Working with the Unix OS

C Programming – Basics
struct tnode *talloc(void)
{
return (struct tnode *)malloc(sizeof(struct tnode));
}
char *strdup(char *s) /* make a duplicate of s */
{
char *p;
p = (char *)malloc(strlen(s)+1); /* +1 for ’\0’ */
if (p != NULL)
strcpy(p, s);
return p;
}
Unions
A variable that holds, at different times, objects of different types and sizes, i.e. different data in a single area of
storage.
union u_tag {
int ival;
float fval;
char *sval;
}u;
if (utype == INT)
printf("%d\n", u.ival);
else if (utype == FLOAT)
printf("%f\n", u.fval);
else if (utype == STRING)
printf("%s\n", u.sval);
else
printf(" bad type id in utype\n", utype);
A union is a structure in which all members have an offset zero from the base. Can only initialise value first
member.
Bit fields
When storage space is at a premium, it may be necessary to pack several objects into a single machine word.
The usual way this done us to define a set of "masks" corresponding to the relevant positions, as in
#define KEYWORD 01
#define EXTERNAL 02
#define STATIC 04
or
enum { KEYWORD = 01, EXTERNAL = 02; STATIC = 04 );
flags |= EXTERNAL | STATIC; /* turns bits on */
flags & = ~(EXTERNAL | STATIC); /* turns bits off */
if((flags &(EXTERNAL | STATIC)) == 0) /* both true */
As an alternative C offers the capability of directly defining and accessing fields within a word.
struct{ /* 3 one bit fields */
unsigned int is_keyword : 1;
unsigned int is_extern : 1;
unsigned int is_static : 1;
}flags;
flags.is extern = flags.is static = 1, /* bits on */
flags.is extern = flags.is static = 0; /* bits off */
if (flags.is extern == 0 && flags.is static == 0)
Table Lookup
Table lookup code is typically found in the symbol table management routines of a macro processor or a compiler.
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