example of C program that writes data to itself. Works under Linux only.

main.c

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

#include <unistd.h>
#include <fcntl.h>


typedef int var_t;

var_t getSavedVar(const char * exec_filename);
void saveNewVar(const var_t newVar, const char * exec_filename);

int main(int argc, char * argv[]) {
    const var_t savedVar = getSavedVar(argv[0]);
    printf("saved var = %d\n", savedVar);
    
    var_t newVar = 0;
    if ( argc > 1 ) {
        newVar = atoi(argv[1]);
    }
    
    if ( newVar > 0 ) {
        printf("new var value = %d\n", newVar);
        saveNewVar(newVar, argv[0]);
    }
}


var_t getSavedVar(const char * exec_filename) {
    FILE * fd_this_exec = fopen(exec_filename, "r");
    fseek(fd_this_exec, -(sizeof(var_t)), SEEK_END);
    
    var_t savedVar = -1;

    size_t read_res = fread((void *)&savedVar, sizeof(var_t), 1, fd_this_exec);
    assert(read_res == 1);

    return savedVar;
}


#define TEMP_STRING_LEN 3
const char g_temp_file_prefix_str[TEMP_STRING_LEN] = "_cp";

int copy_file(const char *source, const char * dest);

void saveNewVar(const var_t newVar, const char * exec_filename) {
        char tmp_filename[strlen(exec_filename)+TEMP_STRING_LEN]; 
        strncpy(tmp_filename, exec_filename, strlen(exec_filename));
        strncat(tmp_filename, g_temp_file_prefix_str, TEMP_STRING_LEN);

        int cp_res = copy_file(exec_filename, tmp_filename);
        assert(cp_res == 0);

        FILE * tmp_file = fopen(tmp_filename, "a");
        if ( tmp_file > 0 ) {
            fwrite((void *)&newVar, sizeof(newVar), 1, tmp_file);
            fclose(tmp_file);
        } else {
            perror("fopen()");
            return;
        }

        int remove_res = remove(exec_filename);
        if ( remove_res != 0 ) {
            perror("remove()");
            return;
        }

        int rename_res = rename(tmp_filename, exec_filename);
        if ( rename_res != 0 ) {
            perror("rename()");
            return;
        }
}


int copy_file(const char *source, const char * dest) {
    int fd_source = open(source, O_RDONLY);
    int fd_dest   = open(dest, O_CREAT|O_WRONLY, S_IRWXU);

    if ( fd_source < 0 || fd_dest < 0 ) return -1;

    char buf[8192];
    while(1) {
        ssize_t read_res = read(fd_source, &buf[0], sizeof(buf));
        if ( read_res < 0 ) {
            perror("read()");
            return -1;
        } else if ( read_res == 0 ) {
            break;
        }

        ssize_t write_res = write(fd_dest, &buf[0], read_res);
        if ( write_res == -1 ) {
            perror("write()");
            return -1;
        }     
    }

    close(fd_source);
    close(fd_dest);

    return 0;
}