Thread: Write to file using struct

While programming with the struct data type, I have a .txt that I wish to write to and I am unable to do so. Below I showcase my code, the problem is the contents of my file become overridden whereas I wish to simply add onto it.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


// a struct to read and write
struct person
{
    int id;
    char fname[20];
    char lname[20];
};


int main ()
{
    FILE *outfile;


    // open file for writing
    outfile = fopen ("person.txt", "w");
    if (outfile == NULL)
    {
        fprintf(stderr, "\nError opened file\n");
        exit (1);
    }


    struct person input1 = {1, "rohan", "sharma"};
    struct person input2 = {2, "mahendra", "dhoni"};


    // write struct to file
    //fwrite (&input1, sizeof(struct person), 1, outfile);
    //fwrite (&input2, sizeof(struct person), 1, outfile);


    //Why does the above erase the contents already in my file
    //I want to keep them and to add the above lines to the file


    if(&fwrite != 0)
        printf("contents to file written successfully !\n");
    else
        printf("error writing file !\n");


    // close file
    fclose (outfile);


    return 0;
}

//Why does the above erase the contents already in my file
//I want to keep them and to add the above lines to the file

First of all, you should open the file in "a" Append mode. If the file does not exist, it will create the file. If the file exists then the file will open and new lines will be written to the end of the file, appending the new data to the existing file. Please see man fopen online or in any Linux command line.

Secondly, I would write out the members of the struct individually, formatting the output on one line or each member on separate lines.

Also...

Code:

   if(&fwrite!= 0)

Is saying "if the fwrite function has an address that isn't zero".

Maybe use

Code:

    if(fprintf(f,"%s, %s\n", input1.fname, input1.lname) == 2) {
       // wrote 2 things to the file (fname and lname)
    } else {
        // unable to write - so error
    }

These suggestions I found helpful, only hamster_nz I do not know what you meant by the parameter 'f' as it gives me an error. Now I am also wondering like you said rstanley how to hardcode struct members individually. Currently they are being added one after the other on the last line of the file, do I have choices other than using '\n' to define their output?

ufl:

You don't say how you will use the data file, once you have written to a file. Will you read the data back into the same program or into a second program?

Yes you could write out the struct as binary data to a file, using fwrite(), or if transferring data between two programs, you might want to write out the three elements of the struct as text lines using fprintf().

From hamster_nz:

fprintf(f,"%s, %s\n", input1.fname, input1.lname)

The first argument to fprintf(), "f" represents the file pointer. in your code, your file pointer is "outfile".