error conventions

if(alloc_memory() != 0 )
        return -ENOMEM;
if (user_parameter_valid()!=0)
        return -EINVAL;

/* 
Some of the examples of the macros are 

#define	ENOTEMPTY	39	/* Directory not empty */
#define	ELOOP		40	/* Too many symbolic links encountered */
#define	EWOULDBLOCK	EAGAIN	/* Operation would block */
#define	ENOMSG		42	/* No message of desired type */
#define	EIDRM		43	/* Identifier removed */
#define	ECHRNG		44	/* Channel number out of range */
#define	EL2NSYNC	45	/* Level 2 not synchronized */
#define	EL3HLT		46	/* Level 3 halted */
#define	EL3RST		47	/* Level 3 reset */
#define	ELNRNG		48	/* Link number out of range */
#define	EUNATCH		49	/* Protocol driver not attached */

If you want more details about these Errors you can find it here 
/usr/include/asm-generic/errno.h 
/usr/include/asm-generic/errno-base.h 
*/