How to use malloc for initialize a char array in C?

Question

i try to allocate a char array with malloc but the size is always "8" :

char *d = malloc(2356 << 1);
printf("size of *d: %d\n", sizeof(d));

output: size of *d: 8

char *d = malloc(2356 * sizeof(*d);
printf("size of *d: %d\n", sizeof(d));

output: size of *d: 8

char *d = malloc(2356 * sizeof(char));
printf("size of *d: %d\n", sizeof(d));

size of *d: 8

Please help me, what's is wrong with my code?

Thanks!

`d` is not a character array, it is a pointer to a character. When you allocate memory with malloc like this `d` is a pointer to the first item in that allocated array. — Nathan Wride, Oct 21 '20 at 08:05
I think there's no way you can find the size of memory allocated by malloc by C compilers. Although, there are some extensions that can tell you the size of the allocated array. More information here: https://stackoverflow.com/questions/1281686/determine-size-of-dynamically-allocated-memory-in-c — iwrestledthebeartwice, Oct 21 '20 at 08:14

score 1 · Answer 1 · answered Oct 21 '20 at 08:23

1

You cannot find the size of memory allocated by malloc using sizeof() method. But, instead, there are OS-specific functions that can be used to find the allocated size.

If you are in windows you can use _msize() function in malloc.h header file. It will return the size in bytes.

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

int main(){
    char *d = malloc(2356 << 1);
    printf("size of *d: %d\n", _msize(d));
    //size of *d: 4712

    char *e = malloc(2356 * sizeof(*e));
    printf("size of *d: %d\n", _msize(e));
    //size of *d: 2356

    char *f = malloc(2356 * sizeof(char));
    printf("size of *d: %d\n", _msize(f));
    //size of *d: 2356
    return 0;
}

You can find more info about other OS-specific functions in this thread: Determine size of dynamically allocated memory in C

answered Oct 21 '20 at 08:23

iwrestledthebeartwice

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Please don't recommend platform-specific tricks to new programmers. – n. 1.8e9-where's-my-share m. Oct 21 '20 at 08:30
@n.'pronouns'm. I agree with you but this would be the right choice for OP to get the job done. – iwrestledthebeartwice Oct 21 '20 at 08:32
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If I were OP's professor, they would get an F for this, which is quite different from getting the job done. – n. 1.8e9-where's-my-share m. Oct 21 '20 at 08:38
So, you want OP to define his own function to keep track of memory allocated? If that's the case it would be awesome to learn what's happening in the backend for OP. – iwrestledthebeartwice Oct 21 '20 at 08:42
1

Yes, keeping track of your own allocations is the proper practice. If I ever encounter anything like `_msize` it in a pull request, that pull request goes straight to the bin, no questions asked. Do you use `_msize` in your software? If so please tell us where you work. – n. 1.8e9-where's-my-share m. Oct 21 '20 at 08:51
@n.'pronouns'm. I'm an undergrad student. I was messing around with malloc a year ago and found these platform-specific functions. Thanks for enlightening me and pointing me on to the right direction. Have a good day :) – iwrestledthebeartwice Oct 21 '20 at 09:33
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Great, please do not use these functions in real code, ever. In real code we use alternative `malloc` implementations all the time. These functions will silently return wrong results and your code will fail in strange and unpredictable ways. – n. 1.8e9-where's-my-share m. Oct 21 '20 at 09:48

How to use malloc for initialize a char array in C?

1 Answers1