Using 'printf' on a 'struct'

skywalker27182 · 12-13-2003, 07:33 AM

hi,
i have a C structure, say, with the following contents:

struct abc
{
int x;
char ch[16];
int y;
} mnp[4];

from a program, i get the values of struct abc mnp[1]. i enter the values as:

x = 1;
ch = "abcdef";
y = 100;
i want the values in a file, so i use fprintf. but in the file, i see the output as:

1abcdef100

i there any way i can get the values exactly as the sizes of int and char, like
x should occupy 4 bytes(size of int) in the output file, char ch[] should occupy 16 bytes, and the y should occupy 4 bytes?
i want this because, say, i have a record of all 5 employees and i want to access the details of 4th employee. i must be able to access the details using lseek = (n - 1)*sizeof record.

kev82 · 12-13-2003, 07:48 AM

use fwrite to write out the binary representation of the data, the following example code should help

Code:

#include <stdio.h>
#include <string.h>

typedef struct {
        int x;
        char ch[16];
        int y;
} C;

int main()
{
        FILE *f=NULL;
        C c;

        c.x=1;
        strcpy(c.ch, "abcdef");
        c.y=100;

        f=fopen("filename", "w");
        fwrite(&c, sizeof(c), 1, f);
        fclose(f);

        return 0;
}

ive not tested it but it looks ok, if it doesnt work write back.

dorian33 · 12-13-2003, 07:53 AM

Use width & precision in the format string f.ex.

Code:

fprintf(fpnt, "%4.4i%16.16s%4.4i",mnp[1].x,mnp[1].ch,mnp[1].y);

or more complicated formatting with asterisks:

Code:

fprintf(fpnt, "%*.*i%*.*s%*.*i",
                   <len_of_x_field>,<len_of_x_field>,mnp[1].x,
                   <len_of_ch_field>,<len_of_ch_field>,mnp[1].ch,
                   <len_of_y_field>,<len_of_x_field>,mnp[1].y);

where <len_of_..._field>s can be a variables also.

skywalker27182 · 12-14-2003, 10:38 PM

hi kev82 and dorian33,
i tried out your methods.

kev, i saw a few junk characters in the output file like ^@^B etc, i tried opening the outfile with "a+" and by fwrite-ing one member at a time to the file, but the results were the same.

dorian, ur method worked. i didn't know this syntax, i learned something new.

thanks a lot to both of u.

kev82 · 12-15-2003, 07:30 AM

by skywalker27182
i there any way i can get the values exactly as the sizes of int and char, like
x should occupy 4 bytes(size of int) in the output file, char ch[] should occupy 16 bytes, and the y should occupy 4 bytes?

thats what my method did, if you run hexdump on the file produced then you will see something like

Code:

00000000  01 00 00 00 61 62 63 64 65 66 00 42 5e 83 04 08
00000010  18 2e 13 42 64 00 00 00
00000018

the black numbers are offsets from start of file, blue is first integer, red is the 16 byte character array, and green is the second integer. if you want to read the data back into the program then simply declare enough memory and use fread() if you do want your data stored in ASCII form in the file then do as dorian33 said but its a big pain in the ass to read it back in. thats why binary files are so popular.

skywalker27182 · 12-15-2003, 11:40 AM

so the junk characters won't make any difference? i can still read the data with open, lseek and read, right?

h/w · 12-15-2003, 12:59 PM

maybe i didnt get your question right, but ...
when u frprintf, cant you append a '\0' or a '\n' to your variables, and make it look like

Code:

1
abcdef
4

?
then you can read your file in a loop based on '\n' or '\0'.
also, since you have 2 ints there, cant u just put those two in a struct of their own, and then just have the char array as the other element?

i fail to remember, but are nested structs allowed in C (im thinking it is)?

kev82 · 12-15-2003, 01:06 PM

by skywalker27182
so the junk characters won't make any difference?

there not junk characters, they are the ascii representation of your data in binary form.

im not sure why you wish to use open/read/write when you have C-standard streams available but you can use fopen, fread, fwrite, fseek, fclose or their unistd.h alternatives to work with and move through binary files, check out the man pages or any good book on C. if your still stuck with the idea of binary files i'll write you an example program when i get back home on wednesday.

skywalker27182 · 12-17-2003, 03:36 AM

pls do include a sample program kev82. thanks.

h/w, i have a record of, say, a 100 employees of a company and the details are name, id, salary etc. it becomes a lot easier to open the record file and go to the nth employee using lseek and reading the rest of the line if all lines of the file are of same size. if it isn't i have to hunt for the right employee number, which might be same as the salary. i hope i made it clear

kev82 · 12-18-2003, 08:50 AM

ok here ya go, the file format is quite simple, first it saves the number of records in the file(a 4-byte integer) then its just all the records one after another

code to create the file

Code:

#include <stdio.h>

typedef struct {
        int a;
        char b[16];
        int c;
} D;

int main()
{
        FILE *f=NULL;
        D *data=NULL;
        int n;
        int numEmployees=0;

        f=fopen("filename","w");

        printf("How many records?\n");
        scanf("%i", &numEmployees);

        fwrite(&numEmployees, sizeof(int), 1, f);

        for(n=0;n<numEmployees;n++) {
                D temp;

                printf("first integer?\n");
                scanf("%i", &temp.a);

                printf("character string?\n");
                scanf("%s", temp.b);

                printf("second integer?\n");
                scanf("%i", &temp.c);

                fwrite(&temp, sizeof(D), 1, f);
        }

        fclose(f);
        return 0;
}
~

code to read the file back

Code:

#include <stdio.h>

typedef struct {
        int a;
        char b[16];
        int c;
} D;

int main()
{
        FILE *f=NULL;
        int numRecs=0;
        int rec=0;
        D data;

        f=fopen("filename","r");
        fread(&numRecs, sizeof(numRecs), 1, f);

        while(1) {
                printf("file has %i records(0-%i) which one to look at?", numRecs, numRecs-1);
                scanf("%i", &rec);
                if(rec<0) break;

                fseek(f, sizeof(int)+rec*sizeof(D), SEEK_SET);
                fread(&data, sizeof(D), 1, f);

                printf("first integer %i, character string %s, second integer %i\n", data.a, data.b, data.c);
        }

        fclose(f);
        return 0;
}
~

i havnt programmed in C for a while so ive probably done something wrong but it seems to work. it needs A LOT of error checking code added if you wanted to use it for anything other than demonstration purposes.

instead of using streams you could quite as easily use open, close, read,write,lseek i dont think it makes any difference to the program.

you might consider using mmap() to access your records all the seeking would be done automatically for you and it would make adding/deleting records much easier.

ananthbv · 12-20-2003, 12:06 PM

thanks once again, kev82, i understand it much better now! it is simple yet elegant.