Programming

C/C++: A small tip for freeing dynamic memory

 in C / C++  tagged free by

Taking into account the behavior of the free() function, it is a good practice to set your pointer to NULL right after you free it.

By doing so, you can rest assured that in case you accidentally call free() more than one times on the same variable (with no reallocation or reassignment in between), then no bad side-effects will happen (besides any logical issues that your code might be dealing with).

You can include free() from malloc.h and it will have the following signature extern void free(void *__ptr);.

Description of operation:

Free a block allocated by malloc, realloc or calloc.
The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc(), or realloc().  Otherwise, if free(ptr) has  already been called before, undefined behavior occurs.  If ptr is NULL, no operation is performed.

Working examples:


#include <stdio.h>
#include <malloc.h>

int main()
{
  printf("Hello, World!\n");

  void * c = malloc (sizeof(char) * 10);

  free(c);
  c = NULL;
  free(c);

  return 0;
}



#include <iostream>

int main()
{
  std::cout << "Hello, World!" << std::endl;

  void * c = malloc (sizeof(char) * 10);

  free(c);
  c = NULL;
  free(c);

  return 0;
}

C: Code to time execution with accuracy greater than a second

 in C  tagged C++ / clock / rand / srand / time by

The following application computes the time needed for a process to finish using the method clock().
The result of the application is the time in seconds as a floating number (where 1.0 = 1 second).
It provides greater accuracy than seconds as the estimation is done using processor time used by the program.

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

int main()
{

    /* clock_t clock(void)
     The clock() function returns an approximation of processor time used by the program.
     The value returned is the CPU time used so far as a clock_t,
     to get the number of seconds used, divide by CLOCKS_PER_SEC.
     On error it returns -1. */
    const clock_t start = clock();

    /* svoid srand(unsigned int __seed)
     The srand() function sets its argument as the seed for a new sequence of pseudo-random
     integers to be returned by rand(). These sequences are repeatable by calling srand() with the
     same seed value.
     If no seed value is provided, the rand() function is automatically seeded with a value of 1. */
    /* time_t time(time_t *__timer)
     time() returns the time since the Epoch (00:00:00 UTC, January 1, 1970), measured in seconds.
     If the __timer variable is not NULL, the return value is also stored there. */
    srand(time(NULL));
    unsigned long i;
    for (i = 0; i < 10000000; i++)
    {
        /* int rand(void)
         The rand() function returns a pseudo-random integer in the range 0 to RAND_MAX inclusive. */
        rand();
    }
    const clock_t end = clock();

    /* ISO/IEC 9899:1999 7.23.1: Components of time
    The macro `CLOCKS_PER_SEC' is an expression with type `clock_t' that is
    the number per second of the value returned by the `clock' function. */
    /* CAE XSH, Issue 4, Version 2: <time.h>
    The value of CLOCKS_PER_SEC is required to be 1 million on all
    XSI-conformant systems. */
    const float seconds = (float) (end - start) / CLOCKS_PER_SEC;

    printf("Seconds elapsed %f\n", seconds);
    return 0;
}

Make building with cmake verbose 1

 in Programming  tagged cmake / make / verbose by

Ever wanted to get more information out of a build process controlled by CMake?
We sure did and this is how we did it:

Option 1 – Change your CMakeLists.txt files

As our first option, we present updating your CMakeLists.txt file to include the following configuration line:

set(CMAKE_VERBOSE_MAKEFILE ON)

This option by itself is enough to enable verbosity.
A caveat with this option is that the configuration is not passed on to other CMakeLists.txt files that are included to the build using the command add_subdirectory () from the master CMakeLists.txt file.
Thus, you need to copy the configuration file to each CMakeLists.txt file you want to be verbose.

Option 2 – Add the variable VERBOSE=1 to your make command

Assuming you are using a terminal and you are in the folder where you want to build your project.
After you execute the command

cmake $path_to_project_source;

execute your make command using the VERBOSE=1 variable as follows

make VERBOSE=1;

The caveat of this solution is that EVERYTHING becomes verbose, so you could have too many output data.

Option 3 – Add the variable -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON to your cmake command

Adding the option -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON to the cmake command, it will enable verbosity on all generated Makefiles permanently.
So, assuming you are in the folder where you want to make the build, execute the following to generate the Makefiles:

cmake -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON $path_to_project_source;

and then just issue make to perform the build with verbose output.
Please note, you cannot disable verbose output using make VERBOSE=0 after you enable it through cmake, you need to execute the cmake command again without the -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON option.

Bonus

To remove the ‘building’ and ‘linking’ lines from the output

e.g.

