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Example
FILE *f1, *f2, *f3, *f4;
To open text file c:\abc.txt for ready only:
f1 = fopen("c:\\abc.txt", "r");
To open text file c:\list.dat for write only:
f2 = fopen("c:\\list.dat", "w");
To open text file c:\abc.txt for read-write:
f3 = fopen("c:\\abc.txt", "r+");
To open binary file c:\liststudent.dat for write only:
f4 = fopen("c:\\liststudent.dat", "wb");
The file pointer will be used with all other functions that operate on the file and it must never be altered or the object it points to.
File checking
if (file_pointer_name == NULL)
{printf("Error opening file.");<Action for error>}
else{<Action for success>}
Before using an input/output file it is worth checking that the file has been correctly opened first. A call to fopen() may result in an error due to a number of reasons including:
If the operation is successful, fopen () returns an address which can be used as a stream. If a file is not successfully opened, the value NULL is returned. An error opening a file can occur if the file was to be opened for reading and did not exist, or a file opened for writing could not be created due to lack of disk space. It is important to always check that the file has opened correctly before proceeding in the program.
FILE *fp;
if ((fp = fopen("myfile", "r")) ==NULL){printf("Error opening file\n");
exit(1);} Once a file has been opened, depending upon its mode, you may read and/or write bytes to or from it.
When writing data to text files, C provides three functions: fprintf() , fputs() , fputc() .
The fprintf() function prototype is as follows:
int fprintf(FILE *fp, char *format, ...);
This function writes to the file specified by file pointer fp a sequence of data formatted as the format argument specifies. After the format parameter, the function expects at least as many additional arguments as specified in format. Depending on the format string, the function may expect a sequence of additional arguments, each containing one value to be inserted instead of each %-tag specified in the format parameter, if any. There should be the same number of these arguments as the number of %-tags that expect a value.
Return value: On success, the total number of characters written is returned. On failure, a negative number is returned.
#include<stdio.h>int main ()
{FILE * fp;
int n;char name [50];fp = fopen ("myfile.txt","w");
for (n=0 ; n<3 ; n++)
{puts ("Please, enter a name: ");
gets (name);fprintf (fp, "Name %d [%-10.10s]\n",n,name);}
fclose (fp);return 0;
} This example prompts 3 times the user for a name and then writes them to myfile.txt each one in a line with a fixed length (a total of 19 characters + newline). Two format tags are used: %d : signed decimal integer, %-10.10s : left aligned (-), minimum of ten characters (10), maximum of ten characters (.10), String (s).
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