C: write (and then read) a string to file using a hash function to locate the byte's position -

i want write 1kb file, , want use hash function calculate position of byte i'm going write. want able later retrieve data in correct order.

to locate position of byte read/write use hash function

f = (index*prime) mod 1024 

where index index in string, , prime prime number need avoid collisions, i.e. not rewriting 2 times in same position. f, strlen (b first create file

dd bs=1024 count=1 if=/dev/zero of=test.fs 

and after compile , run program passing "w" parameter

$ ./a.out w 

now, seems me write() function job correctly, because if do...

$ cat test.fs or $ hexdump test.fs 

... can see content of file consistent string inserted!

however, if run read-mode passing "r" parameter, curious random output, seems me if i'm reading thrash memory. cannot understand read() function fails, thank in advance.

the c code follows:

#include <stdio.h> #include <string.h>  #define prime 7919  int main(int argc, char *argv[]) {         int mode;          if (argc < 2)                 return 1;         else if (strcmp (argv[1], "r") == 0)                 mode = 1;         else if (strcmp (argv[1], "w") == 0)                 mode = 2;         else                 return 1;           file *fs;         char buf[1024];         int val;         int i;         char c;          if (mode == 1) {                 fs = fopen("test.fs", "rb");                 val = read (buf, 1024, fs);                 if (val != 1024)                         fprintf(stderr, "either error occurred, or eof reached.\n");                 printf("content read: %s\n", buf);         }         else if (mode == 2) {                 fs = fopen("test.fs", "wb");                 printf ("enter string want write disk: ");                 while ((c = getchar()) != eof)                          buf[i++] = c;                 buf[i] = eof;                  val = write (fs, buf, strlen (buf));                 if (val == eof)                         fprintf(stderr, "an error occurred while writing.\n");                 printf("%d bytes written disk.\n");         }          fclose (fs); }   int write(file *f, char *str, long len) {         int i;         int err;         (i=0; < len && err != eof; i++) {                 fseek(f, (i*prime)%1024, seek_set);                 err = fputc(str[i], f);         }         if (err != eof)                 return i;         else                 return eof; }   int read(char *buffer, long len, file *f) {         int i;         int br = 1;         (i=0; < len && br != 0; i++) {                 fseek(f, (i*prime)%1024, seek_set);                 br = fread (&buffer[i],1,1,f);         }         if (ferror (f))                 return eof;         else                 return i; } 

buffer not string, array of characters. when incorrectly print string:

printf("content read: %s\n", buf); 

you buffer overflow since array isn't null terminated.