Common C Pitfalls

C is a very powerful language - it is fast and close to the machine.  However, it is also very dangerous.  Ask C to do illegal things, it will dutifully perform them, causing the OS (technically, the shell) to gleefully inform you what a wonderful job it did following your instructions:

Segmentation fault

Below are some common pitfalls:

Buffer overflow

Probably the most common cause for a seg fault is some kind of buffer overflow. Code like this:

char line[10];
printf("enter a number:\n");
gets(line);

is just a time bomb waiting to go off. What if the user's input is more than 10 characters? Even worse, gets() doesn't even check how big your buffer is! That's likely why the compiler decided to generate this warning:

In function `printSum()':
the `gets' function is dangerous and should not be used.

Some common causes for buffer over-run are forgetting to allocate space for the null-terminator during string manipulations:

char * copy;
int len;
len=strlen(passedStr);
copy = (char *)malloc(len*sizeof(char));
strcpy(copy,passedStr); //corrupt 1 byte at the end

or off by 1 on array indexing:

int buf[10],i;
for(i=1;i<=10;i++)
    buf[i]=0; //arrays are 0 based. corrupts last 4 bytes (size of int)

Memory allocation on the stack

void b(char **p)
{
    char * str="print this string";
    *p = str;
}

int main(void)
{
    char * s;
    b(&s);
    s[0]='j'; //crash, since the memory for str is allocated on the stack, 
                //and the call to b has already returned, the memory pointed to by str is no longer valid.
    return 0;
}

Memory corruption bugs are about the nastiest types of bugs to try to track down, because the symptoms don't always show up right away. If you are lucky, the program will crash right away, and you can figure out where the corruption occurred and fix it. If you're unlucky, however, the program will continue running (because you corrupted data that does belong to your program, so no seg fault is generated...yet). At some later point, you may find that completely innocent looking statements start behaving strangely, but for the life of you couldn't tell why ("what the #@$%!! is going on??!!" is a common expression at times like these :)). So please, be very careful when manipulating memory, lest you want to spend hours and hours conversing with gdb.

Also check out this Top ten (or is that 20) list