I recently started using Arduino so I still have to adapt and find the differences between C/C++ and the Arduino language.
So I have a question for you.
When I see someone using a C-style string in Arduino (char *str), they always initialize it like this (and never free it) :
char *str = "Hello World";
In pure C, I would have done something like this:
int my_strlen(char const *str)
{
int i = 0;
while (str[i]) {
i++;
}
return (i);
}
char *my_strcpy(char *dest, char const *src)
{
char *it = dest;
while (*src != 0) {
*it = *src;
it++;
src++;
}
return (dest);
}
char *my_strdup(char const *s)
{
char *result = NULL;
int length = my_strlen(s);
result = my_malloc(sizeof(char const) * (length + 1));
if (result == NULL) {
return (NULL);
}
my_strcpy(result, s);
return (result);
}
and then initialize it like this:
char *str = my_strdup("Hello World");
my_free(str);
So here is my question, on C-style Arduino strings, is malloc optional or these people just got it wrong?
Thank you for your answers.
In C++ it's better to use new[]/delete[] and not mix it with malloc/free.
In the Arduino there is also String class, that hides those allocations from you.
However using dynamic memory allocations in such restrained platform has its pitfalls, like heap fragmentation (mainly because String overloads + operator, so everyone is overusing it like: Serial.println(String{"something : "} + a + b + c + d + ......) and then wonders about mysterious crashes.
More about it on Majenko's blog: The Evils of Arduino String class (Majenko has highest reputation on the arduino stackexchange)
Basically with the String class your strdup code would be simple as this:
String str{"Hello World"};
String copyOfStr = str;
I basically wrote a code in which I take two command line arguments one being the type of file that I want to search in my directory and they other being the amount I want(which is not implemented yet, but I can fix that)
The code is like so:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#define sizeFileName 500
#define filesMax 5000
int cmpfunc( const void *a, const void *b) {
return *(char*)a + *(char*)b;
}
int main( int argc, char ** argv) {
FILE * fp = popen( "find . -type f", "r");
char * type = argv[1];
char * extension = ".";
char* tExtension;
tExtension = malloc(strlen(type)+1+4);
strcpy(tExtension, extension);
strcat(tExtension, type);
// printf("%s\n",tExtension);
int amount = atoi(argv[2]);
//printf("%d\n",amount);
char buff[sizeFileName];
int nFiles = 0;
char * files[filesMax];
while(fgets(buff,sizeFileName,fp)) {
int leng = strlen(buff) - 1;
if (strncmp(buff + leng - 4, tExtension, 4) == 0){
files[nFiles] = strndup(buff,leng);
//printf("\t%s\n", files[nFiles]);
nFiles ++;
}
}
fclose(fp);
printf("Found %d files\n", nFiles);
long long totalBytes = 0;
struct stat st;
// sorting based on byte size from greatest to least
qsort(files, (size_t) strlen(files), (size_t) sizeof(char), cmpfunc);
for(int i = 0;i< nFiles; i ++) {
if(0!= stat(files[i],&st)){
perror("stat failed:");
exit(-1);
}
totalBytes += st.st_size;
printf("%s : %ld\n",files[i],st.st_size);
}
printf("Total size: %lld\n", totalBytes);
// clean up
for(int i = 0; i < nFiles ; i ++ ) {
free(files[i]);
}
return 0;
}
So far I have every section set up properly, upon running the code say $./find ini 5, it would print out all the ini files followed by their byte size(it's currently ignore the 5). However, for the qsort(), I'm not exactly sure how I would sort the contents of char * files as while it holds the pathnames, I had to use stat to get the byte sizes, how would I print out a sorted version of my print statements featuring the first statement being the most bytes and finishes at the least bytes?
If we suppose your input is valid, your question could be simplified with:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define filesMax 5000
int cmpfunc(const void const *a, const void *b) { return *(char *)a + *(char *)b; }
int main(void) {
int nFiles = 4;
char *files[filesMax] = {"amazing", "hello", "this is a file", "I'm a bad file"};
qsort(files, strlen(files), sizeof(char), cmpfunc);
for (int i = 0; i < nFiles;; i++) {
printf("%s\n", files[i]);
}
}
If you compile with warning that give you:
source_file.c:11:23: warning: incompatible pointer types passing 'char *[5000]' to parameter of type 'const char *' [-Wincompatible-pointer-types]
qsort(files, strlen(files), sizeof(char), cmpfunc);
^~~~~
qsort() expect the size of your array (or in your case a subsize) and it's also expect the size of one element of your array. In both you wrongly give it to it. Also, your compare function doesn't compare anything, you are currently adding the first bytes of both pointer of char, that doesn't make a lot of sense.
