Why does this test of getline's return value result in an infinite loop?-CodePudding

The following program is supposed to read a particular text file and print its contents line by line. However, it results in an infinite loop. Why?

#define __STDC_WANT_LIB_EXT2__ 1
#include <stdio.h>

int main(void)
{
    char *line = NULL;
    size_t buf_len;
    size_t line_len;

    FILE *fp = fopen("README.txt", "r");

    while (1) {
        line_len = getline(&line, &buf_len, fp);
        if (line_len < 0)
            break;
        printf("%s", line);
    }

    return 0;
}

I noticed that I could fix the problem by changing line_len < 0 to line_len == -1, but I don't understand the reason why this fixes the problem. Isn't -1 less than 0?

(C compiler: gcc 9.4.0 on Ubuntu 20.04).

CodePudding user response：

line_len is of type size_t which should link to an unsigned long, since it is a unsigned value, it will never drop below 0 and instead wrap around to the highest value possible.

This is probably because -1 does not have any kind of type annotation:

unsigned long test1 = 18446744073709551615;
long test2 = -1;

printf("%d\n", test1 == test2);

returns true because the bit values are the same.

C will do some type conversion (for instance 10.0f == 10), but if the value is inlined it cannot know what type you need it to be.