[ 10%] Building C object libs/segmentation/CMakeFiles/segments.dir/list_helpers.c.o
and
[ 30%] Linking C static library libsegments.a

add the option -DCMAKE_RULE_MESSAGES:BOOL=OFF to your cmake command to disable them.
e.g.

cmake -DCMAKE_RULE_MESSAGES:BOOL=OFF $path_to_project_source;

To remove the ‘Entering directory’ and ‘Leaving directory’ lines from the output

e.g.

make[2]: Leaving directory '/home/george/Projects/3rd Party/segments222bit/cmake-build-debug'
and
make[2]: Entering directory '/home/george/Projects/3rd Party/segments222bit/cmake-build-debug'

add the option --no-print-directory to your make command to disable them.
e.g.

make --no-print-directory;

Reading data from /dev/i2c-%d

 in C / GNU/Linux  tagged errno / fcntl / i2c / i2c_smbus_read_byte_data / i2c-dev.h / ioctl by

The following code will read a byte from position 0x10, of the register at 0x3f of the device /dev/i2c-2.

To compile this code, you need the helper library i2c-dev.h which can be found in the download package here: [download id=”3312″]

main.c

//Based on https://www.kernel.org/doc/Documentation/i2c/dev-interface

#include "linux/i2c-dev.h"
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>

int main() {
    const int adapter_nr = 2;
    char filename[20];
    snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
    const int file = open(filename, O_RDWR);
    if (file < 0) {
        printf("Oh dear, something went wrong with open()! %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    // The I2C address. Got it from `i2cdetect -y 2;`
    const int addr = 0x3f;

    if (ioctl(file, I2C_SLAVE, addr) < 0) {
        printf("Oh dear, something went wrong with ioctl()! %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    // Device register to access.
    const __u8 reg = 0x10;
    // Using SMBus commands
    const __s32 result = i2c_smbus_read_byte_data(file, reg);
    if (result < 0) {
         // ERROR HANDLING: i2c transaction failed
         printf("Oh dear, something went wrong with i2c_smbus_read_byte_data()>i2c_smbus_access()>ioctl()! %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    } else {
        // res contains the read word
        printf("0x%+02x\n", result);
    }

    close(file);

    return(EXIT_SUCCESS);
}

linux/i2c-dev.h

/*
    i2c-dev.h - i2c-bus driver, char device interface

    Copyright (C) 1995-97 Simon G. Vogl
    Copyright (C) 1998-99 Frodo Looijaard <[email protected]>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
    MA 02110-1301 USA.
*/

#ifndef _LINUX_I2C_DEV_H
#define _LINUX_I2C_DEV_H

#include <linux/types.h>
#include <sys/ioctl.h>
#include <stddef.h>


/* -- i2c.h -- */


/*
 * I2C Message - used for pure i2c transaction, also from /dev interface
 */
struct i2c_msg {
    __u16 addr;    /* slave address            */
    unsigned short flags;
#define I2C_M_TEN    0x10    /* we have a ten bit chip address    */
#define I2C_M_RD    0x01
#define I2C_M_NOSTART    0x4000
#define I2C_M_REV_DIR_ADDR    0x2000
#define I2C_M_IGNORE_NAK    0x1000
#define I2C_M_NO_RD_ACK        0x0800
    short len;        /* msg length                */
    char *buf;        /* pointer to msg data            */
};

/* To determine what functionality is present */

#define I2C_FUNC_I2C            0x00000001
#define I2C_FUNC_10BIT_ADDR        0x00000002
#define I2C_FUNC_PROTOCOL_MANGLING    0x00000004 /* I2C_M_{REV_DIR_ADDR,NOSTART,..} */
#define I2C_FUNC_SMBUS_PEC        0x00000008
#define I2C_FUNC_SMBUS_BLOCK_PROC_CALL    0x00008000 /* SMBus 2.0 */
#define I2C_FUNC_SMBUS_QUICK        0x00010000
#define I2C_FUNC_SMBUS_READ_BYTE    0x00020000
#define I2C_FUNC_SMBUS_WRITE_BYTE    0x00040000
#define I2C_FUNC_SMBUS_READ_BYTE_DATA    0x00080000
#define I2C_FUNC_SMBUS_WRITE_BYTE_DATA    0x00100000
#define I2C_FUNC_SMBUS_READ_WORD_DATA    0x00200000
#define I2C_FUNC_SMBUS_WRITE_WORD_DATA    0x00400000
#define I2C_FUNC_SMBUS_PROC_CALL    0x00800000
#define I2C_FUNC_SMBUS_READ_BLOCK_DATA    0x01000000
#define I2C_FUNC_SMBUS_WRITE_BLOCK_DATA 0x02000000
#define I2C_FUNC_SMBUS_READ_I2C_BLOCK    0x04000000 /* I2C-like block xfer  */
#define I2C_FUNC_SMBUS_WRITE_I2C_BLOCK    0x08000000 /* w/ 1-byte reg. addr. */

