use fgets in c language to read windows text file, but cannot read a line of content. The text file content protects NUL special characters such as ^ @

-sharpinclude<stdio.h>
-sharpinclude<stdlib.h>
-sharpinclude<string.h>

char *filter(char *str)
{
    int len, i, j=0;
    char content[256];

    len = strlen(str);
    printf("=>input len:%d\n", len);
    for (i=0; i< len; iPP) {
        //32=>
        if ((str[i] < "0" || str[i] > "z") && str[i] != 32) {
            continue;
        }
        content[j]=str[i];
        jPP;
    }

    strcpy(str, content);

    return str;
}

int main()
{
    char *filename = "a.txt"; //"b.txt";
    char content[256], *content_cpy;
    FILE *fp;

    fp = fopen(filename, "r");

    while(NULL != fgets(content, 1024, fp)){
        content_cpy = filter(content);       

        printf("=>%s\n", content_origin);
    }

    fclose(fp);

    return 0;
}

text file content is similar:

"[^@S^@y^@s^@t^@e^@m^@]^@^M^@"