read this page. http://www.cprogramming.com/tutorial/cfileio.html

FILE FUNCTIONS

1. fopen() //for opening a file

FILE *fopen(const char *path, const char *mode);

The fopen() function is used to open a file and associates an I/O stream with it. This function takes two arguments. The first argument is a pointer to a string containing name of the file to be opened while the second argument is the mode in which the file is to be opened. The mode can be :

• ‘r’ : Open text file for reading. The stream is positioned at the beginning of the file.

• ‘r+’ : Open for reading and writing. The stream is positioned at the beginning of the file.

• ‘w’ : Truncate file to zero length or create text file for writing. The stream is positioned at the beginning of the file.

• ‘w+’ : Open for reading and writing. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file.

• ‘a’ : Open for appending (writing at end of file). The file is created if it does not exist. The stream is positioned at the end of the file.

• ‘a+’ : Open for reading and appending (writing at end of file). The file is created if it does not exist. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

The fopen() function returns a FILE stream pointer on success while it returns NULL in case of a failure.

2. fread() and fwrite() //for reading/writing from/into a file

The functions fread/fwrite are used for reading/writing data from/to the file opened by fopen function. These functions accept three arguments. The first argument is a pointer to buffer used for reading/writing the data. The data read/written is in the form of ‘nmemb’ elements each ‘size’ bytes long.

In case of success, fread/fwrite return the number of bytes actually read/written from/to the stream opened by fopen function. In case of failure, a lesser number of byes (then requested to read/write) is returned.

Other function pairs for reading/writing files include the following, which are not discussed in the example program.

• fgets()/fputs()

• fgetc()/fputc()

• fscanf()/fprintf()

3. fseek() //used to manipulate the position of the pointer within the file, once the file has been opened

int fseek(FILE *stream, long offset, int whence);

The fseek() function is used to set the file position indicator for the stream to a new position. This function accepts three arguments. The first argument is the FILE stream pointer returned by the fopen() function. The second argument ‘offset’ tells the amount of bytes to seek. The third argument ‘whence’ tells from where the seek of ‘offset’ number of bytes is to be done. The available values for whence are SEEK_SET, SEEK_CUR, or SEEK_END. These three values (in order) depict the start of the file, the current position and the end of the file.

Upon success, this function returns 0, otherwise it returns -1.

4. fclose() //used to close the handle to the file

int fclose(FILE *fp);

The fclose() function first flushes the stream opened by fopen() and then closes the underlying descriptor. Upon successful completion this function returns 0 else end of file (eof) is returned. In case of failure, if the stream is accessed further then the behavior remains undefined.

5. Example Code

#include

#include

#define SIZE 1

#define NUMELEM 5

int main(void)

{

FILE* fd = NULL; // Declaring a file pointer, will be used later as a handle to the desired file

char buff[100];

memset(buff,0,sizeof(buff));

fd = fopen("test.txt","rw+");//Opening a file using the handle described previously. Assumes file "test.txt" exists in the same directory from where the executable will run. "rw+" defines the opening mode

if(NULL == fd) //file pointer returns NULL, when the file cannot be opened using the specified mode

{

printf("\n fopen() Error!!!\n");

return 1;

}

printf("\n File opened successfully through fopen()\n");

if(SIZE*NUMELEM != fread(buff,SIZE,NUMELEM,fd)) // Checking for successful read, you can use other conditions to check or other functions to read

{

printf("\n fread() failed\n");

return 1;

}

printf("\n Some bytes successfully read through fread()\n");

printf("\n The bytes read are [%s]\n",buff);

if(0 != fseek(fd,11,SEEK_CUR)) //Manipulating the pointer to some other location within the file

{

printf("\n fseek() failed\n");

return 1;

}

printf("\n fseek() successful\n");

if(SIZE*NUMELEM != fwrite(buff,SIZE,strlen(buff),fd)) // Checking for successful write, you can use other conditions or other functions to write

{

printf("\n fwrite() failed\n");

return 1;

}

printf("\n fwrite() successful, data written to text file\n");

fclose(fd); //Close the File Handle to close the actual file

printf("\n File stream closed through fclose()\n");

return 0;

}

>>>>Initially the content in file is :

$ cat test.txt

hello everybody

>>>>Executing The Code

$ ./fileHandling

>>>>Output

File opened successfully through fopen()

Some bytes successfully read through fread()

The bytes read are [hello]

fseek() successful

fwrite() successful, data written to text file

File stream closed through fclose()

>>>>Again check the contents of the file test.txt. As you see below, the content of the file was modified.

$ cat test.txt

hello everybody

hello