Add a simple pager program

progs/base/Makefile

@@ -10,6 +10,7 @@ LDFLAGS?=-lgcc
 O=build
 
 BINS=$(O)/bin/ls \
+	 $(O)/bin/pager \
 	 $(O)/bin/cat \
 	 $(O)/bin/login \
 	 $(O)/bin/date \

progs/base/bin/pager.c

@@ -0,0 +1,131 @@
+#include <sys/ioctl.h>
+#include <termios.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <stdio.h>
+
+static void reprint(const char *buf, size_t start, size_t count, size_t limit) {
+    // Clear screen
+    fputs("\033[2J\033[1;1f", stdout);
+    fflush(stdout);
+
+    // Assuming cursor position is at start
+    for (size_t i = start; i < start + count && i < limit; ++i) {
+        const char *line = &buf[i * 256];
+        printf("%s", line);
+    }
+    fflush(stdout);
+}
+
+int main(int argc, char **argv) {
+    // Alloc initial buffer
+    struct termios termios, old_termios;
+    struct winsize winsz;
+    size_t height;
+    ssize_t len;
+    char *buf;
+    char key[256];
+    size_t lines, cap, scroll;
+    int tty_fd;
+
+    if (!isatty(STDOUT_FILENO)) {
+        fprintf(stderr, "stdout is not a tty\n");
+        return -1;
+    }
+
+    if (ioctl(STDOUT_FILENO, TIOCGWINSZ, &winsz) != 0) {
+        perror("window size");
+        return -1;
+    }
+    height = winsz.ws_row - 1;
+
+    // For keyboard input
+    tty_fd = open("/dev/tty", O_RDONLY, 0);
+    if (tty_fd < 0) {
+        perror("/dev/tty");
+        return -1;
+    }
+
+    // Set non-canonical term mode
+    if (tcgetattr(tty_fd, &termios) != 0) {
+        perror("tcgetattr()");
+        return -1;
+    }
+
+    memcpy(&old_termios, &termios, sizeof(struct termios));
+    termios.c_iflag &= ~ICANON;
+    termios.c_lflag &= ~(ECHO | ECHONL | ECHOE | ECHOK);
+
+    if (tcsetattr(tty_fd, TCSANOW, &termios) != 0) {
+        perror("tcsetattr()");
+        return -1;
+    }
+
+    // Read lines until screen full
+    lines = 0;
+    cap = height;
+    // Max line length: 256
+    buf = malloc(256 * cap);
+
+    while (lines < height) {
+        if (fgets(&buf[lines * 256], 256, stdin) == NULL) {
+            break;
+        }
+        ++lines;
+    }
+
+    scroll = 0;
+
+    reprint(buf, 0, lines, lines);
+
+    while ((len = read(tty_fd, key, sizeof(key))) > 0) {
+        if (key[0] == 'q') {
+            break;
+        }
+
+        if (key[0] == 'j') {
+            if (!scroll) {
+                // Just read the line and print it at the end
+                if (lines == cap) {
+                    // Grow a single page to the buffer
+                    cap += 256 * height;
+                    buf = realloc(buf, cap);
+                    if (!buf) {
+                        return -1;
+                    }
+                }
+
+                // End of file reached?
+                if (fgets(&buf[lines * 256], 256, stdin) == NULL) {
+                    continue;
+                }
+
+                printf("%s", &buf[lines * 256]);
+                fflush(stdout);
+
+                ++lines;
+            } else {
+                --scroll;
+                reprint(buf, lines - height - scroll, height, lines);
+            }
+
+            continue;
+        } else if (key[0] == 'k') {
+            if (lines >= height) {
+                size_t max_scroll = lines - height;
+                if (scroll < max_scroll) {
+                    ++scroll;
+
+                    reprint(buf, lines - height - scroll, height, lines);
+                }
+            }
+        }
+    }
+
+    tcsetattr(tty_fd, TCSANOW, &old_termios);
+    close(tty_fd);
+
+    return 0;
+}