Weeks 9-10

1. Weeks 9-10 • IO • Standard IO (stdin, stdout) and Pipes • Formatted IO • File IO • System Calls • System interface to obtain services from the OS, include many subsystems • Memory • Scheduler • File/Storage System • Inter-process Communication and Network, etc • A popular subsystem: File system • File descriptors • Low level IO • File Management • Examples

2. Standard IO and Pipes • stdin, stdout cmd < inputfile cmd > outputfile • Numbered file descriptors • 0: stdin • 1: stdout • 2: stderr … • Pipes cmd1| cmd2 | cmd3 • IO Redirection cmd1 < inputfile > outputfile: specifies the input and output file cmd1 >> outputfile: append output to outputfile

3. Formatted IO • Output: printf • Input: scanf • Arguments: a format string, followed by the arguments printf (char *fmt, arg1, arg2, …) scanf (char *fmt, arg1, arg2, …) • Arguments for scanf have to be memory addresses

4. Formatting • Output goes to stdout • Input comes from stdin • Format String: regular string + conversion specification • Start of a format specification: % • Width, precision, adjustment: • Between ‘%’ and the conversion character, in order • ‘-’: left adjustment • A number: minimum field width • ‘.’: separates the field width from the precision • A number: precision • l or h: long or short. • Go to web page http://www.cplusplus.com/reference/clibrary/cstdio/printf/ for more details

5. String-based input • String-based output • int sprintf ( char * str, const char * format, ... ); • String-based input • int sscanf (char *line, char *fmt, arg1, arg2, …); • ex: char st[]=“3.21, 4.33”; • float x,y; • sscanf(st,”%f,%f”,&x,&y);//x=3.21, y=4.33 • char newst[32]; • sprintf(newst,”%f,%f”,y,x);//newst=“4.33,3.21”

6. Variable length arguments • Variable length arguments int printf(char *fmt, …); • <stdarg.h> • a new datatype: va_list • Associated macro functions: • va_start(ap, last) • Initialize ap to be the va_list after the argument: last. • va_arg(ap, int) • Expand ap to an expression that has type/value that match int • ap moves to the next argument in the variable list • va_end(ap) • Cleanup the variable argument list when done • Example: http://www.cprogramming.com/tutorial/c/lesson17.html

7. File Accesses • Files and file descriptors: • FILE *fp; • FILE *fopen(char *name, char *mode); • Name can be a long path • Mode: a combination of ‘r’, ‘w’, ‘x’, or ‘a’ • File input/output • int getc(FILE *fp) • int putc(int c, FILE *fp); • These works with default stdin/stdout: • getchar() • putchar()

8. Line-oriented input/output • int getline(char *line, size_t n, FILE *fp) • char *fgets(char *line, size_t n, FILE *fp); • gets(char *line, size_t n); /* buggy, never use it */ • char *fputs(char *line, FILE *fp);

9. Error handling • fprintf(stderr, char *fmt, arg1, arg2, …) • exit(1): exit with non-zero status • ferror(FILE *fp): test for any error on fp • feof(FILE *fp) : test for end-of-file • perror(char *s): • print the error message, s, to stderr for the last system or library calls