fopen() for an existing file in write mode

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.

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#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;
}

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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.

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#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;
}

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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

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