#define I2C_FUNC_SMBUS_BYTE (I2C_FUNC_SMBUS_READ_BYTE | \
                             I2C_FUNC_SMBUS_WRITE_BYTE)
#define I2C_FUNC_SMBUS_BYTE_DATA (I2C_FUNC_SMBUS_READ_BYTE_DATA | \
                                  I2C_FUNC_SMBUS_WRITE_BYTE_DATA)
#define I2C_FUNC_SMBUS_WORD_DATA (I2C_FUNC_SMBUS_READ_WORD_DATA | \
                                  I2C_FUNC_SMBUS_WRITE_WORD_DATA)
#define I2C_FUNC_SMBUS_BLOCK_DATA (I2C_FUNC_SMBUS_READ_BLOCK_DATA | \
                                   I2C_FUNC_SMBUS_WRITE_BLOCK_DATA)
#define I2C_FUNC_SMBUS_I2C_BLOCK (I2C_FUNC_SMBUS_READ_I2C_BLOCK | \
                                  I2C_FUNC_SMBUS_WRITE_I2C_BLOCK)

/* Old name, for compatibility */
#define I2C_FUNC_SMBUS_HWPEC_CALC    I2C_FUNC_SMBUS_PEC

/*
 * Data for SMBus Messages
 */
#define I2C_SMBUS_BLOCK_MAX    32    /* As specified in SMBus standard */
#define I2C_SMBUS_I2C_BLOCK_MAX    32    /* Not specified but we use same structure */
union i2c_smbus_data {
    __u8 byte;
    __u16 word;
    __u8 block[I2C_SMBUS_BLOCK_MAX + 2]; /* block[0] is used for length */
                                                /* and one more for PEC */
};

/* smbus_access read or write markers */
#define I2C_SMBUS_READ    1
#define I2C_SMBUS_WRITE    0

/* SMBus transaction types (size parameter in the above functions)
   Note: these no longer correspond to the (arbitrary) PIIX4 internal codes! */
#define I2C_SMBUS_QUICK            0
#define I2C_SMBUS_BYTE            1
#define I2C_SMBUS_BYTE_DATA        2
#define I2C_SMBUS_WORD_DATA        3
#define I2C_SMBUS_PROC_CALL        4
#define I2C_SMBUS_BLOCK_DATA        5
#define I2C_SMBUS_I2C_BLOCK_BROKEN  6
#define I2C_SMBUS_BLOCK_PROC_CALL   7        /* SMBus 2.0 */
#define I2C_SMBUS_I2C_BLOCK_DATA    8


/* /dev/i2c-X ioctl commands.  The ioctl's parameter is always an
 * unsigned long, except for:
 *    - I2C_FUNCS, takes pointer to an unsigned long
 *    - I2C_RDWR, takes pointer to struct i2c_rdwr_ioctl_data
 *    - I2C_SMBUS, takes pointer to struct i2c_smbus_ioctl_data
 */
#define I2C_RETRIES    0x0701    /* number of times a device address should
                   be polled when not acknowledging */
#define I2C_TIMEOUT    0x0702    /* set timeout in units of 10 ms */

/* NOTE: Slave address is 7 or 10 bits, but 10-bit addresses
 * are NOT supported! (due to code brokenness)
 */
#define I2C_SLAVE    0x0703    /* Use this slave address */
#define I2C_SLAVE_FORCE    0x0706    /* Use this slave address, even if it
                   is already in use by a driver! */
#define I2C_TENBIT    0x0704    /* 0 for 7 bit addrs, != 0 for 10 bit */

#define I2C_FUNCS    0x0705    /* Get the adapter functionality mask */

#define I2C_RDWR    0x0707    /* Combined R/W transfer (one STOP only) */

#define I2C_PEC        0x0708    /* != 0 to use PEC with SMBus */
#define I2C_SMBUS    0x0720    /* SMBus transfer */


/* This is the structure as used in the I2C_SMBUS ioctl call */
struct i2c_smbus_ioctl_data {
    __u8 read_write;
    __u8 command;
    __u32 size;
    union i2c_smbus_data *data;
};

