c中改变配置文件的某一个配置的值

int get_set_variable_val(unsigned char* file_name, unsigned char* variable_name, unsigned char* dest_val, char mode)

{
    int i=0,t1=0;
    FILE *p;
    char ch;
    int Count=0;
    int dest_len = strlen(variable_name);
    int file_size = -1;
    unsigned char* orig_file;
    int switch_pos = -1;
    unsigned char pos_char = 0;
    unsigned char *tmp = NULL;

    if(mode == 0){
        memset(dest_val, 0, 32);
    }
    else if(mode == 1){
        if(NULL == dest_val)
            return -1;
    }

    if((p=fopen(file_name, "rb"))==NULL)
    {
        fprintf(stderr, "can not open file %s \n", file_name);
        return -1;
    }
    fseek( p, 0L, SEEK_END );
    file_size = ftell(p);
    fseek( p, 0L, SEEK_SET );

    ch=fgetc(p);
    while( !feof(p))
    {
        if(t1 < dest_len){
            if(ch==variable_name[t1])
                t1++;
            else
                t1=0;
        }
        else{
            if(ch == 0xa || ch == 0xd || (pos_char != 0 && ch == pos_char))
                break;
            if(ch != '='){
                if(mode == 0){
                    dest_val[Count++] = ch;
                }
                else if(mode == 1 && switch_pos == -1){
                    if(ch != '\'' && ch != '\"')
                        switch_pos = i;
                    else{
                        pos_char = ch;
                        switch_pos = i;
                    }
                }
            }
        }
        i++;
        ch=fgetc(p);
    }

    if(mode == 1){
        orig_file = (unsigned char*)malloc(file_size + (strlen(dest_val) - (i - switch_pos)) + 5);

        if(orig_file == NULL){
            fprintf(stderr, "malloc memory error, exit \n");
            return -1;
        }
        memset(orig_file, 0, file_size + (strlen(dest_val) - (i - switch_pos)) + 4);
        fseek(p, 0, SEEK_SET);

        if(pos_char != 0){
            fread(orig_file, switch_pos+1, 1, p);
        }
        else{
            fread(orig_file, switch_pos, 1, p);
        }

        strcat(orig_file, dest_val);
        fseek(p, i, SEEK_SET);
        tmp = (unsigned char*)malloc(file_size - i + 2);
        memset(tmp, 0, file_size - i);
        fread(tmp, 1, file_size - i, p);
        tmp[file_size-i] = '\0';
        //fread(orig_file + strlen(orig_file) + 1, 1, file_size - i, p);
        strcat(orig_file, tmp);
        //dbg("get_set_variable_val fist call free \n");

        free(tmp);
        fclose(p);
        update_file(file_name, orig_file, strlen(orig_file));
        //dbg("get_set_variable_val second call free \n