C: pointers libraries structs

Question 1

what is static lifetime vs auto lifetime?

Answer 1

Variables declared outside of a function have storage class static. Lifetime begins when a program is loaded, ends when program ens.

Variables declared inside of a function by default have storage class auto. Dies when function call returns.

Variables declared inside of a function may be explicitly declared static. Such variables have local scope, but the global lifetime of static variables.

Question 2

Where in the structure of a program do auto variables live? Can you trust the values of initialized variables here?

Answer 2

The stack. Values of uninitialized variables are random.

Question 3

What data goes into the heap?

Answer 3

Long term malloc()’d data

Question 4

Where do uninitialized static variables live? Can you trust the values stored here by default?

Answer 4

BSS. Always initialized to 0.

Question 5

Where do initalized static variables live?

Answer 5

.data

Question 6

are C functions call by value or address?

Answer 6

value. bypass w/ pointers

Question 7

$ a.out 1 23 “four five”

what is argc, argv?

Answer 7

argc: 4 (includes program name)

argv: [1, 23, “four five”]

Question 8

Is this legal?

char *makeBig(char *s) {
s[0] = toupper(s[0]);
return s;
}
…
makeBig(“a cat”);
…

Answer 8

No. this code will likely fail because compilers typically place literal strings in a read-only area of the address space

Question 9

Why is strncpy safer than strcpy?

char *strncpy(char *dest, char *source, int num)

Answer 9

stops copying after num chars, and appends a /0
less likely to go beyond bounds

Question 10

is there a better way to declare this struct to save memory?

struct {
char x;
int y;
char z;
} s1;

Answer 10

Yes. If the chars are placed next to each other in memory, there probably wont be as much padding around the chars for alignment.

Question 11

typedef struct {
unsigned int version:2;
unsigned int p:1;
unsigned int cc:4;
unsigned int m:1;
unsigned int pt:7;
u_int16 seq;
u_int32 ts;
} rtp_hdr_t;

What is this an example of?

Answer 11

Bit fields. version, for example, is declared to be 2 bits long, and will only take up 1 byte of memory.

Question 12

union u_tag {
int ival;
float fval;
char *sval;
} u;

what is sizeof(u)? in terms of its attributes.

Answer 12

sizeof(float)

Question 13

where does getchar() read from?

Answer 13

stdin

Question 14

what is a buffer?

Answer 14

“buffer” typically refers to a memory area used for temporary storage. To declare a buffer, you can use an array. For example:

char buffer[256];

Question 15

In C, the fflush function is used to flush (clear) the output buffer. It ensures that any data that was buffered for output is actually written to the file or console. Here’s a concise example:

```c
fflush(stdout); // Flushes the standard output buffer.
~~~

This is particularly useful when you want to make sure that data is immediately written to a file or displayed on the screen instead of being held in a buffer.

Answer 15

the fflush function is used to flush (clear) the output buffer. It ensures that any data that was buffered for output is actually written to the file or console. Here’s a concise example:

fflush(stdout); // Flushes the standard output buffer.

Essentially, “save” progress, and get ready for whats next.