file_write_read.c: move from cpp-cheat

Improve README C section with example tree.

userland/arch/x86_64/dec.S

@@ -5,5 +5,5 @@
 LKMC_PROLOGUE
     mov $3, %rax
     dec %rax
-    LKMC_ASSERT_EQ_32(%rax, $2)
+    LKMC_ASSERT_EQ(%rax, $2)
 LKMC_EPILOGUE

userland/c/file_write_read.c

@@ -0,0 +1,136 @@
+#include <lkmc.h>
+
+#include <assert.h>
+#include <string.h>
+#include <stdio.h>
+
+/* Returns the size of the given open `FILE*`.
+ *
+ * If an error occurs, returns `-1L`.
+ *
+ * Does not work for pipes.
+ */
+long fget_file_size(FILE *fp) {
+    long oldpos;
+    long return_value;
+    oldpos = ftell(fp);
+    if (oldpos == -1L) {
+        return -1L;
+    }
+    if (fseek(fp, 0, SEEK_END) != 0) {
+        return -1L;
+    }
+    return_value = ftell(fp);
+    if (return_value == -1L) {
+        return -1L;
+    }
+    /* Restore the old position. */
+    if (fseek(fp, oldpos , SEEK_SET) != 0) {
+        return -1L;
+    }
+    return return_value;
+}
+
+/* Same as `file_size`, but takes the path instead of a `FILE*`. */
+long file_size(char *path) {
+    FILE *fp;
+    long return_value;
+    fp = fopen(path, "r");
+    if (fp == NULL) {
+        return -1L;
+    }
+    return_value = fget_file_size(fp);
+    if (fclose(fp) == EOF) {
+        return -1L;
+    }
+    return return_value;
+}
+
+/* Read the entire file to a char[] dynamically allocated inside this function.
+ *
+ * Returns a pointer to the start of that array.
+ *
+ * In case of any error, returns NULL.
+ *
+ * http://stackoverflow.com/questions/174531/easiest-way-to-get-files-contents-in-c
+ */
+char *file_read(char *path) {
+    FILE *fp;
+    char *buffer;
+    long fsize;
+
+    fp = fopen(path , "rb");
+    if (fp == NULL) {
+        return NULL;
+    }
+    fsize = fget_file_size(fp);
+    if (fsize < 0){
+        fprintf(stderr, "could not determine lenght of:\n%s\n", path);
+        return NULL;
+    }
+    buffer = (char*)malloc(fsize);
+    if (buffer == NULL) {
+        return NULL;
+    }
+    if (fread(buffer, 1, fsize, fp) != (size_t)fsize) {
+        return NULL;
+    }
+    if (fclose(fp) == EOF){
+        return NULL;
+    }
+    return buffer;
+}
+
+/* Write a null terminated string to file
+ *
+ * Return -1 on failure, 0 on success.
+ */
+int file_write(char *path, char *write_string) {
+    long len;
+    FILE *fp;
+
+    fp = fopen(path, "wb");
+    if (fp == NULL) {
+        return -1;
+    }
+    len = strlen(write_string);
+    /* copy the file into the buffer: */
+    if (fwrite(write_string, 1, len, fp) != (size_t)len) {
+        return -1;
+    }
+    if (fclose(fp) == EOF) {
+        return -1;
+    }
+    return 0;
+}
+
+int main(void) {
+    char *path = LKMC_TMP_FILE;
+    char *input = "asdf\nqwer\n";
+
+    /* Write entire string to file at once. */
+    if (file_write(path, input) == -1) {
+        LKMC_IO_ERROR("file_write", path);
+    }
+
+    /* Read entire file at once to a string. */
+    {
+        char *output = file_read(path);
+        if (output == NULL) {
+            LKMC_IO_ERROR("file_read", path);
+        }
+        assert(strcmp(input, output) == 0);
+        free(output);
+    }
+
+    /* Get file size. */
+    {
+        long size = file_size(path);
+        if (size == -1) {
+            LKMC_IO_ERROR("file_size", path);
+        }
+        assert((size_t)size == strlen(input));
+    }
+
+    return EXIT_SUCCESS;
+}

userland/c/infinite_loop.c

@@ -1,4 +1,6 @@
-/* Loop infinitely. Print an integer whenever a period is reached:
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#c
+ *
+ * Loop infinitely. Print an integer whenever a period is reached:
  *
  * ....
  * ./infinite_loop [period]