File

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define ERROR(arg)          \
    { fprintf(stderr, "%s", arg);   \
      exit(EXIT_FAILURE);       \
    }

typedef struct data
{
    int id;
    char *message;
} data_t;

enum {
    WRITE_ = 0,
    READ_
} action;

int data_manager(FILE *fp, data_t *buffer, size_t size, char *args,
         signed char option, unsigned int id)
{
    signed char ret;

    buffer->message = malloc((size+1) * sizeof(char));
    if (buffer->message == NULL)
        ERROR("memory can't be allocated");
    buffer->id = 0;
    memset(buffer->message, 0, size);

    if(option == WRITE_) {
        buffer->id = id;
        strcpy(buffer->message, args);

        /* Write a stream on the
         * file specified.
         */
        ret = fwrite((data_t *)buffer, sizeof(data_t), 1, fp);
    } else if (option == READ_)
        /* Read a stream from the
         * file specified.
         */
        ret = fread((data_t *)buffer, sizeof(data_t), 1, fp);
    else
        return -1;

    if (ret < 0)
        return -1;

    return 0;
}

int main(int argc, char *argv[])
{
    FILE *fp;
    signed char ret, c;
    size_t size;
    data_t send, receive;

    if (argc != 3) {
        fprintf(stderr, "usage: %s file message\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    /* Open for reading and writing
     * (overwrite file).
     */
    fp = fopen(argv[1], "w+");
    if (fp == NULL)
        ERROR("Could not open file!\n");

    size = strlen(argv[2]);
    ret = data_manager(fp, &send, size, argv[2], WRITE_, 1);
    if(ret < 0)
        ERROR("'fwrite' does not work");

    ret = fseek(fp, 0, SEEK_SET);
    if(ret < 0)
        ERROR("'fseek' does not work");

    ret = data_manager(fp, &receive, size, NULL, READ_, 0);
    if(ret < 0)
        ERROR("'fread' does not work");

    fprintf(stdout, "File content: %d %d %s\n", READ_, receive.id, receive.message);

    free(receive.message);
    fclose(fp);
    return EXIT_SUCCESS;
}