To fix your code you must write:
qsort(files, nFiles, sizeof *files, &cmpfunc);
and also fix your compare function:
int cmpfunc_aux(char * const *a, char * const *b) { return strcmp(*a, *b); }
int cmpfunc(void const *a, void const *b) { return cmpfunc_aux(a, b); }
also size should be of type size_t:
size_t nFiles = 0;
Don't forget that all informations about how to use a function are write in their doc.
how would I print out a sorted version of my print statements featuring the first statement being the most bytes and finishes at the least bytes?
Your code don't show any clue that your are trying to do that, you are currently storing name file and only that. How do you expect sort your file with an information you didn't acquired ?
However, that simple create a struct that contain both file name and size, acquire information needed to sort it and sort it:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <inttypes.h>
struct file {
off_t size;
char *name;
};
int cmpfunc_aux(struct file const *a, struct file const *b) {
if (a->size > b->size) {
return -1;
} else if (a->size < b->size) {
return 1;
} else {
return 0;
}
}
int cmpfunc(void const *a, void const *b) { return cmpfunc_aux(a, b); }
#define filesMax 5000
int main(void) {
size_t nFiles = 4;
struct file files[filesMax] = {{42, "amazing"},
{21, "hello"},
{168, "this is a file"},
{84, "I'm a bad file"}};
qsort(files, nFiles, sizeof *files, &cmpfunc);
for (size_t i = 0; i < nFiles; i++) {
printf("%s, %" PRId64 "\n", files[i].name, (intmax_t)files[i].size);
}
}
The function cmpfunc() provided adds the first character of each string, and that's not a proper comparison function (it should give a opposite sign value when you switch the parameters, e.g. if "a" and "b" are the strings to compare, it adds the first two characters of both strings, giving 97+98 == 195, which is positive on unsigned chars, then calling with "b" and "a" should give a negative number (and it again gives you 98 + 97 == 195), more on, it always gives the same result ---even with signed chars--- so it cannot be used as a sorting comparator)
As you are comparing strings, why not to use the standard library function strcmp(3) which is a valid comparison function? It gives a negative number if first string is less lexicographically than the second, 0 if both are equal, and positive if first is greater lexicographically than the second.
if your function has to check (and sort) by the lenght of the filenames, then you can define it as:
int cmpfunc(char *a, char *b) /* yes, you can define parameters as char * */
{
return strlen(a) - strlen(b);
}
or, first based on file length, then lexicographically:
int cmpfunc(char *a, char *b)
{
int la = strlen(a), lb = strlen(b);
if (la != lb) return la - lb;
/* la == lb, so we must check lexicographycally */
return strcmp(a, b);
}
Now, to continue helping you, I need to know why do you need to sort anything, as you say that you want to search a directory for a file, where does the sorting take place in the problem?
In the below code scanf() is working for getting the name from the user but fgets() is not working pls someone help me to understand why it's not working
#include <stdio.h>
#include <stdlib.h>
typedef struct university{
int roll_no;
char name[16];
}uni;
int main()
{
uni *ptr[5],soome;char i,j=0;
for(i=0;i<5;i++)
{
ptr[i]=(uni*)calloc(1,20);
if(ptr[i]==NULL)
{
printf("memory allocation failure");
}
printf("enter the roll no and name \n");
printf("ur going to enter at the address%u \n",ptr[i]);
scanf("%d",&ptr[i]->roll_no);
//scanf("%s",&ptr[i]->name);
fgets(&ptr[i]->name,16,stdin);
}
while(*(ptr+j))
{
printf("%d %s\n",ptr[j]->roll_no,ptr[j]->name);
j++;
}
return 0;
}
First of all, fgets(char *s, int n, FILE *stream) takes three argument: a pointer s to the beginning of a character array, a count n, and an input stream.
In the original application you used the address operator & to get the pointer not to the first element of the name[16] array, but to something else (to use the address operator, you should have referenced the first char in the array: name[0]).
You use a lot of magic numbers in your application (e.g. 20 as the size of the uni struct). In my sample I'm using sizeof as much as possible.
