Why does fputc() append to my file and not overwrite it?

I'm writing a C Program that changes all uppercase characters to lowercase from a certain file. The problem is, after storing all characters (processed) in a string, and trying to print all characters inside that file, they are appended to that file, and not overwritten. I'm using "r+" as a permission.

This is my code:

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

#define MAX 1024

int main(int argc, char **argv)
{
    if (argc != 2) {
        printf("Invalid number of arguments.\n");
        exit(1);
    }

    FILE *f = fopen(argv[1], "r+");
    if (f == NULL)
    {
        printf("Could not open specified file.\n");
        exit(2);
    }

    char buf[MAX];
    int len = 0;
    
    while(!feof(f))
    {
        int c = fgetc(f);
        if (c >= 'A' && c <= 'Z')
            c = tolower(c);
        if (c != EOF) {
            buf[len] = c;
            len++;
        }
    }

    for (int i = 0; buf[i] != '\0'; i++)
        fputc(buf[i], f);

    fclose(f);

    return 0;
}

Same problem with fputs(buf, f). Also even after appending, some "strange" characters appear at the end of the selected file.

What's the problem, exactly? How can I fix it? Thank you. Help would be appreciated.

Solution

Your program has incorrect and even undefined behavior for multiple reasons:

while(!feof(f)) does not detect end of file reliably, feof() is set after an attempt at read from f fails. You should instead write:
```
  while ((c = getc(f)) != EOF) {
      if (isupper(c))
          c = tolower(c);
      ...
```
testing for uppercase with if (c >= 'A' && c <= 'Z') works for ASCII but would fail for EBCDIC. You could instead use isupper(c) or unconditionally use tolower(c).
reading and writing to the same file, open for read and update mode with "r+", requires a call to fseek() or rewind() to change from reading to writing and vice versa.
you do not check if len stays within the boundaries of buf, causing undefined behavior when attempting to write beyond the end of the array for sufficiently long input.
you do not set a null terminator at buf[len], hence calling fputs(buf, f) has undefined behavior because buf is not a proper C string. Similarly, the loop in the posted code iterates while buf[i] != '\0' which causes undefined behavior as this null terminator has not been set at the end of the data read from the file.

Here is a modified version:

#include <ctype.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
    FILE *f;
    int c;

    if (argc != 2) {
        printf("Invalid number of arguments.\n");
        return 1;
    }

    if ((f = fopen(argv[1], "r+")) == NULL) {
        printf("Could not open file %s.\n", argv[1]);
        return 2;
    }

    while ((c = getc(f)) != EOF) {
        if (isupper(c) {
            fseek(f, -1L, SEEK_CUR);
            putc(tolower(c), f);
            fseek(f, 0L, SEEK_CUR);
        }
    }
    fclose(f);
    return 0;
}

Note that it would be more efficient and reliable to read from the file and write to a different file. You can try this simplistic filter and compare with the above code for a large file:

#include <ctype.h>
#include <stdio.h>

int main() {
    int c;
    while ((c = getchar()) != EOF) {
        putchar(tolower(c));
    }
    return 0;
}