CPS 346 & 444/544 Lecture notes: System Libraries and I/O

---

I/O in C

---

• scanf and printf
  • what do they return?
  • we must develop the habit of checking return values
• opening and closing files: fopen and fclose
• file pointer: FILE*
• fscanf is the C analog of the C++ extraction operator (>>)
• fprintf is the C analog of the C++ insertion operator (<<)
• conversion specifiers
  • %d for decimal
  • %f for floating-point
  • %c for single character
  • %s for string
  • %x for hex
• formatted output ([CPL] §7.2, pp. 153-155), between % and conversion character, there may be, in order
  • a minus sign, - indicating left justification
  • a minimum field-width
  • a period which separates the field-width from the precision
  • a precision
• EOF
  • a #defined constant in stdio.h
  • is <crtl-d> character on UNIX system
• getchar
• declared in stdio.h: the C analog of iostream in C++

---

Effect of a Successful Open on a File

---

(ref. [C] 7-17)



	"r" read	"w" write	"a" append
File Exists	- Old contents discarded -   Error     File created     File created
File Does Not Exist




• "r+", "w+", "a+"         Updating; allows reading and writing
  
  
• "r+"                                Commonly used to read and change an existing file

---

Analogs from C++ to C

---

C++	C
iostream	stdio.h
cin	stdin
cout	stdout
>>	fscanf
<<	fprintf


Also, unlike C++, in C you must declare all variables in any function prior to any other code; in other words, you cannot declare variables in lexically scoped blocks in C.

---

Review of standard I/O functions

---

(ref. [C] 7-7)



	stdin and stdout	file I/O
character	getchar putchar	getc putc fgetc fputc ungetc
line	gets puts	fgets fputs
formatted	scanf printf	fscanf fprintf
record	- -	fread fwrite

---

Developing cat in C

---

(ref. [CPL] Chapter 7, §§7.5-7.6, pp. 160-164)

/* ref. [CPL] Chapter 7, 7.5, p. 162 with minor modification by Perugini */
#include<stdio.h>

/* cat: version 1 */
void filecopy (FILE* ifp, FILE* ofp) {

   char c;

   while ((c = getc (ifp)) != EOF)
      putc (c, ofp);
}

int main (int argc, char** argv) {

   FILE* fp = NULL;

   if (argc == 1)
      filecopy (stdin, stdout);
   else
      while (--argc > 0)
         if ((fp = fopen (*(++argv), "r")) == NULL) {
             printf ("cat: can't open %s\n", *argv);
         } else {
             filecopy (fp, stdout);
             fclose (fp);
           }

   return 0;
}

/* ref. [CPL] Chapter 7, 7.6, p. 163 with minor modifications by Perugini */
#include<stdio.h>
#include<stdlib.h>

/* cat: version 2 */
int main (int argc, char** argv) {

   void filecopy (FILE* ifs, FILE* ofs);

   int exit_status = 0;

   char* prog = *argv;

   FILE* fp = NULL;
                                 
   if (argc == 1)
      filecopy (stdin, stdout);
   else
      while (--argc > 0)
         if ((fp = fopen (*(++argv), "r")) == NULL) {
            fprintf (stderr, "%s: can't open %s\n", prog, *argv);
            exit (1);
            /* or use following line to continue processing */
            exit_status = 1;
         } else {
              filecopy (fp, stdout);
              fclose (fp);
           }

   if (ferror (stdout)) {
      fprintf (stderr, "%s: error writing stdout\n", prog);
      exit_status = 2;
   }

   exit (exit_status);
}

void filecopy (FILE* ifp, FILE* ofp) {

   int c;

   while ((c = getc (ifp)) != EOF)
      putc (c, ofp);
}

---

Portability (safety)

---

char c;
while ((c = getchar()) != EOF) { ... }

• use /* C-style comments */ vs. // C++-style comments
• also, do not use TABs in your code

---

String copy code from first day of class

---

#include <stdio.h>

main() {

   char* q = "copy this";
   char* p = (char*) malloc (sizeof (char)*10);
   char* r = p;

   printf ("%s\n", q);
   while (*p++ = *q++);
   *p = '\0';  /* necessary? no */
   printf ("%s\n", r);
}

---

String functions

---

• prototyped, not defined, in <string.h>
• int strlen (char*),
• int strcmp (char*, char*), int strncmp (char*, char*, int)
• char* strcpy (char*, char*), char* strncpy (char*, char*, n),
• char* strcat (char*, char*), char* strncat(char*, char*)
• char* strdup (const char*)
• when copying or concatenating strings, make sure destination string has sufficient space (memory)

---

`s' family of printf/scanf functions

---

• sprintf
• snprintf
• sscanf

---

Command-line arguments

---

• argc (argument count; command name is included)
• argv (argument vector, termined by null pointer; argv[0] is command name)
• main (int argc, char* argv[]) or main (int argc, char** argv)
• echoargs.c

  #include<stdio.h>
  #include<stdlib.h>
  
  int main (int argc, char* argv[]) {
     int i;
  
     printf ("argc is %d\n", argc);
  
     for (i = 0; i < argc; i++)
        printf ("argv[%1d] is %s\n", i, argv[i]);
  
     exit (0);
  }
  

• echopargs.c

  #include<stdio.h>
  #include<stdlib.h>
  
  int main (int argc, char** argv) {
  
     printf ("argc is %d\n", argc);
  
     for (; *argv; argv++)
        printf ("Next argument is %s\n", *argv);
  
     exit (0);
  }
  

---

argv array for the call ./a.out -wlc myfile

---

---

Using a pointer to traverse an array

---

---

References

[C]	C Language for Experienced Programmers, Version 2.0.0, AT&T, 1988.
[CPL]	B.W. Kernighan and D.M. Ritchie. The C Programming Language. Prentice Hall, Upper Saddle River, NJ, Second edition, 1988.
[UPE]	B.W. Kernighan and R. Pike. The UNIX Programming Environment. Prentice Hall, Upper Saddle River, NJ, Second edition, 1984.
[USP]	K.A. Robbins and S. Robbins. UNIX Systems Programming: Concurrency, Communication, and Threads. Prentice Hall, Upper Saddle River, NJ, Second edition, 2003.