Changing a c code to work line by line

Question

#include <mhash.h>
#include <stdio.h>
#include <stdlib.h>

int main(void)
 {
        int i;
        MHASH td;
        unsigned char buffer;
        unsigned char *hash;

        td = mhash_init(MHASH_WHIRLPOOL);

        if (td == MHASH_FAILED) exit(1);

        while (fread(&buffer, 1, 1, stdin) == 1) {
             mhash(td, &buffer, 1);
        }

        hash = mhash_end(td);

        for (i = 0; i < mhash_get_block_size(MHASH_WHIRLPOOL); i++) {
            printf("%.2x", hash[i]);
        }
        printf("\n");

        exit(0);
 }

Hi, I have above code from the mhash example page. I need to change it, so It will keep reading from stdin, and calculate the hash line by line, instead of waiting for EOF

cat textfile | whirlpool_line_hash

My understanding is that I keep the while loop (which waits for the EOF) and make the hash calculation and print after I received a 10 (0x0a). After the print mhash needs to be reset, right? I am not into C at all, but I need a fast program, so I want to do it in C. I already fail at comparing the pointer to an integer ;-) Can someone please help?

Answer 1

It works except one little caveat, the hash buffer returned by mhash_end is a dynamically allocated buffer, so it's better to free it when you are done using it:

free(hash);

You can use fgets which will read one line at the most. In terms of performance, read and feed into hash a char each time probably is not the best thing to do, instead, you can read an entire line and feed the block into hash function for update. Try this:

char line[4096];
int len;
while (fgets(line, sizeof line, stdin) != NULL) { // read a line each time
    len = strlen(line);
    char *p = strrchr(line, '\n');
    if (p != NULL)
        mhash(td, line, len - 1);  // strip the new line
    else
        mhash(td, line, len);
}

hash = mhash_end(td);
for (i = 0; i < mhash_get_block_size(MHASH_WHIRLPOOL); i++) {
    printf("%.2x", hash[i]);
}
free(hash);

Answer 2

got it done ;-)

#include <mhash.h> //mhash
#include <stdio.h>
#include <stdlib.h> //exit
#include <unistd.h> //getopt


int main(void)
 {

        int i;
        MHASH td;
        unsigned char buffer;
        unsigned char *hash;

        td = mhash_init(MHASH_WHIRLPOOL);

        if (td == MHASH_FAILED) exit(1);

        while (fread(&buffer, 1, 1, stdin) == 1) { // read from stdin until receive EOF

        if ( buffer != '\n' ) { mhash(td, &buffer, 1); } //dont calculate line break
        if ( buffer == '\n' ) {  //received line break
        hash = mhash_end(td);

        for (i = 0; i < mhash_get_block_size(MHASH_WHIRLPOOL); i++) { printf("%.2x", hash[i]); }
        printf("\n");
        td = mhash_init(MHASH_WHIRLPOOL);

}
}
        exit(0);
 }