When a file that already exists is opened in append mode the files existing contents are not erased?

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In C, fopen() is used to open a file in different modes. To open a file in write mode, “w” is specified.  When mode “w” is specified, it creates an empty file for output operations. What if the file already exists? If a file with the same name already exists, its contents are discarded and the file is treated as a new empty file. For example, in the following program, if “test.txt” already exists, its contents are removed and “GeeksforGeeks” is written to it. 

C++

#include <iostream>

#include <cstdlib>

int main()

{

    FILE *fp = fopen("test.txt", "w");

    if (fp == NULL)

    {

        puts("Couldn't open file");

        exit(0);

    }

    else

    {

        fputs("GeeksforGeeks", fp);

        puts("Done");

        fclose(fp);

    }

    return 0;

} 

C

#include <stdio.h>

#include <stdlib.h>

int main()

{

    FILE *fp = fopen("test.txt", "w");

    if (fp == NULL)

    {

        puts("Couldn't open file");

        exit(0);

    }

    else

    {

        fputs("GeeksforGeeks", fp);

        puts("Done");

        fclose(fp);

    }

    return 0;

}

The above behavior may lead to unexpected results. If programmer’s intention was to create a new file and a file with same name already exists, the existing file’s contents are overwritten. The latest C standard C11 provides a new mode “x” which is exclusive create-and-open mode. Mode “x” can be used with any “w” specifier, like “wx”, “wbx”. When x is used with w, fopen() returns NULL if file already exists or could not open. Following is modified C11 program that doesn’t overwrite an existing file. 

C++

#include <iostream>

#include <cstdlib>

int main()

{

    FILE *fp = fopen("test.txt", "wx");

    if (fp == NULL)

    {

        puts("Couldn't open file or file already exists");

        exit(0);

    }

    else

    {

        fputs("GeeksforGeeks", fp);

        puts("Done");

        fclose(fp);

    }

    return 0;

} 

C

#include <stdio.h>

#include <stdlib.h>

int main()

{

    FILE *fp = fopen("test.txt", "wx");

    if (fp == NULL)

    {

        puts("Couldn't open file or file already exists");

        exit(0);

    }

    else

    {

        fputs("GeeksforGeeks", fp);

        puts("Done");

        fclose(fp);

    }

    return 0;

}

References: Do not make assumptions about fopen() and file creation http://en.wikipedia.org/wiki/C11_(C_standard_revision) http://www.cplusplus.com/reference/cstdio/freopen/ This article is compiled by Abhay Rathi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above