Given that you use calloc, I've used the fact that the first parameter is the number of elements of size equal to the second parameter to preallocate all the five uni struct at once.
Final result is:
#include <stdio.h>
#include <stdlib.h>
#define NUM_ITEMS (5)
#define NAME_LENGTH (16)
typedef struct university{
int roll_no;
char name[NAME_LENGTH];
} uni;
int main()
{
uni *ptr;
int i;
ptr = (uni*)calloc(NUM_ITEMS, sizeof(uni));
if(NULL == ptr) {
printf("memory allocation failure");
return -1;
}
for(i=0; i<NUM_ITEMS; i++) {
printf("enter the roll no and name \n");
printf("You're going to enter at the address: 0x%X \n",(unsigned int)&ptr[i]);
scanf("%d",&ptr[i].roll_no);
fgets(ptr[i].name, NAME_LENGTH, stdin);
}
for(i=0; i<NUM_ITEMS; i++) {
printf("%d - %s",ptr[i].roll_no,ptr[i].name);
}
free(ptr);
return 0;
}
Note: I've added a call to free(ptr); to free the memory allocated by calloc at the end of the application and a different return code if it's not possible to allocate the memory.
I'm trying to copy char array word to char pointer s[1].c, an then another word to char pointer s[2].c but when i'm trying to do that , the second word appears to be copied in all two pointers . How can i fix that ? I don't want to use strings .
struct Stud {
char *c;
} s[100];
char word[32];
int main()
{
strcpy(word,"one");
s[1].c=word;
word={0};
strcpy(word,"two");
s[2].c=word;
cout<<s[1].c<<" "<<s[2].c;
return 0;
}
In your code you are setting s[1].c = word; which means you are setting s[1].c to the address of word. Then you set s[2].c = word; which is the same exact memory location. (With c strings, (char *)s1 = (char *)2 does not do a string copy as you might expect. It just assigns one pointer to another).
With strdup you allocate a new memory block and then copy the string into the allocated space.
Here's your code modified.
struct Stud
{
char *c;
} s[100];
int main()
{
char word[32];
strcpy(word, "one");
s[0].c = strdup(word); // In C/C++ the first array index is 0
strcpy(word, "two");
s[1].c = strdup(word);
// Should check to make sure s[0].c and s[1].c are not NULL....
cout << s[0].c << " " <<s [1].c;
free(s[0].c);
free(s[1].c);
return 0;
}
This code:
#include <stdlib.h>
#include <stdio.h>
int j_btree_create (int fn_initial_nodes);
typedef struct {
int depth;
int value;
void *item;
void *left_pointer;
void *right_pointer;
} j_btree_node_int;
typedef struct {
int nodes;
int available_nodes;
int btree_extension;
} j_btree_descriptor_int;
int j_btree_create (int fn_initial_nodes) {
int *free_btree_node;
int loop_counter;
j_btree_descriptor_int *btree_start;
btree_start = (j_btree_descriptor_int *) malloc (((sizeof(j_btree_node_int) + sizeof(free_btree_node)) * fn_initial_nodes) + sizeof(j_btree_descriptor_int));
printf ("btree_start: " . btree_start);
/* *btree_start.nodes = fn_initial_nodes;
*btree_start.available_nodes = fn_initial_nodes;
*btree_start.extension = NULL; */
for (loop_counter = 0; loop_counter < fn_initial_nodes; loop_counter++) {
printf ("loop_test:" . loop_counter);
}
}
Produces this error:
/home/jamie/aws/btree_int.c||In function ‘j_btree_create’:|
/home/jamie/aws/btree_int.c|28|error: request for member ‘btree_start’ in something not a structure or union|
/home/jamie/aws/btree_int.c|33|error: request for member ‘loop_counter’ in something not a structure or union|
||=== Build finished: 2 errors, 0 warnings ===|
When compiled with CodeBlocks. I have not managed to find an exact answer to my problem (I have looked), does anyone know roughly what I am doing wrong? Probably more than one thing given I am fairly new to C.
printf ("btree_start: " . btree_start);
This is not how the things are done in c. There's no . concatenation operator and you do not concatenate strings (pointers to characters) and pointers to structures. If you want to print out the pointer, it's
printf("btree_start: %p\n",btree_start);
For the loop counter it's
printf("loop_test: %d",loop_counter);