Introduction to C / C++ Programming
Additional File Input and Output

Accreditation

Some content here was drawn from "C Programming, A Modern Approach" by K.N. King, Chapter 22.

Some material on this page was drawn from linuxmanpages.com and the C Reference section of cppreference.com

Review

The following topics were covered in Formatted I/O

• printf
• scanf

The following topics were covered in Basic Text File Inpt and Output

• Binary versus Text I/O
• FILE * data type
  • Standard FILE *s stdin, stdout, stderr
• fopen
• fprintf
• fscanf
• fclose

The following topics were covered in Character Strings:

• sscanf
• sprintf
• puts
• gets
• fgets

Binary File I/O using FILE pointers

FILE *s are pointers to objects of type FILE, which will make more sense to us after we have studied structs

• fread
  • size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
  • The function fread reads nmemb elements of data, each size bytes long, from the stream pointed to by stream, storing them at the location given by ptr.
  • Example:

        double data[ 100 ];
        FILE *infile = fopen( "input.dat", "r" );
        
        if( fread( data, sizeof( double ), 100, infile ) != 100 ) {
            fprintf( stderr, "Error: fread failed!\n" );
            exit( -1 );
        }

• fwrite
• size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
• size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
• The return value for both fread and fwrite is the number of ITEMS transferred successfully, not the number of bytes. On error the return value will be less than what was requested, possibly but not necessarily zero.

Low-Level Character and String I/O

• putc, fputc, puts, fputs, putchar
  • int fputc(int c, FILE *stream);
  • int fputs(const char *s, FILE *stream);
  • int putc(int c, FILE *stream);
  • int putchar(int c);
  • int puts(const char *s);
• getc, gets, fgetc, fgets, getchar, ungetc
  • int fgetc(FILE *stream);
  • char *fgets(char *s, int size, FILE *stream);
  • int getc(FILE *stream);
  • int getchar(void);
  • char *gets(char *s);
  • int ungetc(int c, FILE *stream);

I/O Using File Descriptors

File descriptors are basically ints, that refer to entries in a ( system ) table

• open
  • int open(const char *pathname, int flags);
  • int open(const char *pathname, int flags, mode_t mode);
  • int creat(const char *pathname, mode_t mode);
• close
  • int close(int fd);
• read
  • ssize_t read(int fd, void *buf, size_t count);
  • read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
  • read returns the number of BYTES transferred successfully, or -1 on error
  • Example:

       double data[ 100 ];
       int fd = open( "inputfile.dat", O_RDWR )
        
       if( read( fd, data, 100 * sizeof( double ) ) != 100 * sizeof( double ) ) {
           fprintf( stderr, "Error: read failed!\n" );
           exit( -1 );
       }

• write
  • ssize_t write(int fd, const void *buf, size_t count);

File Manipulations

• feof, ferror, clearerr, fileno - For detecting the end of the file and other file error conditions
  • int feof(FILE *stream); - Checks whether the files EOF bit has been set, which happens AFTER an attempt has been made to read past the end of file. ( If a file contains 100 bytes, then reading all 100 bytes does not set EOF. Attempting to read the 101st byte does. )
  • int ferror(FILE *stream); - Checks the error bit for a stream.
  • void clearerr(FILE *stream); - Clears the error bit for a stream.
  • int fileno(FILE *stream); - Returns the file descriptor associated with a given FILE *
• fseek, ftell, rewind, fsetpos, fgetpos
  • int fseek(FILE *stream, long offset, int whence); - Jumps to a particular position in a file.
  • long ftell(FILE *stream); - Returns the current position in a file. ( For use later in fseek )
  • void rewind(FILE *stream);
  • int fgetpos(FILE *stream, fpos_t *pos);
  • int fsetpos(FILE *stream, fpos_t *pos);
• freopen
  • FILE *fopen(const char *path, const char *mode);
  • FILE *fdopen(int fildes, const char *mode); - Returns a FILE * associated with a given file descriptor
  • FILE *freopen(const char *path, const char *mode, FILE *stream);
• tmpfile
  • FILE *tmpfile (void);
• tmpnam
  • char *tmpnam(char *s);
• fflush
  • int fflush(FILE *stream);
• setbuf
• setvbuf
• remove
• rename

New Material

• getline
• snprintf - Like sprintf, but limits the number of character printed