How to see contents of char * since printf won't work?

Question

Here is a snippet of my code:

#include <stdio.h>
#include "uhash.h"
#include <openssl/evp.h>


char * hash(item  a)
{
    const char * str= a.k;
    int len= strlen(str);

    int md_len;
    unsigned char md_value[EVP_MAX_MD_SIZE];    /* Buff to store change result */
        EVP_MD_CTX *mdctx;              /* Digest data structure declaration */
        const EVP_MD *md;  
    OpenSSL_add_all_digests();
        md = EVP_get_digestbyname("SHA256");
        mdctx = EVP_MD_CTX_create();
        EVP_DigestInit_ex(mdctx, md, NULL);
        EVP_DigestUpdate(mdctx, str, len);
        EVP_DigestFinal_ex(mdctx, md_value, &md_len);
        EVP_MD_CTX_destroy(mdctx);

    char md5str[33];
    for(int i=0;i<md_len;++i)
    {
        sprintf(&md5str[i*2],"%02x",(unsigned int)md_value[i]);
    }
    printf("%s\n", md5str);
    return md5str;
}
int main(int argv, char **argc)
{
    char *c;
    if (argv>0)
    {
        int i=0;
        int s=0;
        for(i=1;i<argv;i++)
        {
            s+=strlen(argc[i]);
        }
        c=(char *)(malloc(sizeof(char)*s*(argv-2)+1));
        s=0;
        for(i=1;i<argv;i++)
        {
            char *t=c+s;
            memcpy(t,argc[i],strlen(argc[i]));
            if(i!=argv-1){
                    printf("%d\n", argv);
                    t[strlen(argc[i])]=' ';
                s++;
            }
            s+=strlen(argc[i]);
        }
        *(c+s)='\0';
    }
    printf("%s\n", c);
    item * kee= malloc(sizeof(item));
    kee->k=c;
    kee->v=10;
    char *res= hash(*kee);
    fflush(stdout);
    if(res==NULL)
        printf("result is null...");
    else
        printf("%s\n",res);
    fflush(stdout);
}

So the main function takes the arguments fine(tested) and passes it fine(tested) but the hash() function although computing the hash, does not either return the right value or I can't print the result in main. I've been trying to check for errors since yesterday but I'm not very smart so any help would be greatly appreciated!

The hash is probably a mix that includes non-printable characters and maybe `'\0'` too. I'd recommend scanning over the buffer and printing in hex -- but you need to know how many bytes there are in the `res` string. — Jonathan Leffler, Apr 11 '14 at 16:22
@JonathanLeffler is likely right. If you can give us your header file as well (`uhash.h`) we may be able to provide further assistance. — Cloud, Apr 11 '14 at 16:51

score 2 · Accepted Answer · answered Apr 11 '14 at 16:58

2

Root cause: You are returning something that has been created on the stack and once it returns it is wiped. res is pointing into your stack.

Fix: Put the declaration of md5str outside the routine hash or make it static within the routine hash.

answered Apr 11 '14 at 16:58

cup

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How to see contents of char * since printf won't work?

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