.o fakemalloc.o -o mycode $(OBJS) $(LDFLAGS) $(LIBS)

So with that as background, we move to the example test script.


while [ $ct -lt  500 ]
  echo '===== START TEST'
  rm -f core
  # rm  $targ so the test executable gets rebuild.
  rm -f $targ
  sed -e "s/FAILCOUNT/$ct/"  < > fakemalloc.c
  echo TEST $ct
  # This is just so you can see the sed worked.
  grep if fakemalloc.c
  # Next line presumably does the compile of fakemalloc.c 
  # and the build of mycode.
  $targ  "place args here" > junk.x
  # to see the output on stdout.
  cat junk.x
  if [ -f core ]
    echo "CORE FILE EXISTS, test" > $ct
  rm -f core
  echo '===== END TEST'
  ct=`expr $ct + 1`

I hope you find this useful for your testing.


Important C Documents

Some key writings on C for C programmers (from Henry Spencer, Dennis Richie, and others) is at C programmers will enjoy this. Take a look.


POSIX Signals, C and C++

Occasionally folks are unaware of or misunderstand the rules on writing POSIX signal handlers in C and C++. So I've written a brief overview of POSIX signal handling pitfalls. Corrections and suggestions welcome.


Hex Dump Utility

When viewing the binary representation of a file I prefer to use a format I first saw in a utility provided by IBM in OS360 in 1964. So I wrote C code I call hxdump that prepares output in a similar format. Download the hxdump.c C source. To compile it cc -g hxdump.c -o hxdump should suffice. The source has always said there are no restrictions on its use.


Binary Patch Utility

Sometimes (rarely) one has a file one wants to patch directly, rather than use an editor. Years ago I wrote (and made available via ftp) a program I call binpatch. Download the binpatch.c C source. To compile it cc -g binpatch.c -o binpatch should suffice. The source has always said there are no restrictions on its use.


Palm Data Extractor

See the page describing a palm extractor in C. It is a way to get data out of a palm data file (palm provides no way to do this!).


Tsearch Search Example In C

A standard based search is quite useful, but tsearch has its peculiarities, so perhaps an example is of interest. See the page describing a tsearch example in C.


Tsearch Implementation Source in C

I wanted a set of search calls that I could use everywhere. Tsearch, for example, is widely but not universally available. And the functionality to completely and easily take down a tsearch tree is even less available. So I implemented several tree search algorithms and added a hash algorithm and placed the source to all of them in the libdwarf source (see dwarf.html in in the tsearch directory.

The implemented tree searches are of binary tree, binary tree with Eppinger delete, balanced tree, and red-black tree, The hash search uses the same interfaces. To avoid conflicts with your standard library, the function names are prefixed with dwarf_. The implemented functions (for every version) are dwarf_tsearch(), dwarf_tfind(),dwarf_tdelete(), dwarf_tdestroy(), dwarf_twalk(), and dwarf_tdump().


Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.