Check file handle in C using system calls

chp2/select_regular_files.c

@@ -0,0 +1,132 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <sys/time.h>
+#include <sys/select.h>
+
+#define FILE_NUMBER 3
+#define FILENAME_MAX_LEN 10
+#define TIMEOUT 10
+#define BUF_LEN 1024
+
+int create_tmp_file(char *filename);
+void cleanup(char *filenames[]);
+
+int main(void) {
+	int exit_code = 0;
+	char *filename_template = "test%d.txt";
+	char *filenames[FILE_NUMBER] = {NULL};
+	int fds[FILE_NUMBER] = {0};
+
+	// Create a bunch of number ephemeral files
+	for (int i = 0; i < FILE_NUMBER; i++) {
+		char *filename = (char *)malloc(FILENAME_MAX_LEN*sizeof(char)); 
+		if (!filename) {
+			perror("malloc failed");
+			exit_code = -1;
+			goto exit;
+		}
+		snprintf(filename, FILENAME_MAX_LEN, filename_template, i+1);
+		filenames[i] = filename;
+		printf("Opening file %s...\n", filename);
+		fds[i] = create_tmp_file(filename);
+		if (fds[i] == -1) {
+			fprintf(stderr, "Error to open file %s: %s\n", filename, strerror(errno));
+			exit_code = -1;
+			goto exit;
+		}
+		printf("Open file %s with file descriptor %d\n", filename, fds[i]);
+
+		// write something in the files
+		dprintf(fds[i], "Hello I am %s\n", filename);
+	}
+	
+	fd_set readfds;
+	FD_ZERO(&readfds);
+	for (int i = 0; i < FILE_NUMBER; i++) {
+		FD_SET(fds[i], &readfds);
+	}
+	
+	struct timeval *timeout = (struct timeval *)malloc(sizeof(struct timeval));
+	timeout->tv_sec = TIMEOUT;
+	timeout->tv_usec = 0;
+	
+	/* This function do not work with regular files because for the operating system,
+	 * a regular file is ready always.
+         * This example is wrong the best way to use select() is with socket, pipe or other channel types.
+	 */
+	int ret = select(
+		FILE_NUMBER + 1,
+		&readfds,
+		NULL,
+		NULL,
+		timeout
+	);
+
+	if (ret == -1) {
+		fprintf(stderr, "Error to select: %s\n", strerror(errno));
+		exit_code = 1;
+		goto exit;
+	} else if (!ret) {
+		fprintf(stderr, "%d seconds elapsed.\n", TIMEOUT);
+		exit_code = 1;
+		goto exit;
+	}
+
+	for (int i = 0; i < FILE_NUMBER; i++) {
+		int fd = fds[i];
+		if (FD_ISSET(fd, &readfds)) {
+			char buf[BUF_LEN+1];
+			int len = read(fd, buf, BUF_LEN);
+			if (len == -1) {
+				fprintf(stderr, "Error reading file %s: %s\n", filenames[i], strerror(errno));
+				exit_code = 1;
+				goto exit;
+			}
+			if (len) {
+				buf[len] = '\0';
+				printf("%s:\n%s\n", filenames[i], buf);
+			}
+		}
+	}
+
+exit:
+	// Closing and remove all creating files
+	for (int i = 0; i < FILE_NUMBER; i++) {
+		int fd = fds[i];
+		printf("Close file with file descriptor %d\n", fd);
+		if (close(fd) == -1) { 
+			fprintf(stderr, "Error to close file %d: %s\n", fd, strerror(errno));
+		}
+	}
+
+	free(timeout);
+	timeout = NULL;
+
+	printf("Cleanup files\n");
+	cleanup(filenames);
+	
+	return exit_code;
+}
+
+int create_tmp_file(char *filename) {
+	int fd = open(filename, O_CREAT | O_EXCL | O_RDWR, 0644);
+	if (fd == -1) {
+		return -1;
+	}
+	return fd;
+}
+
+void cleanup(char *filenames[]) {
+	for (int j = 0; j < FILE_NUMBER; j++) {
+		if (filenames[j] != NULL) {
+			if (remove(filenames[j]) == -1) 
+				fprintf(stderr, "Failed to remove %s: %s\n", filenames[j], strerror(errno));
+			free(filenames[j]);
+			filenames[j] = NULL;
+		}
+	}
+}