A few more pointer points Prints to stdout formatted Beware null - PDF document

int printf(char *fmt, a1 , a2 , …) A few more pointer points � Prints to stdout – formatted � Beware null & wayward pointers! (Learn from Binky) – Same as fprintf(stdout, char *fmt, a1, …) � Command line arguments – Variable length argument list after format – one for – int main(int argc, char *argv[]) {…} each % in format string (in order) – Equivalent argv declaration: char **argv � % [ - ][width][ . ][precision]character – Either way, argv[i] refers to i th argument – ‘-’ specifies left justification � Pointers to functions – function name is a pointer – width – maximum field width in characters – Can pass to another function as an argument – [.][precision] – for floating point nums only – void other ( int (*func)( func’s parameters ) ) – Character – specific for type to convert � See libqsort.c in ~cs60/demo03 for example � d, i, o, x, u – for integers � Complicated declarations – read from right to left � f, e, g – for floating point – Decipher using K&R’s dcl.c (in ~cs60/demo03) � s for strings, and c for chars Line input and output int scanf(char *fmt, a1 , a2 , …) � Formatted input from stdin � Note: K&R getline is non-standard – better to � For all except %c – skips white space use fgets from <stdio.h> : � Arguments corresponding to conversion char *fgets(char *line, int max, FILE *fp); characters must be pointers: – Reads at most max – 1 characters, including ‘\n’ int x; – The array, line , must be able to hold max chars char word[20]; scanf(“%d %s”, &x, word); � But do not use gets(…) – it’s dangerous – Note – word is already a pointer, so no & � fputs – alternative to fprintf to output lines: – Another note – word array must be large enough int fputs(char *line, FILE *fp); /* returns EOF if error */ � Also sscanf and fscanf – for input from a – Or just use puts(…) for stdout string or a file (i.e., like sprintf and fprintf ) Variable-length argument lists File input/output � #include <stdarg.h> � FILE *fp; /* declare a file pointer */ � va_list ap; � fp = fopen(“filename”, mode ); /* first: declare pointer to unnamed args */ /* associate a file with the pointer */ � va_start(ap, last named-arg ); – mode is char * – either “r” , “w” , or “a” /* aim pointer at first unnamed argument � Input or output using the file pointer: (note: must be at least one named argument) */ – getc(fp); /* returns next int from file */ � type value = va_arg(ap, type ) – putc(intValue, fp); /* outputs value to file */ /* get current unnamed argument, and increment */ � va_end(ap); – fscanf(fp, format, …); /* input from file */ /* must be called when done – before returning */ – fprintf(fp, format, …); /* output to file */ 1

Error handling basics C structures – some basics � Structures are user-defined types with multiple data fields � Do NOT print errors to stdout � e.g., define structure to hold a char and a double : – Print error messages to stderr instead: struct example{ /* type is 2 words: struct example */ char c; fprintf(stderr, “message”, args…); double d; � Often need to terminate execution due to errors }; /* the semicolon is mandatory */ – In main – return EXIT_FAILURE; /* or any non-zero */ � Create a structure, and declare and assign a pointer struct example e, *ep = &e; – In other functions – exit(EXIT_FAILURE); � Or, if won’t need to refer to type name again: � Sometimes want to check error status of file ( fp ) struct { /* can leave off the “tag” */ – General error – ferror(fp); /* returns 0 if no errors */ char c; double d; – End-of-file – feof(fp); /* returns non-0 if end of file */ } e, *ep; More structure basics typedef � Can precede any declaration with typedef � Access fields with the dot ‘ . ’ operator – Defines a name for the given type: – By using the structure’s name: e.d = 2.5; typedef struct example ExampleType; – Or the pointer: (*ep).c = ‘a’; /* parens needed */ ExampleType e, *ep; /* e, ep same as prior slide */ � Easier to use the arrow ‘ -> ’ operator for pointers � Can even use a defined type to define another: – ep->c = ‘a’; typedef ExampleType *ETPointer; � Can initialize all fields in one step: ETPointer ep; /* ep same as above */ – struct example e2 = { ‘c’, 97.14 }; � Note – can simplify code with macros too: � Note – size of structure >= sum of field sizes: #define C(p) (p)->c – sizeof e >= sizeof(char) + sizeof(double) /* preprocessor substitutes correct code */ C(ep) = ‘b’; Structures and functions Collections of structures � Arrays – an alternative to “parallel arrays” � Usually better to pass pointers – avoid copy costs – Mostly handle like all other array types – But gives function access to original fields � ExampleType array[ /*size*/ ], *p; � p = array + 2; /* p points at third struct */ � Note: const not guaranteed in C – See keyword counting programs, K&R p. 134 & 137 � Do not return pointers to local structure variables (today’s demo programs in ~cs60/demo04/) – In this case, accept the copying costs � Linked data structures – a.k.a. self-referential – But okay if dynamically allocate memory for structure typedef struct node { � Returning defined types aids readability: DataType data; struct node *next; /* a pointer to next node */ – ETPointer someFunction() is easy to read } ListNode; – struct example *someFunction() is not – Also see binary search tree program, K&R pp. 140-2 2

Unions More library functions � Can hold different data types/sizes (at different times) � Become familiar with K&R appendix B! � e.g., define union to hold an int or a double : � <string.h> – to deal with char * data union myValue{ � <ctype.h> – to handle individual char s int x; � <math.h> – trig functions, logs, many more double d; – Note: usually must link to libm.a – use -lm } u, *up; /* u is a union, up can point to one */ � Access x or d by u. or up-> just like structures � <stdlib.h> – various utilities � sizeof u is size of largest field in union – Inc. atoi , qsort , rand , malloc , exit , system , … – Equals sizeof(double) in this case � <assert.h> – one cool macro: assert(int) � Often store inside a structure, with a key to identify type � <time.h> , <limits.h> , … – check them out! – Otherwise might be no way to know which field to access 3