CODEWITHSUNDEEP
carrayspointers
John Boe
John Boe

Reputation: 3611

how to access unsigned char array from inside function in C?

I am trying to get access the buffer called buf from function readfile. When I print sizeof(buf) I see that buf has 4 bytes (pointer). On the other hand, if I paste the printf command on the readFiles, I can see that the size is 2916. Actually, I don't understand why it is not 729, but it is obvious I cannot access the buf inside the readfile which I need. So the question is; where is the problem and how to correct it?

void readfiles(FILES * files){
    unsigned char * buf [1*729];
    int skip_lines = 14;
    int shift = 0;
    char * filename = "file.txt";
    // printf("buf size %d", sizeof(buf));
    readfile(filename, skip_lines, buf, shift);
}
int readfile(char * name, int skip, unsigned char * buf, int shift ){
 // buf is (unsigned char *) on address 0x22dd18 ""
    printf("buf size %d", sizeof(buf));
}

Upvotes: 1

Views: 503

Answers (2)

Code-Apprentice
Code-Apprentice

Reputation: 83527

In readfile(),

printf("buf size %d", sizeof(buf));

This prints 4 because buf is a pointer, as you seem to understand. The same print statement in readfiles(). prints a larger number because there buf is an array.

As pm100 stated, you cannot get the size of an array from a pointer to that array. Additionally, you need to be very careful about accessing pointers. If they do not point at a valid memory address, then you will have a lot of bugs. These are difficult to track down because the problems will occur in other parts of your code than where the pointer errors are. I suggest you learn about malloc() and free() in order to dynamically allocate memory for your pointers.

Upvotes: 0

pm100
pm100

Reputation: 50190

if you pass an array as a pointer into a C function you cannot detect its length.

int readfile(char * name, int skip, unsigned char * buf, int shift ){
// nothing you do in here can tell how big buf is
}

if you need to know the length of buf you have to pass it in as a paramter

int readfile(char * name, int skip, unsigned char * buf,int blen, int shift ){
...
}

Upvotes: 5

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