Mmap_file

mmap_file/.gitignore

@@ -0,0 +1 @@
+mapped.txt

mmap_file/Makefile

@@ -0,0 +1,12 @@
+CC ?= gcc
+CFLAGS ?= -Wall -O2
+
+TARGET = main.elf
+SRC = mmap_file.c
+
+$(TARGET): $(SRC)
+	@echo CC $@
+	@$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+clean:
+	rm -f $(TARGET)

mmap_file/mmap_file.c

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+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#define FILE_PATH "mapped.txt"
+#define FILE_SIZE 4096
+
+int main() {
+    int fd = open(FILE_PATH, O_RDWR | O_CREAT, 0644);
+    if (fd == -1) {
+        perror("open");
+        return 1;
+    }
+
+    /* We want some space to write */
+    if (ftruncate(fd, FILE_SIZE) == -1) {
+        perror("ftruncate");
+        close(fd);
+        return 1;
+    }
+
+    /* NULL means we do not care where the mapping goes, anywhere in canonical memory will do
+     * PROT_READ/WRITE are just protection flags that allows our process rw rights
+     * MAP_SHARED means our (real time, or close to) changes are visible to other processes mmping the same file */
+    void *file_memory = mmap(NULL, FILE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+    if (file_memory == MAP_FAILED) {
+        perror("mmap");
+        close(fd);
+        return 1;
+    }
+
+    const char *message = "This is written to file!";
+    strncpy(file_memory, message, strlen(message) + 1);
+
+    /* Re-truncate the file to known length, otherwise the entire buffer
+     * (4k, with garbage from whatever the kernel gave us) will be in the file*/
+    if (ftruncate(fd, strnlen(message, FILE_SIZE + 1)) == -1) {
+        perror("ftruncate");
+        close(fd);
+        return 1;
+    }
+
+    printf("Mapped region content: %s\n", (char *)file_memory);
+
+    munmap(file_memory, FILE_SIZE);
+
+    close(fd);
+
+    /* Optional finishing step, but i assume you want to inspect the file */
+    // remove(file_path);
+
+    return 0;
+}