Question

I’ve got a block of C++ code that reads text from a named pipe. The relevant section is:

char s[256];
int num, fd;
string command;

command.erase(); //Clear the command string
cout << "Command0:" << endl << command << endl;
fd = open(FIFO_NAME, O_RDONLY); //Will block until a writer connects
do {
    if ((num = read(fd, s, 255)) == -1)
        perror("read");
    else {
        cout << "Command1:" << endl << command << endl;
        s[num+1] = '\0';
        cout << "Command2:" << endl << command << endl;
        //printf("read %d bytes: \"%s\"\n", num, s);
        command += s;
    }
} while (num > 0);
cout << "Command:" << endl << command << endl;

The “commandX” printfs are some debug code that I’ll reference the output of in a second. The chunks of text that get read into s print just fine, but after it null-terminates the char array, I end up with binary junk in the “command” string:

Command0:

Command1:

Command2:

Other than that, everything appears to work fine. The char[] concatenates onto the command string correctly, so I end up with the complete string, just with some extra binary on the front end. Am I having a weird array out of bounds problem here that is writing to command’s memory?

Answer 1

In the below condition,

if ((num = read(fd, s, 255))

If num = 255; then

s[num+1] = '\0';

will set s[256] = 0; which is out of range for s[0] to s[255].