Linux - Memory Mapped File

Question

I am trying to implement the caesar cipher with mmap. I think the cipher works so fine but the problem is, the mmap. The idea of it is to update the file, if there was a change. So far it isn't working. I can just read the memory mapped file and print it. But if I make any modificaiton, I get a segmentation fault. Unfortunately, I couldn't solve the problem myself. So, I would appreciate it, if you could help me with it.

Here is the code.

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

    if(argc != 5)
       fprintf(stdeer, "usage: ./cipher (encrypt|decrypt) <file name> (casar| vigenere) <key>\n");

    // (encrypt / decrypt) can be found in argv[1]
    // filename in argv[2]
    // encryption method in argv[3]
    // key in argv[4]
    int fd = open(argv[2], O_RDWR, S_IWRITE | S_IREAD);
    if (fd < 0)
        hanle_error("open");

    off_t len = lseek(fd, 0, SEEK_END);
    if (len == (off_t)-1)
        handle_error("lseek");

    unsigned char* data = mmap(0, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); // Add PROT_WRITE
    if (data == MAP_FAILED)
        handle_error("mmap");

    char c = *argv[4];
    int key = 0;

    if(strcmp(argv[3], "caesar") == 0) {
        key = c - 48;
        if(strcmp(argv[1], "decrpyt") == 0)
            key = -key;
        int num = 0;
        for(int size_t i = 0; i < (size_t)len; i++) {
            if(data[i] >= 97 && data[i] <= 122) {
                num = data[i];
                num +=key;
                if(num > 'z') {
                    num -= 26;
                    data[i] = num + '0';
                } else if (num < 'a') {
                    num += 26;
                    data[i] = num + '0';
                } else {
                    data[i] = num + '0';
                }
            } else {
                continue;
            }
        }
    }

    return 0;
}

A possible input can be anything eg

SsWd asdas
qwmkfd aw.

The algorithm above should just modify the lower case letters and leave the rest as it is.

I hope someone might be able to help me.

Also, I only implemented the caesar cipher.

EDIT: The seg fault is gone after I added PROT_WRITE. But know I get weird question marks for the modified lower case letters. Does anyone know why?

Answer 1

If you want to write to the file too, then

unsigned char* data = mmap(0, len, PROT_READ, MAP_SHARED, fd, 0);

should be

unsigned char* data = mmap(0, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

That may not be the only issue, but would explain the seg fault.

The reason is self explanatory!