Week 6 #

Programming in C (cont.) #

File I/O in C #

• Content can be accessed as a steam (seq. read/write)
• File functions work with FILE *
  • FILE: Type rep. stream state, definition varies by platform, most likely a struct

Open #

FILE *fopen(const char *filename, const char *mode)

• Modes: "r", "w", "a", "r+", "w+", "a+"
  • Read, Write, Append, Read & Write, Write & Read, Append & Read
  • Appending and Writing to a file will create a new one if the file does not exist
  • Reading to a non-existent file will cause an error (returns NULL)

Close #

int fclose(FILE *stream)

• Close stream when finished with the file
• Returns 0 if success, EOF if error
• Why should you close the file as soon as you’re finished:
  • There is a limit on how many streams are open per process
  • Writing may be buffered until closing
  • No two process can open the same file (Windows only)

Formatted I/O #

// printf and scanf but for a given stream
int fprintf(FILE *stream, const char *format, ...) 
int fscanf(FILE *stream, const char *format, ...)

• printf is just fprintf but stdout is specified

  scanf(format, args) == fscanf(stdin, format, args) //same for scanf
  

• You do not need to manually close stdin, stdout, stderr streams

Character and String I/O #

• One single character: returns EOF if error or not found

  int putchar(int c)  /* stdout */
  int putc(int c, FILE *stream)
  int getchar(void)    /* stdin */
  int getc(FILE *stream)
  

• String:

  int fputs(const char *string, FILE *stream)   // does not put a newline at the end
  char *fgets(char *dest, int n, FILE *stream)  // Reads at most (n-1) chars or until (and including) newline
  

Arbitrary Data I/O #

size_t fread(void *dest, size_t s, size_t n, FILE *stream)
size_t fwrite(const void *data, size_t s, size_t n,FILE *stream)

• Reads/Writes n items, each of s bytes
• Returns how many items have been read/written
• Potential Use Cases:
  • A whole array
  • struct
  • Raw bytes (array of unsigned char)

Error versus End-of-Stream Disambiguation #

• getc, scanf return EOF on error
• fgets returns NULL
• fread returns < n

int feof(FILE *stream)      // returns true if end-of-stream
int ferror(FILE *stream)    // returns true if error
void clearerr(FILE *stream) // clears end-of-stream and error status

Error Information #

#include <errno.h>

• Global variable stores error reason of most recent error

  int errno;
  

• Many possible values:

  ENOENT  // File does not exist
  EACCESS // No permission
  EDOM    // sqrt(-3.0)
  

• Usually, we just use perror to print the error message to stderr:

  void perror(const char *prefix)
  

Buffering #

• C delays file writing
  • Accumulates data in a buffer until it is large, then requests the kernel to write that chunk
• Also hastens reading
  • Requests kernel to read a large chunk into the buffer, then serves read requests from said buffer

Buffer Operations #

int fflush(FILE *stream) // Returns 0 if success, EOF if error

• Writes the buffer for the output stream
• Clears the buffer for the input stream

int setvbuf(FILE *stream, char *buf, int mode, size_t n)

Default Buffering of stdin, stdout, stderr #

• If Terminal:

  • stdin: line buffer
  • stdout: no buffer
  • stderr: complicated

• If in a file or pipelined:

  • stdin: full buffer
  • stdout: no buffer
  • stderr: full buffer

Seeking #

• Ask current position:

  long int ftell(FILE *stream) // returns -1L if error
  

• Seek:

  int fseek(FILE *stream, long i, int origin) // non-zero return if error
  

  origin	go to i bytes from
  SEEK_SET	beginning
  SEEK_END	end
  SEEK_CUR	current position