C programming, flaw in using BUFSIZ,malloc and memset

Question

I came across this question in C programming:

Question: what is the flaw in the code snippet below that fills a buffer with zeros? How can the problem be fixed?

char*buf;
buf=malloc(BUFSIZ);
memset(buf,0,BUFSIZ);

I have tried to find the flaw, but everything works fine for me. Please let me know if there is any flaw in the above C program.

`malloc` could fail. In that case, your program will set `0`s to uninitialized variable and leads to undefined behaviour. — P.P, Sep 12 '14 at 07:21
There are only two flaws: You not defining `BUFSIZ` and that `malloc` may fail and you don't check for that. — Some programmer dude, Sep 12 '14 at 07:22
_How can the problem be fixed?_... This can be fix by checking the return type of `buf`, it should not be `NULL`. — ani627, Sep 12 '14 at 07:31
See this Secure Coding book snapshot. same question http://books.google.com/books?id=dD60dU9Xvj4C&pg=PA77&lpg=PA77 — Arjun Sreedharan, Sep 12 '14 at 08:51

Ivan Ivanov · Answer 1 · 2014-09-12T08:41:24.603

//somewhere
const size_t BUFSIZE = 1000; //or it could be 0!
...
char *buf = NULL;
if (BUFSIZE > 0) {
    buf = (char*) malloc(BUFSIZ); //cast is not necessary on most compilers

    if (buf == NULL) {
        //some error processing
    } else {
        memset(buf, 0, BUFSIZ);
    }

    free(buf);
    buf = NULL;
}

set pointer to NULL
malloc positive number of bytes, because behaviour for 0 is implementation-specific
check it is not null after malloc
free memory. If it is NULL nothing happens
nullify pointer after free
cast malloc, some compilers will warn, not an error. Well, really, do not cast
maybe there are some standards like MISRA, so this code is absolutely invalid for that standard
and also this can be thread-unsafe

score 0 · Answer 2 · answered Sep 12 '14 at 08:29

0

First thing is that malloc returns a pointer of type 'void' so better to typecast it to (char *). Second thing "malloc" can fail, in that condition it will return "NULL" pointer, check for this condition. I am sure BUFSIZ is defined in code somewhere!

answered Sep 12 '14 at 08:29

Kapil

31
2

2

About your first point : It's not a good habit to cast malloc return values in C : http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc – Logar Sep 12 '14 at 08:38

C programming, flaw in using BUFSIZ,malloc and memset

2 Answers2