Why does this program fail to write the character in the file properly and instead get stuck in an infinite loop?

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

int main() {
    FILE *fp;
    char ch;

    fp = fopen("sample.txt", "r+");
    if (fp == NULL) {
        printf("Problem accessing the file!");
        exit(1);
    }

    while ((ch = fgetc(fp)) != EOF) {
        if (ch == ' ') {
            fseek(fp, -1l, SEEK_CUR);
            fputc('_', fp);
        }
    }

    fclose(fp);

    return 0;
}

In this program, I wanted the all the white-space characters in the opened file to be replaced with underscore characters. But this program is behaving in a very unexpected manner and getting stuck in an infinite loop. How do I make it work properly?

I tried to flush the stream fp on each iteration of the while loop but still it doesn't give the intended output.

ch must be defined with type int to accommodate all byte values read by fgetc() and the special negative value of the EOF macro.

Using char makes it impossible to test EOF on your platform because type char seems to be unsigned, hence the value EOF is converted to a positive value 255 when stored into ch, which compares different from EOF in the test.

Furthermore, you must issue a call to fseek() or rewind() before switching from reading from the stream to writing to it, which you do, but also to switch back to reading, which you don't.

Finally, you should open the file as binary for fseek to operate correctly with absolute numbers such as -1.

Here is a modified version:

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

int main(void) {
    FILE *fp;
    int ch;

    fp = fopen("sample.txt", "rw+");
    if (fp == NULL) {
        printf("Problem accessing the file!");
        exit(1);
    }

    while ((ch = fgetc(fp)) != EOF) {
        if (ch == ' ') {
            fseek(fp, -1l, SEEK_CUR);
            fputc('_', fp);
            fseek(fp, 0L, SEEK_CUR);
        }
    }

    fclose(fp);

    return 0;
}

There are several errors, but all of them are related to this declaration:

...
    char ch;
...
    while ((ch = fgetc(fp)) != EOF) {
...

the function fgetc() requires to use an int variable, as it returns the input byte as a number between 0 and 255 (all of them positive) and an extra value (EOF, normally defined as -1) this cannot be handled by a char.

The second thing is that, char in your computer, is probably defined as unsigned so you get in ch a number between 0 and 255 (with -1 converted into one of these values by an Undefined Behavior you have incurred in your declaration)

The third thing is that comparing a number in the range (0..255) against -1 makes the left operand to be converted into an integer between 0 and 255, with results in the while test to be true always, this makes the while loop to run forever.

fgetc() is a function that returns 257 values, not 256. The ones used for all the possible byte values, plus one extra for indicating EOF (and this is not a charater, indeed, it is an indicator of what happened in the file, it is a common mistake considering that there's an end of file character, which there's not.