Difference between revisions of "C programming notes"

Revision as of 20:25, 12 April 2024

^ Overview

This page to hold notes and links to C programming resources and reference materials. A language standards document is available at https://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf.

A good tool to aid writing C code is the indexing utility known as `ctags`:

• https://kulkarniamit.github.io/whatwhyhow/howto/use-vim-ctags.html

^ Data Type and Limits

See the following C library headers for important data type definitions, such as the definition of `uint32_t` and CHAR_BIT.

• zephyr/lib/libc/minimal/include/stdint.h

• zephyr/lib/libc/minimal/include/limits.h

^ C Qualifiers and Type Casting

• https://www.microchip.com/forums/m633052.aspx

^ Serial Port Use in C

• https://tldp.org/HOWTO/Serial-Programming-HOWTO/
• https://tldp.org/HOWTO/Serial-Programming-HOWTO/x115.html

// Example 3.2 code from https://tldp.org/HOWTO/Serial-Programming-HOWTO/x115.html

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>

#include <stdlib.h>                //echo "z" >> /dev/ttyS1 to provide exit()

// https://man7.org/linux/man-pages/man3/bzero.3.html
#include <strings.h>               // to provide bzero()

// https://man7.org/linux/man-pages/man2/read.2.html
// https://linux.die.net/man/3/read
#include <unistd.h>                // to provide read()


    
#define BAUDRATE B38400
//#define MODEMDEVICE "/dev/ttyS1"
#define MODEMDEVICE "/dev/ttyUSB0"
#define _POSIX_SOURCE 1 /* POSIX compliant source */
#define FALSE 0
#define TRUE 1
    
volatile int STOP=FALSE; 
    
int main(void)
{
        int fd, c, res;
        struct termios oldtio,newtio;
        char buf[255];
    
    printf("Starting,\nopening serial device '%s' . . .\n", MODEMDEVICE);

        fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );  
        if (fd <0) {perror(MODEMDEVICE); exit(-1); }

        tcgetattr(fd,&oldtio); /* save current port settings */

        bzero(&newtio, sizeof(newtio));
//        newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
        newtio.c_cflag = BAUDRATE | CS8 | CLOCAL | CREAD;
        newtio.c_iflag = IGNPAR;
        newtio.c_oflag = 0;

        /* set input mode (non-canonical, no echo,...) */
        newtio.c_lflag = 0;

        newtio.c_cc[VTIME]    = 0;   /* inter-character timer unused */
        newtio.c_cc[VMIN]     = 5;   /* blocking read until 5 chars received */

        tcflush(fd, TCIFLUSH);
        tcsetattr(fd,TCSANOW,&newtio);


    printf("looping to read from serial port . . .\n");
        while (STOP==FALSE) {       /* loop for input */
          res = read(fd,buf,255);   /* returns after 5 chars have been input */
          buf[res]=0;               /* so we can printf... */
          printf(":%s:%d\n", buf, res);
          if (buf[0]=='z') STOP=TRUE;
        }
        tcsetattr(fd,TCSANOW,&oldtio);

    return 0;
}

^ Co-routines

Interesting article by Simon Tatham on co-routines and a couple ways of implementing them in C language; accessible good style of writing:

• https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html

Quote from the article:

<! -- not intended use for ul tag --> "Any coding standard which insists on syntactic clarity at the expense of algorithmic clarity should be rewritten."  - Simon Tatham

Floating point convert and notes on-line, Javascript converter:

• https://www.h-schmidt.net/FloatConverter/IEEE754.html

^ C Pointers and dereference syntax

This section the beginning of references, links and personal notes on some of the most difficult and important C language constructs, often used in schedulers and RTOS implementations:

• https://riptutorial.com/c/example/31818/typedef-for-function-pointers

Arrays of strings and arrays of pointers to strings:

• https://stackoverflow.com/questions/1088622/how-do-i-create-an-array-of-strings-in-c/1095006

^ Simple C examples

Program 1 - use of C comma operator, a binary operator:

#include <stdio.h>
#include <stdlib.h>

// Compile with:
//
//    $ gcc -Wall main.c
//
// As of 2022-06-09 Builds with 'gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0'

int main(int argc, char *argv[])
{
    int z = 3;

    int y[(z++, (z + 1))];

    y[0] = 1;

    if ( z == y[0] ) { } 

#define SIZE_OF_160_BYTES (160)
    char lbuf[SIZE_OF_160_BYTES] = { 0 };
    snprintf(lbuf, SIZE_OF_160_BYTES, "2022-06-09 main.c test of C comma operator\narray y[] has size of %lu\n",
      sizeof(y));
    printf("%s", lbuf);

    return 0;
}

^ C Macros

This section likely to grow as C macros have a development history somewhat separate from the development of C language itself, and extensive use being both powerful and prone to pitfalls.

Articles on the use of C language 'do while(0)' construct in macros. The do while(0) construct is the only C syntax which expands correctly from C macros regardless of the use of curly braces and semicolons around the given macro.

• https://hownot2code.wordpress.com/2016/12/05/do-while-0-in-macros/

A useful macro pair, and this must be a macro pair, is the following set of defines. These defines will accept a string and put double quotes around it:

  #define MAKE_STRING_QUOTED(string) #string
  #define WRAPPER_NEEDED_TO_QUOTE_STRING(token) MAKE_STRING_QUOTED(string)

One Guy Rutenberg explains this C pre-processor pattern:

• https://www.guyrutenberg.com/2008/12/20/expanding-macros-into-string-constants-in-c/

This C macro pattern is explained a number of other places as well.

The `__fallthrough()` macro appears to be a C++ macro. It may be found among Zephyr RTOS 3.4.0 source tree contents. An article on the concept of "fall through" in C switch statements:

• https://www.quora.com/What-is-the-fall-through-behavior-in-C-programming-languages-switch-case-statements

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^ References

• https://en.wikipedia.org/wiki/Duff%27s_device

• https://en.wikipedia.org/wiki/Comma_operator