Function to decrypt file in C? - c

Question

I have a file with a large number of 'prospects', so each line has an encrypted last name, an encrypted first name, a 12 digit id code, and then 4 ratings (3 ints, 1 float). Encryption is shifting every character of the names by the value of the last number in the file (found to be 310).
Attempted to create a function to decrypt 1 character, then another function to decrypt a string (name) using this function, but am getting errors and segmentation faults, please help!
#include <stdio.h>
#include <stdlib.h>
#define MSTRLEN 20
#define MAX_SIZE 1000
/* structure prototype */
typedef struct {
char lastname[MSTRLEN];
char firstname[MSTRLEN];
char secretcode[13];
int rank1;
int rank2;
float rank3;
int rank4;
} prospect_t;
int main (void)
{
FILE *ifile;
prospect_t *prospects;
char last[MSTRLEN],first[MSTRLEN],code[13],last_name,first_name;
int r1,r2,r4,num_prospects,shift,i,j;
float r3;
char unencrypt_letter(char *letter, int shift);
char unencrypt_name(char name[MSTRLEN], int shift);
/*finding how many prospects and last integer*/
ifile = fopen("prospects.txt","r");
num_prospects = 0;
if (ifile == NULL){
printf("File not found!\n");
return (-1);
}
while (fscanf(ifile,"%s %s %s %d %d %f %d",last,first,code,&r1,&r2,&r3,&r4)!=EOF){
num_prospects++;
}
shift = r4%26;
fclose(ifile);
/*--------------------------------------*/
/* dynamic memory allocation */
prospects = (prospect_t*)malloc(num_prospects*sizeof(prospect_t));
ifile = fopen("prospects.txt","r");
if (ifile == NULL){
printf("File not found!\n");
return (-1);
}
for(i=0;i<num_prospects;i++){
fscanf(ifile,"%s %s %s %d %d %f %d", prospects[i].lastname,prospects[i].firstname,prospects[i].secretcode,&prospects[i].rank1,&prospects[i].rank2,&prospects[i].rank3,&prospects[i].rank4);
}
/* to be used once get working
for(j=0;j<num_prospects;j++){
prospects[j].lastname = unencrypt_name(prospects[j].lastname,shift);
prospects[j].firstname = unencrypt_name(prospects[j].firstname,shift);
}
*/
/* to be taken out once working */
last_name = unencrypt_name(prospects[0].lastname,shift);
first_name = unencrypt_name(prospects[0].firstname,shift);
printf("%s %s\n",last_name,first_name);
fclose(ifile);
free(prospects);
return(0);
}
/* function to unencrypt one letter */
char unencrypt_letter(char *letter, int shift)
{
char *new_letter;
if ((*letter - shift) < 'a')
*new_letter = (char)((*letter - shift) + 26);
else
*new_letter = (char)(*letter - shift);
return(*new_letter);
}
/* function to unencrypt a name */
char unencrypt_name(char name[MSTRLEN],int shift)
{
char new_name[MSTRLEN];
int k;
k = 0;
while (name[k] != '\0'){
new_name[k] = unencrypt_letter(name[k],shift);
k++;
}
return(*new_name);
}
From terminal, I am getting the following:
la2.c: In function ‘main’:
la2.c:68:2: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘int’ [-Wformat]
la2.c:68:2: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘int’ [-Wformat]
la2.c: In function ‘unencrypt_name’:
la2.c:99:3: warning: passing argument 1 of ‘unencrypt_letter’ makes pointer from integer without a cast [enabled by default]
la2.c:79:6: note: expected ‘char *’ but argument is of type ‘char’
** Link phase
gcc -o la2 la2.o
Compile and link completed successfully
Your binary can be run by typing:
la2
engs20-1:~/engs20/workspace$ la2
Segmentation fault

Answer 1

Read the warnings again, they are quite clear:
la2.c:68:2: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘int’
It tells you that the second argument to the printf call on line 68 is expected to be a string (char *) but you pass an integer (actually a single char, but the compiler casts it to int) as that argument instead.
Later when you run the program printf uses that integer as a pointer to a string, and as that integer is not a proper integer the program crashes.

Answer 2

Assuming the 2nd fscanf is line 68
fscanf(ifile,"%s %s %s %d %d %f %d", prospects[i].lastname,prospects[i].firstname,prospects[i].secretcode,&prospects[i].rank1,&prospects[i].rank2,&prospects[i].rank3,&prospects[i].rank4);
I'd stick some () in to make sure you are getting the address of what you think.
&(prospects[i].lastname), ...
You don't pre declare char unencrypt_letter(char *letter, int shift) before you call it, so the compiler assumes it returns an int.
Add a predeclaration of it before main().