CS 241Section Week #1About Sections• Each week:– We’ll spend additional time on topics that the instructors feel should be reviewed.– We’ll prepare you for the upcoming homework or MP submissions.– We’ll provide extra review/guidance for upcoming exams.Topics This Section• Reading and Writing to the Console• Memory• Precedence• Casting• StringsConsole I/O• In lecture, you saw the printf() command.– printf(“%s: %d”, str, i);– printf(“%c%c%c”, c1, c2, c3);–…• In C I/O, you will provide a format string with a parameter list of values to populate the string with.Console I/O• In lecture, you saw the printf() command.– printf(“%s: %d”, str, i);– printf(“%c%c%c”, c1, c2, c3);–…• The embedded format tags tell C how to format the variables you provide.Console I/O• The printf() man page describes all the different types of specifies you can use.• Common specifies:– %c A single character – %d An integer value– %f A floating point value– %s A string– %p A pointerConsole I/O• Example #1:char *s = “the cat and the hat”;printf(“%s”, s);printf(“%c”, s);Console I/O• Example #1:char *s = “the cat and the hat”;printf(“%s”, s); the cat and the hatprintf(“%c”, s); tConsole I/O• Example #1:char *s = “the cat and the hat”;printf(“%s”, s); the cat and the hatprintf(“%c”, s); tWhy?Console I/O• Example #2:int i = 42;printf(“%d”, i);printf(“%c”, i);Console I/O• Example #2:int i = 42;printf(“%d”, i); 42printf(“%c”, i); *Console I/O• Example #2:int i = 42;printf(“%d”, i); 42printf(“%c”, i); *Why?Console I/O• Reading from the console uses nearly the same I/O format as writing to the console.– Key difference: All variables must be references.• printf(“%d”, i); Writes i to console.• scanf(“%d”, &i); Reads an int from consoleinto i.Arrayschar a[8]; /* array of bytes */char b[4][2]; /* 2-dimensional array */int c[2]; /* array of 32-bit words */..................MemoryMemory is just a big array of bytesPointers are indicies into memoryThe type of a pointer determines whether the memory it indexes is viewed as a char, an int, etc.void indicates the no-value type.void *p = ...;(char *) p(int *) p..................Referencing and dereferencingThe & operator creates a pointer to a variable (takes the address of the memory location holding the variable), while the *operator reads the data which a pointer references:int x;int *xptr = &x;/* xptr = 0xAF981DF8 */int y = *xptr;/* y = x */Process memory layoutprogram code(“text segment”)initialized static datauninitialized data (“bss”)dynamic memorygapstacksystem data: argv, envkernel memorylow memoryNULL: 0x00x0804A0080x080496B8&argv: 0xAFA47DD4argv: 0xAFA47E44Pointer arithmeticchar a[8]; /* array of bytes */char *p = a; /* p, a: 0xAF99EFDC */char *q = a+3; /* q: 0xAF99EFDF */..................Pointer arithmetic (2)char a[8]; /* array of bytes */char *q = a+3; /* q: 0xAF99EFDF */char *r = &a[3]; /* r: 0xAF99EFDF */..................Pointer arithmetic (3)int b[2]; /* array of 4-byte words */int *q = b+1; /* q: 0xAF99EFE0 */char *r = &b[1]; /* r: 0xAF99EFE0 */..................Memory• Three main categories of memory that we’ll concern ourselves with in CS 241:– Static Memory:• Memory that is declared with the ‘static’keyword.• Memory is only allocated once.• Memory is always of fixed size.• Memory is never freed.Memory• Three main categories of memory that we’ll concern ourselves with in CS 241:– Heap Allocated Memory:• Memory that is allocated with memory-allocating functions.– malloc(), calloc(), etc• Allocated only when the memory-allocating function is called.• Freed only when free() is called.Memory• Three main categories of memory that we’ll concern ourselves with in CS 241:– Stack Allocated Memory:• Memory that is allocated within the scope of a function.• Stores local variables and function parameters • Allocated when the function begins execution.• Freed when the function finishes execution.• The stack memory associated with a given function is referred to as a “stack frame”.Memory• Code Execution:• Memory:void foo(int myInt){int *x = (int *)malloc(sizeof(int));free(x);}Memory• Code Execution:• Memory:void foo(int myInt){int *x = (int *)malloc(sizeof(int));free(x);}myIntMemory• Code Execution:• Memory:void foo(int myInt){int *x = (int *)malloc(sizeof(int));free(x);}myInt*x4 bytesMemory• Code Execution:• Memory:void foo(int myInt){int *x = (int *)malloc(sizeof(int));free(x);}myInt*xMemory• Code Execution:• Memory:void foo(int myInt){int *x = (int *)malloc(sizeof(int));free(x);}Memory• Code Execution:… start with dog();• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z = 12;cat(s, z);}Memory• Code Execution:• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z = 12;cat(s, z);}Memory• Code Execution:• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z = 12;cat(s, z);}*s“my cat”Memory• Code Execution:• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z = 12;cat(s, z);}*s“my cat”z = 12Memory• Code Execution:• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z = 12;cat(s, z);}*s“my cat”z = 12*xi = 12cat()Stack Framedog()Stack FrameMemory• Code Execution:• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z = 12;cat(s, z);}*s“my cat”z = 12*xi = 12r = 4cat()Stack Framedog()Stack FrameStatic MemoryMemory• Code Execution:• Memory:char *cat(char *x, int i){static int r = 4;x[3] = 'h';char *result = (char *)malloc(20);sprintf(result, "%s x %d", x, i);return result; }void dog(){char s[] = "my cat";int z =
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