Setting a pointer value inside a function [duplicate]

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void pointer as argument

I'm trying to make a simple function in C, but I get an empty output and I can't figure out why:

int encrypt(unsigned char *message, char *key, unsigned char *buffered_message) { /* ... */ buffered_message = calloc(1, (blocks * block_size)); /* ... */ printf("Message: %s\n", buffered_message); return strlen(buffered_message); }

Inside the function, the message is printed out without problems. But when I try to use my function in main, something goes wrong.

int main() { /* ... */ unsigned char *encrypted; int len = encrypt(message, key, encrypted); if (len > 0) { printf("The encrypted message %s\n", encrypted); } return 0; }

See here. That answer applies verbatim to your buffered_message. Further duplicates: #1, #2, #3. — Kerrek SB
– Kerrek SB, Commented Sep 26, 2012 at 9:33
@Kerrek So do I get this right, my buffered_message exists only in the function encrypt and thats why I cant use it in main? Sorry if it's a stupid question, but how can I get the encrypted message. I tryed to return buffered_message in encrypt, but then I get: warning: return makes pointer from integer without a cast — user1656466
– user1656466, Commented Sep 26, 2012 at 9:46
You have to pass a pointer to the pointer and dereference it in the function. Like I explained in every one of my other answers. You need to change the argument type to unsigned char **. — Kerrek SB
– Kerrek SB, Commented Sep 26, 2012 at 9:47
I've taken the liberty to trim your question down to the important content. — Kerrek SB
– Kerrek SB, Commented Sep 26, 2012 at 9:52
Oh, by the way, strlen looks dodgy. The encrypted message may well contain null bytes. Since you're encrypting entire blocks anyway, there shouldn't be a need to communicate a string length, since the caller already knows the expected length. — Kerrek SB
– Kerrek SB, Commented Sep 26, 2012 at 10:01

fkl · Accepted Answer · 2012-09-26 10:13:56Z

1

You need to pass double pointer since you are allocating memory and copying to the pointer local to the function.

The following should work, though i haven't compiled.

int encrypt(unsigned char *message, char *key, unsigned char **buffered_message) { /* ... */ *buffered_message = calloc(1, (blocks * block_size)); /* ... */ printf("Message: %s\n", *buffered_message); return strlen(*buffered_message); }

From main

unsigned char *encrypted; int len = encrypt(message, key, &encrypted); printf("The encrypted message %s\n", encrypted);

edited Sep 26, 2012 at 10:13

answered Sep 26, 2012 at 10:05

fkl

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4 Comments

fkl Over a year ago

And yes, as already mentioned, using strlen is a bad idea on an encrypted message because strlen would return length upto first \0, which might be very common in an encrypted message.

Armen Tsirunyan Over a year ago

The main function is wrong. You should define unsigned char * encrypted and then pass &encrypted to the function. Otherwise you end up dereferencing an unintialized pointer

fkl Over a year ago

@ArmenTsirunyan. Yeah i only second thought that, and was editing answer on the way. Thanks for confirming. Edited the answer

user1656466 Over a year ago

Thanks everybody. I guess I have to read a litle more about pointers to pointers. But I'm very greatfull to all of you, for answering so fast.

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