CODEWITHSUNDEEP
c
Deependra singh
Deependra singh

Reputation: 95

What does "buffer[gotten] = '\0';" does in this code?

#include<stdio.h>
#include<fcntl.h>
int main(int argc, char *argv[])
{
    char buffer[6];
    int gotten;
    printf("%s",argv[1]);
    int fh = open(argv[1],O_RDONLY);
    printf("File handle %d\n", fh);
    while (gotten = read(fh, buffer, 6)) {
        buffer[gotten] = '\0';
        printf("%s", buffer);
    }
    return 0;
}

This part takes a file as input and prints the content of the file. The text file I am providing contains "hello". What does buffer[gotten] = '\0'; do in this code?

Upvotes: 2

Views: 1083

Answers (6)

sehe
sehe

Reputation: 392833

It makes sure there is a NUL character to terminate a C-style string, to make it safe for use with printf: https://en.wikipedia.org/wiki/Null-terminated_string

In C++ there are better types, like std::string:

Live On Coliru

#include <fcntl.h>
#include <unistd.h>
#include <cstdio>
#include <array>
#include <iostream>

int main(int argc, char *argv[]) {

    if (argc < 2)
        return -1;

    std::array<char, 6> buffer;
    printf("%s\n", argv[1]);

    int fh = open(argv[1], O_RDONLY);

    printf("File handle %d\n", fh);

    while (int gotten = read(fh, buffer.data(), buffer.size())) {
        std::cout.write(buffer.data(), gotten);
    }
}

Upvotes: 1

Some programmer dude
Some programmer dude

Reputation: 409136

In C, char strings are really called null terminated character strings. That '\0' character is the "null terminator" (not to be confused with a null pointer).

If that's missing, all standard string functions will not work, as they will go out of bounds looking for it.

Because of the terminator, any string of e.g. 6 characters need space for at least 7 characters, to able to fit the terminator.

Upvotes: 0

Mureinik
Mureinik

Reputation: 310993

Strings in are char arrays where the last character of the string is the \0 character which marks the end of the string. Since reading the characters from the file doesn't include the \0 character, it needs to be added manually so it can be treated as a normal string.

Upvotes: 0

twain249
twain249

Reputation: 5706

Null terminates the string.

'\0' is the null-termination character. gotten is set by the read() function which will return the number of characters read so buffer[gotten] will be the next spot in the array (since it's 0-based).

The null-termination allows C to know when the string ends. Since you are reusing the buffer for every read it's impossible to know what will be in the buffer beforehand so manually null-terminating is the best option.

Upvotes: 0

9y7h0n
9y7h0n

Reputation: 326

The '\0' character is known as the string terminator. This lets string functions know that the string in the area of memory you pass to it is done. This helps avoid reading too far in memory.

Upvotes: 0

frslm
frslm

Reputation: 2978

It adds a null character ('\0') to the end of the string you read in. Without it, printing the value in buffer might go past the end of "hello" and potentially print garbage values.

Upvotes: 0

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