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#include <stdio.h>
int main(void)
{
    FILE *fp;
    char buf[256];
        
    fp = fopen("test.txt", "w+");
    fprintf(fp, "?");
    rewind(fp);
    fscanf(fp, "%s", buf);
    printf("in file: %s\n", buf);
    fclose(fp);
    return 0;
}

#include <stdio.h>
int main(void)
{
    FILE *fp;
    char buf[256];
    char buf1[256];    
    fp = fopen("test.txt", "w+");
    printf("vvedite inf v file");
    gets(buf1);
    fprintf(fp,buf1);
    fprintf(fp, "?");
    rewind(fp);
    fscanf(fp, "%s", buf);
    printf("in file: %s\n", buf);
    fclose(fp);
    return 0;
}

#include <windows.h>
#include <stdio.h>
 
void main()
{
    printf("After this mesage all symbols ");
    printf("will be writed into text file\r\n");
    printf("Note : for stop writing press Enter\r\n");
    char ch;FILE *f;
    if(!(f = fopen("test.txt","wb+")))
        printf("error while writing test.txt\r\n");
    else
    {
        while((ch = getchar()) != '\n')
            fprintf(f,"%c",ch);
        fclose(f);
    }
    printf("Press NUM1 for writing again\r\n");
    if(getchar() == '1')
    {
        getchar();//Для Enter после '1'
        main();
    }
}