How to read last byte from file in C++

Use fseek() with offset -1 and mode SEEK_END:

fseek(myfile, -1, SEEK_END);

Ready-to-use function

#include <cstdio>
#include <optional>

/**
 * Read the last byte of a given 
 */
std::optional<char> readLastByteOfFile(const char* filename) {
    FILE* fin = fopen(filename, "r");
    if(fin == nullptr) {
        return std::nullopt;
    }
    fseek(fin, -1, SEEK_END);
    char lastByte;
    if(fread(&lastByte, 1, 1, fin) == 0) {
        return std::nullopt;
    }
    fclose(fin);
    return lastByte;
}

Complete example program

#include <cstdlib>
#include <cstdio>
#include <iostream>
#include <optional>

/**
 * Read the last byte of a given 
 */
std::optional<char> readLastByteOfFile(const char* filename) {
    FILE* fin = fopen(filename, "r");
    if(fin == nullptr) {
        return std::nullopt;
    }
    fseek(fin, -1, SEEK_END);
    char lastByte;
    if(fread(&lastByte, 1, 1, fin) == 0) {
        return std::nullopt;
    }
    fclose(fin);
    return lastByte;
}

int main(int argc, char** argv) {
    if(argc < 2) {
        std::cerr << "Usage: " << argv[0] << " <input file to read from>" << std::endl;
    }

    auto lastByte = readLastByteOfFile(argv[1]);
    if(lastByte) {
        std::cout << lastByte.value() << std::endl;
    } else {
        std::cout << "File error or empty" << std::endl;
    }
}

Generate test data with:

echo -n "abcd" > test.txt
touch test2.txt

Compile using:

g++ -o read-last-byte read-last-byte.cpp --std=c++17

Test using:

$ ./test-last-byte test1.txt
d
$ ./test-last-byte test2.txt
File error or no last byte