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I have this struct
typedef struct{
char **palavras;
}no;
and I want to allocate memory to this array of strings basically
and I can't do this, since it says it expects something before the '('
no *atual;
atual->(*palavras)=calloc(1,sizeof(char*));
You need to do it in several stages:
First, allocate memory to atual,
Next, allocate memory to palavras
Finally, allocate memory to elements of palavras
Assuming that you need to allocate 10 palavras, you can do it like this:
no *atual = malloc(sizeof(no));
atual->palavras = malloc(sizeof(char*)*10);
atual->palavras[0] = malloc(20);
...
You should access palavras by atual->palavras, e.g. atual->palavras = calloc(5, sizeof(char *)), and dereference the char ** by *atual->palavras. You may also allocate memory for the char * pointers by atual->palavras by, say, atual->palavras[0] = malloc(10 * sizeof(char)).
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I want to know if the following piece of code will produce a memory leak:
m = malloc(5);
m = NULL;
Yes, there is a memory leak. The 5 allocated bytes are no longer accessible as you don't have a pointer to them.
If you save the pointer, you can still use, and free, the resources
unsigned char *m = malloc(5);
if (m) {
unsigned char *p = m;
m = NULL; // can no longer access the memory through m
p[2] = 1; // but p is ok
free(p); // p is ok to free
} else {
fprintf(stderr, "Problem! malloc failed!\n");
exit(EXIT_FAILURE);
}
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char *ssid,num;
ssid = malloc(sizeof(char)*6);
memcpy(ssid,"123456",6);
struct stu{
uint8_t head;
char *name;
uint8_t end;
} stu1 = {0x55, ssid, 0xf0 };
//printf("%s",stu1.name);
//how to output the structure data to the serial port.
send_msg((char *)&stu1,sizeof(stu1));
free(ssid);
Since your struct contains a pointer (i.e. name), it makes no sense to send the whole struct to the serial port. A pointer is something local to the program that it doesn't make sense to print.
So you need to print the members one by one. Like:
send_msg(&stu1.head, sizeof stu1.head);
send_msg(stu1.name, 6);
send_msg(&stu1.end, sizeof stu1.end);
BTW....
Notice that name is not zero terminated. That means that name is not a string. I guess you want it to be a string. So you should do:
int len = strlen("123456");
ssid = malloc(len + 1); // notice + 1 for the zero termination
memcpy(ssid,"123456", len + 1); // or strcpy(ssid, "123456");
and then you can do
send_msg(stu1.name, strlen(stu1.name));
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When is it appropriate to use
void* space_to_use = malloc(size);
void* space_to_use = malloc(size);
// malloc always return void pointer that means it can be typecast to any type.
// before using void pointer it is necessary to typecast it into proper type.
// for example:-
// if size is 8 byte.It will allocate 8 byte of memory.
/*
void* space_to_use = malloc(size);
char * ptr = (char*)space_to_use;
*/
// These two line can be combine in one statement.
char * ptr = (char*)malloc(size*sizeeof(char));
// NOTE:sizeof(char) is to make sure platform independent.
// Same for int if we want to store some integer.
int * ptr = (int*)malloc(size*sizeeof(int));
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Is someone possible to explain me what I am doing wrong here?
I am trying to malloc "multi-dimensional" dynamic array like this.
Thank you
enum { MAX_WORDS = 100, MAX_LENGHT = 20 };
char **words;
// it fails here "void * cannot be assigned... type of char**"
words = malloc(MAX_WORDS * sizeof(char*));
for (int i = 0; i < MAX_WORDS; i++) {
words[i] = malloc(MAX_LENGHT * sizeof(char));
}
words = malloc(MAX_WORDS * sizeof(char*));
Should read
words = (char**)malloc(MAX_WORDS * sizeof(char*));
Your compiler is mad because you are attempting to assign a void pointed to a char pointer, so you need to typecast it to char** to work properly.
EDIT: Apparently this is because I am using a C++ compiler. In C you should not be casting the result of malloc(). If you are using C++ you should switch to new and delete if possible.
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I have a code: array of structures that must be sorted. Program works, but:
I can't understand, if Mat is pointer, why not void sort(tArt *sMat), but void sort(tArt sMat[]). I'm really puzzled.
typedef struct{
char data[26];
}tArt;
...
int main(void)
{
FILE* fMat; fMat=fopen..........
tArt* Mat;
...
Mat=malloc(sizeof(tArt));
for(i=0;i<N;i++) fread(&Mat[i],sizeof(tArt),1,fMat);
fclose(fMat);
sort(Mat,N);
...
}
void sort(tArt sMat, int num) {...........}
My guess since we can not see the entire code, is that when you use malloc to dynamically allocate the array you forget to allocate the array for N 'objects'. In other words, I suspect your problem lies in the line
Mat = malloc(sizeof(tArt));
where it should be
Mat = malloc(sizeof(tArt) * N);
On the other hand, when you create explicitly your array with a declaration of the form
tArt Mat[N];
where N is defined somewhere earlier in the ellipses, everything is working as expected.
Hope this helps.
There is no difference between tArt *sMat and tArt sMat[] and tArt sMat[1234], the compiler treat them all as tArt *sMat and ignore the length information.