Why does printf not print anything?

March 16, 2023

I have this code where I take a file name and generate a new file with the same file name but ending in .c

However, when I attempt to transform the file name to end in a .c, I try to print the result to check if it’s working. And printf just prints nothing. If it makes a difference, the file name is a pointer that points to argv[1] from int main(int argc, char *argv[]).

Here is the code:

bool file_genEquiCFile(char *file_name)
{
    // Maybe add a memmove to preserve the original buffer
    printf("file_name before change: %s\n", file_name);
    while (file_name && *file_name != '.')
        file_name++;

    if (file_name)
    {
        if (*file_name++ == '.')
        {
        *file_name++ = 'c';
        *file_name = '\0';

        // For now use "w" mode which makes an empty file for writing.
        FILE *c_file = fopen(file_name, "w");
        fclose(c_file);
        printf("file_name after change: %s\n", file_name);

        return true;
        }
        else
            return false;
    }
        
    return false;
}

By the way, file_name points to a file called hey.txt
Here is what printf printed:

file_name before change: hey.txt
file_name after change:

>Solution :

After *file_name = '\0';, file_name points at a 0-character string because what file_name points at is the terminating NUL character.

You should store the original file_name at the beginning of the function and use it later instead of the modified file_name later.

bool file_genEquiCFile(char *file_name)
{
    char *file_name_orig = file_name;
    // Maybe add a memmove to preserve the original buffer
    printf("file_name before change: %s\n", file_name);
    while (file_name && *file_name != '.')
        file_name++;

    if (file_name)
    {
        if (*file_name++ == '.')
        {
        *file_name++ = 'c';
        *file_name = '\0';

        // For now use "w" mode which makes an empty file for writing.
        FILE *c_file = fopen(file_name_orig, "w");
        fclose(c_file);
        printf("file_name after change: %s\n", file_name_orig);

        return true;
        }
        else
            return false;
    }
        
    return false;
}

Also you may want to use *file_name instead of file_name in the first while and if to check not if the pointer is not NULL but if what the pointer points at is a NUL character.