1. Weeks 9-10 IO System Calls 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 Numbered file descriptors Pipes
cmd < inputfile
cmd > outputfile
Numbered file descriptors
0: stdin
1: stdout
2: stderr …
Pipes
cmd1 | cmd2 | cmd3
IO Redirection
cmd1 < inputfile > outputfile
cmd1 | cmd2 2>&1 | cmd3 (**: not for csh)

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 the 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:

5. String-based input String-based input
int sscanf (char *line, char *fmt, arg1, arg2, …);
Except %s, which tends to be confusing for sscanf, the others are very useful.

6. 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

7. File Accesses Files and file descriptors: File input/output 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