Why the increment step inside the for loop is being executed, since not all conditions were met?-CodePudding

I'm studying C using the "C Programming Language" book, and I got into character arrays (section 1.9) from chapter 1. This is the getline() function it presents:

/* getline: read a line into s, return length */
int getline(char s[], int lim)
{
    int c, i;

    for (i = 0; i < lim-1 && (c = getchar()) != EOF && c != '\n';   i)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
          i;
    }
    s[i] = '\0';
    return i;
}

Take the word "Hi" as example.

In my point of view, s[1] should be equal to '\n', because the i should be only incremented when all conditions are met, however, when c is equal to '\n', the for loop exits, and i should contain a value of 1 and not 2.

So, why it is being incremented with only 2 of 3 conditions met?

I already tried using pre-increment operators and post-increments operators, and testing the i value inside certain lines of code.

CodePudding user response：

The loop variable i is incremented at the end of the loop body. You can write your for-loop:

for(i = 0; i < lim-1 && (c = getchar()) != EOF && c != '\n'; i  )
    s[i] = c;

as an equivalent while-loop:

i = 0;
while(i < lim-1 && (c = getchar()) != EOF && c != '\n') {
    s[i] = c;
    i  ;
}

Also, I suggest you rename your function to not conflict with the exiting function of the same name. Consider swapping the arguments so you can express how the lim and s variables are related and use size_t instead of int as negative lim probalby doesn't make sense:

size_t my_getline(size_t lim, char s[lim]);

The loop condition to i < lim - 1 is wrong as may write 2 bytes after the loop which will overflow the buffer. If lim <= 0 you overflow the buffer. if lim < 1 you have undefined behavior as c is uninitialized. I suggest you handle the newline in the loop like this, and only write the '\0' if s has room for at least 1 byte:

size_t my_getline(size_t lim, char s[lim]) {
    size_t i = 0;
    while(i < lim - 1) {
        int c = getchar();
        if(c == EOF) break;
        s[i  ] = c;
        if(c == '\n') break;
    }
    if(lim) s[i] = '\0';
    return i;
}