/* This is the structure as used in the I2C_RDWR ioctl call */
struct i2c_rdwr_ioctl_data {
    struct i2c_msg *msgs;    /* pointers to i2c_msgs */
    __u32 nmsgs;            /* number of i2c_msgs */
};

#define  I2C_RDRW_IOCTL_MAX_MSGS    42


static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command,
                                     int size, union i2c_smbus_data *data)
{
    struct i2c_smbus_ioctl_data args;

    args.read_write = read_write;
    args.command = command;
    args.size = size;
    args.data = data;
    return ioctl(file,I2C_SMBUS,&args);
}


static inline __s32 i2c_smbus_write_quick(int file, __u8 value)
{
    return i2c_smbus_access(file,value,0,I2C_SMBUS_QUICK,NULL);
}

static inline __s32 i2c_smbus_read_byte(int file)
{
    union i2c_smbus_data data;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,0,I2C_SMBUS_BYTE,&data))
        return -1;
    else
        return 0x0FF & data.byte;
}

static inline __s32 i2c_smbus_write_byte(int file, __u8 value)
{
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,value,
                            I2C_SMBUS_BYTE,NULL);
}

static inline __s32 i2c_smbus_read_byte_data(int file, __u8 command)
{
    union i2c_smbus_data data;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         I2C_SMBUS_BYTE_DATA,&data))
        return -1;
    else
        return 0x0FF & data.byte;
}

static inline __s32 i2c_smbus_write_byte_data(int file, __u8 command,
                                              __u8 value)
{
    union i2c_smbus_data data;
    data.byte = value;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_BYTE_DATA, &data);
}

static inline __s32 i2c_smbus_read_word_data(int file, __u8 command)
{
    union i2c_smbus_data data;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         I2C_SMBUS_WORD_DATA,&data))
        return -1;
    else
        return 0x0FFFF & data.word;
}

static inline __s32 i2c_smbus_write_word_data(int file, __u8 command,
                                              __u16 value)
{
    union i2c_smbus_data data;
    data.word = value;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_WORD_DATA, &data);
}

static inline __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value)
{
    union i2c_smbus_data data;
    data.word = value;
    if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                         I2C_SMBUS_PROC_CALL,&data))
        return -1;
    else
        return 0x0FFFF & data.word;
}


/* Returns the number of read bytes */
static inline __s32 i2c_smbus_read_block_data(int file, __u8 command,
                                              __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         I2C_SMBUS_BLOCK_DATA,&data))
        return -1;
    else {
        for (i = 1; i <= data.block[0]; i++)
            values[i-1] = data.block[i];
        return data.block[0];
    }
}

static inline __s32 i2c_smbus_write_block_data(int file, __u8 command,
                                               __u8 length, const __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (length > 32)
        length = 32;
    for (i = 1; i <= length; i++)
        data.block[i] = values[i-1];
    data.block[0] = length;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_BLOCK_DATA, &data);
}

/* Returns the number of read bytes */
/* Until kernel 2.6.22, the length is hardcoded to 32 bytes. If you
   ask for less than 32 bytes, your code will only work with kernels
   2.6.23 and later. */
static inline __s32 i2c_smbus_read_i2c_block_data(int file, __u8 command,
                                                  __u8 length, __u8 *values)
{
    union i2c_smbus_data data;
    int i;

    if (length > 32)
        length = 32;
    data.block[0] = length;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         length == 32 ? I2C_SMBUS_I2C_BLOCK_BROKEN :
                          I2C_SMBUS_I2C_BLOCK_DATA,&data))
        return -1;
    else {
        for (i = 1; i <= data.block[0]; i++)
            values[i-1] = data.block[i];
        return data.block[0];
    }
}

static inline __s32 i2c_smbus_write_i2c_block_data(int file, __u8 command,
                                                   __u8 length,
                                                   const __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (length > 32)
        length = 32;
    for (i = 1; i <= length; i++)
        data.block[i] = values[i-1];
    data.block[0] = length;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_I2C_BLOCK_BROKEN, &data);
}

/* Returns the number of read bytes */
static inline __s32 i2c_smbus_block_process_call(int file, __u8 command,
                                                 __u8 length, __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (length > 32)
        length = 32;
    for (i = 1; i <= length; i++)
        data.block[i] = values[i-1];
    data.block[0] = length;
    if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                         I2C_SMBUS_BLOCK_PROC_CALL,&data))
        return -1;
    else {
        for (i = 1; i <= data.block[0]; i++)
            values[i-1] = data.block[i];
        return data.block[0];
    }
}


#endif /* _LINUX_I2C_DEV_H */