3.8 - File Input / Output

 File Input / Output

• Sequence of bytes 
• Could be regular or binary file 
• Why?
  
  • Persistent storage 
  • Theoretically unlimited size 
  • Flexibility of storing any type data

Redirection

• General way for feeding and getting the output is using standard input (keyboard) and output (screen) 
• By using redirection we can achieve it with files i.e ./a.out < input_file > output_file 
• The above line feed the input from input_file and output to output_file 
• The above might look useful, but its the part of the OS and the C doesn't work this way 
• C has a general mechanism for reading and writing files, which is more flexible than redirection 
• C abstracts all file operations into operations on streams of bytes, which may be "input streams" or "output streams" 
• No direct support for random-access data files 
• To read from a record in the middle of a file, the programmer must create a stream, seek to the middle of the file, and then read bytes in sequence from the stream 
• Let's discuss some commonly used file I/O functions
  

File Pointer

• stdio.h is used for file I/O library functions
• The data type for file operation generally is

Type

FILE *fp; 

• FILE pointer, which will let the program keep track of the file being accessed
  
• Operations on the files can be

– Open
– File operations
– Close
 

Functions
Prototype

FILE *fopen(const char *filename, const char *mode);
int fclose(FILE *filename);

Where mode are: r - open for reading w - open for writing (file need not exist) a - open for appending (file need not exist) r+ - open for reading and writing, start at beginning w+ - open for reading and writing (overwrite file) a+ - open for reading and writing (append if file exists)  
Example

#include 
int main()
{
    FILE *fp;
    fp = fopen(“test.txt”, “r”);
    fclose(fp);
return 0;
} 

Example

#include 
#include 
int main()
{
    FILE *input_fp;
    input_fp = fopen( "text.txt" , "r" );
    if (input_fp == NULL )
    {
    exit(1);
    }
    fclose(input_fp);
return 0;
}

Functions  
Prototype

int *fgetc(FILE *fp);
int fputc(int c, FILE *fp);

Prototype

int fprintf(char *string, char *format, ...);

Example:

#include 
#include 
int main()
{
    FILE *input_fp;
    input_fp = fopen( "text.txt" , "r" );
    if (input_fp == NULL )
    {
        fprintf(stderr , "Can't open input file text.txt!\n" );
        exit(1);
    }
        fclose(input_fp);
    return 0;
}