course-c

Introduction

• Input and Output in C is provided by several standard library functions and not by the core language itself

• The standard library functions operate on streams (source or destination of data)

• The state of a stream is stored in the FILE structure declared in <stdio.h>

• A FILE structure variable should not be created by the programmer

Standard streams

• The C library provides three standard streams in <stdio.h>

  • Standard input – stdin

  • Standard output – stdout

  • Standard error – stderr

Standard Input

• Stream that represents the standard input; usually this is the keyboard

• The stdin object is the standard input stream; it can be redirected by using the freopen function

• It can also be redirected at the command line

  executable < filename
  

  • content of filename available in stdin of program

  executable1 | executable2
  

  • output of executable1 available in stdin of executable2

Standard Output

• Stream that represents the standard output; usually this is the display

• The stdout object is the standard output stream; it can be redirected using the freopen function

• It can also be redirected at the command line

  executable > filename
  

  • redirects stdout of program to file filename

  executable1 | executable2
  

  • redirects stdout of executable1 to stdin of executable2

Standard Error

• Stream that represents the standard error; usually this is the display

• The stdout object is the standard output stream; it can be redirected using the freopen function

• Program errors should be sent to this stream; this way, when the standard output is redirected to some other file, the error messages will continue to appear to the user on the display

• It can also be redirected at the command line

  executable 2> filename
  

File Access

• Open a file stream

  FILE* fp;
  fp = fopen(name, mode);
  

  • name is a relative or absolute file name and path

  • mode can be "r", "w", "a", "rt", "wt", "at", "rb", "wb", "ab", "r+t", "w+t", "a+t", "r+b", "w+b", or "a+b"

• Use the functions in <stdio.h> to manipulate the content of the file stream

• Call fclose to close the file stream

Formatted Output

• Function printf prints text to stdout

  int printf(char* format, ...)
  

• Function fprintf prints to any open stream

  int fprintf(FILE* fp, char* format, ...)
  

• format is the format string; it contains the text to be printed, interspersed with conversion specifications that are used to convert and print the arguments that follow

Print conversion specification

%[flags][width][.precision][modifiers]type

• flags can be -, +, space, 0, #

• width specifies the minimum field width

• precision specifies different things depending on the type

• modifiers can be h, l or L

• type can be d, i, o, x, X, u, c, s, f, e, E, g, G, p, n, or %

Formatted Input

• Function scanf reads formatted input from stdin

  int scanf(char *format, ...)
  

• Function fscanf reads formatted input from any stream

  int fscanf(FILE *fp; char *format, ...)
  

• format is the format string containing text to be matched against the input

• Blanks and tabs in the format string are ignored

• White-space characters in the input stream act as field separators

Input conversion specification

%[*][width][modifiers]type

• * specifies assignment suppression

• width specifies the maximum width

• modifiers can be h or l

• type can be d, i, o, u, x, c, s, e, f, g, p, n, [...], [^...], or %

  scanf("%d/%d/%d", &day, &month, &year)
  

Variable length argument lists

• A function may contain a variable length argument list

  int printf(const char*, ...)
  

• Header <stdarg.h> contains macro definitions that define how to read the argument list

  • Declare a variable ap of type va_list

  • Call va_start(ap, lastarg) to initialize ap; lastarg is the last argument before ...

  • Call va_arg(ap, type) to read next argument

  • Call va_end(ap) to clean up

Character input and output

int getc (FILE* fp)

• returns next character from stream fp, or EOF

int putc(int c, FILE* fp)

• write character c to stream fp
• returns character written, or EOF on error

getchar()

• same as getc(stdin)

putchar(c)

• same as putc(c, stdout)

Line input and output

char* fgets(char* line, int maxline, FILE* fp)

• reads at most (maxline - 1) characters from file stream fp
• returns line, NULL on error, or EOF

int fputs(char* line, FILE* fp)

• writes the string in line to the file stream fp
• returns zero or EOF if an error occurs

File positioning

fseek(FILE* stream, long offset, int origin)

• sets the file position for the stream; offset may be SEEK_SET, SEEK_CUR or SEEK_END

long ftell(FILE* stream)

• returns the current file position or -1L on error

void rewind(FILE* stream)

• sets the file position to the beginning, this is same as calling fseek(fp, 0L, SEEK_SET)

Error handling

void clearerr(FILE* stream)

• clears end of file and error indicators

int feof(FILE* stream)

• returns non-zero if end of file indicator is set

int ferror(FILE* stream)

• returns non-zero if error indicator is set

void perror(const char*)

• prints error message

Listing directories

• Requires <sys/stat.h> and, on BSD/Linux, <dirent.h>

  DIR* dir;
  struct dirent* item;
  struct stat statbuf;
  dir = opendir(".");
  item = readdir(dir);
  while(item != NULL) {
    stat(item->d_name, &statbuf);
    if(S_ISDIR(statbuf.st_mode)) {
      //...
    }
    item = readdir(dir);
  }
  

Exercise

• Write a program that functions like the Unix tar command. The program should pack all files in the current directory into a single file whose name is specified at the command line. If the program receives the –u flag followed by a file name, it should unpack the content of the file to the current directory

  pack [-u] filename