Modern Computer: Von Neumann

ECEC 201 · Week 7 · Lecture

Four calls move every byte

These are system calls, not the stdio library: no buffering, no FILE *.

ECEC 201 · Week 7 · Lecture

Reading a /proc file

#include <fcntl.h>
#include <unistd.h>
/* Reads path into buf as a string; returns length or -1. */
ssize_t read_text(const char *path, char *buf, size_t size)
{
	int fd = open(path, O_RDONLY);
	if (fd < 0)
		return -1;
	ssize_t n = read(fd, buf, size - 1);
	close(fd);
	if (n >= 0)
		buf[n] = '\0';
	return n;
}

ECEC 201 · Week 7 · Lecture

When a call fails: errno

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>

int main(void)
{
	int fd = open("/no/such/file", O_RDONLY);
	if (fd < 0)
		printf("open: %s\n", strerror(errno));
	return 0;
}

• Read errno right after the failing call: the next call may change it.

ECEC 201 · Week 7 · Lecture

Appending to a log

• open(path, O_WRONLY | O_CREAT | O_APPEND, 0644) creates the file if needed and puts every write at the end.
• The third argument is the new file's permission bits; it is required with O_CREAT.
• Without O_APPEND, each run writes from the start and overwrites the last line.
• Check that write returned the length you asked for.