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Homework / Exam

• HW7 due class 25

• Exam 3 - class 26– Open Book, Open Notes– Covers up through end of K&R 7– and Appendix B Standard Library– Plus UNIX Shells / Processes / Shell Scripts

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Line Input and Output, K&R 7.7

• The standard C library equivalents to getline and putline: fgets and fputs

• Function fgets like getline() from a filechar *fgets(char *line, int maxline, FILE *fp);

• Reads the next input line (including '\n' at the end) from file fp into the char array line– at most maxline-1 chars will be read– then a terminal '\0' will be added.

• Returns ptr to line or NULL (means EOF)

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Line Input and Output

/* sample code to understand fgets – similar to getline */ char *fgets(char *s, int n, FILE *iop){register int c;register char *cs;cs = s;while (--n >0 && (c = getc(iop)) != EOF))

if ((*cs++ =c) == '\n')break;

*cs = '\0''return (c == EOF && cs == s) ? NULL : s; }

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Line Input and Output

• Function fputs writes line to fpint fputs(char *line, FILE *fp);

• It returns EOF if error occurs (disk fills up?) otherwise it returns zero

• Can use perror to print out exact error cause (to stderr, not user screen)

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Miscellaneous Functions, K&R 7.8

• Look at pg. 251-253 where they are covered

• 7.8.1 String Operations (string.h)– Used many of these already– Examples:

size_t strlen(const char *s)

int strcmp(const char *s, const char *t)

char *strcpy(char *s, const char *t)

char *strncpy(char *s, const char *t, size_t n)

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Miscellaneous functions

• 7.8.2 Character Processing Functions (ctype.h)– Used at least one of these already– Examples:

int isalnum(int c)int isalpha (int c)int isdigit (int c)int isxdigit (int c)int isspace (int c)int tolower (int c)int toupper (int c)

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Miscellaneous functions

• 7.8.3 Ungetc (stdio.h)– Had something like this in homework, ungetch() – Only have guarantee that you can push one char

back, but that is usually enough in practice– Example:

int c = getc (stdin) /* expecting an alpha character */

if (!isalpha(c))

ungetc (c, stdin) /* if not alpha - return it */

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Miscellaneous functions• 7.8 4 Command Execution (stdlib.h)

– The function system() See pg 253int system(const char *s);

– The string s contains a UNIX system commandsystem(“pwd”);system(“ls >fname”);system(“prog”);

– Return value depends on the command executed– Learn that from man page for the command

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Miscellaneous functions– In program compiled as prog, determine values for

a and b through argc and argv[ ]

int a, b; char command[MAXCMD];

…

sprintf(command, "prog %d %d > prog.out", a, b);

system(command);– System call doesn't return data from the command,

but by writing > prog.out it creates an output file– Calling program can then open/read “prog.out” file

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Miscellaneous Functions• 7.8.5 Storage Management (stdlib.h)• malloc() andfree()

– Review: Why is this incorrect?for (p = head; p != NULL; p = p->next)

free(p);– Review: Why must it be written like this?

for (p = head; p != NULL; p = q) { q = p->next; free(p);

}

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Miscellaneous Functions

• 7.8.6 Math functions (math.h)– Need to use #include <math.h> in source code– And use math library flag in execution of gcc:

gcc -lm source.c

– See list in appendix, mostly familiar!– Examples:

double pow (double x, double y) /* Not “x**y” */

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Miscellaneous Functions

• 7.8.7 Random number functions (stdlib.h)– Two functions:

rand (void)

srand (unsigned int seed) /* default value is 1 */

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qsort

• Library function qsort prototype (stdlib.h):void qsort(void *base, size_t n, size_t size, int (*cmp) (const void *, const void *));

• It sorts an array of data using quick sort algorithmbase a pointer to the table (an array of unknown type data)n the number of elements in the tablesize the size of each element

• What’s the last argument?– A pointer to a compare function for specific data type

• NOT the same function as qsort in K&R, pg 120

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bsearch• Library function bsearch prototype (stdlib.h):

void *bsearch(const void *key, const void *base,size_t n, size_t size,int (*cmp) (const void *, const void *));

• It searches for element containing key in an array of data of unknown that is already sorted in ascending order

• Returns:– pointer to element (if found) or – NULL (if not found)

• Last argument is a pointer to the compare function• Should use same compare function as was used for sorting

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qsort and bsearch

• qsort and bsearch don’t understand the data type for the elements of the table that it sorts or searches

• How can we tell that?– key (for bsearch only) is a type void *

– base (the pointer to the table for both) is a type void *

– size (the size of each element for both) is provided

– Last argument is a pointer to the correct compare function with a cast of the argument list variables to void *

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Example: qsort and bsearch

/* compare function for integers lowest to highest, we pass a pointer to this function to qsort/bsearch */

int intcompare(int *i, int *j)

{

return (*i - *j);

}

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Example: qsort and bsearch

main ()

{

int i, *ip, *kp, a [10] = {8, 2, 9, 6, 5, 1, 3, 7, 4, 0};

qsort ( (void *) a, sizeof a/sizeof (int), sizeof(int),

(int (*) (void *, void *)) intcompare);

for (i = 0; i < 10; i++)

printf(“%d, ”, a[i]);

/* prints “0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ” */

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Example: qsort and bsearch

kp = &a[3]; ip = (int *) bsearch ( (void *) kp, (void *) a,

sizeof a/sizeof (int), sizeof(int), (int (*) (void *, void *)) intcompare);if (ip != NULL) /* ip will be &a[3] */

printf(“found %d\n”, *ip); /* prints “found 3” */

}

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UNIX Time Representation

• UNIX time is kept as a signed int representing the number of seconds from Jan 1, 1970 UTC

• That date is referred to as the UNIX epoch– Earliest representable date: 12/13/1901– Latest representable date: 01/18/2038

• UNIX will have a Y2.038K problem!

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UNIX Time Representation

clock_t /* elapsed processor time */time_t /* calendar time */struct tm { /* calendar time components */

int tm_sec;int tm_min;int tm_hour;int tm_mday;int tm_mon;int tm_year;. . .

};

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UNIX Time Representation

• Time functions (time.h)clock_t clock(void) /* returns processor time */time_t time (time_t *tp) /* returns current time */double diff_time(time_t time2, time_t time1)

/* returns time difference in seconds */struct tm *gmtime(const time_t *tp)

/* returns *tp as UTC struct tm */struct tm *localtime(const time_t *tp)

/* returns *tp as local time struct tm */char *asctime(const struct tm *tp)

/* returns a string representing *tp, e.g.

Sun Jan 3 13:08:42 1988\n\0 */