CODEWITHSUNDEEP
cstring
user2711681
user2711681

Reputation: 275

C inserting string into char pointer

Hi all I trying to assign string into a char * pointer. Below is how I am doing but I am getting this warning: assignment makes integer from pointer without a cast.

What is the correct why to deal with strings in C ?

char* protoName = "";

if(h->extended_hdr.parsed_pkt.l3_proto==0)
    *protoName = "HOPOPT";  
else if(h->extended_hdr.parsed_pkt.l3_proto==1)
    *protoName = "ICMP";    
else if(h->extended_hdr.parsed_pkt.l3_proto==2)
    *protoName = "IGMP";    
else if(h->extended_hdr.parsed_pkt.l3_proto==3)
    *protoName = "IGMP";    
else if(h->extended_hdr.parsed_pkt.l3_proto==4)
    *protoName = "IGMP";

char all[500];

sprintf(all,"IPv4','%s'",* protoName);

Upvotes: 5

Views: 3810

Answers (5)

dcaswell
dcaswell

Reputation: 3167

Here's some examples:

#include <stdio.h>

const char * protoNameFromPktId(int id) {
    static char* protoName[] = { "HOPOPT", "ICMP", "IGMP", "IGMP","IGMP"};
    return protoName[id];
}

main() {
   printf("%s\n", protoNameFromPktId(2));
   char all[500];
   sprintf(all,"%s", protoNameFromPktId(2));
   strcpy(all, protoNameFromPktId(2));
}

Upvotes: 2

Christos Papoulas
Christos Papoulas

Reputation: 2568

The main problem that I can see is that you dont bind memory for the strings, so you have to use malloc or better use strdup function that allocates memory automatically. because if you assigned big strings then you should have a problem. The problem with the warning answered by the others, so is ok. plz correct me if I'm wrong.

char* protoName;
if(h->extended_hdr.parsed_pkt.l3_proto==0){
   protoName = strdup("HOPOPT");    
}
else if(h->extended_hdr.parsed_pkt.l3_proto==1){
  protoName = strdup("ICMP");  
...
char all[500];
sprintf(all,"IPv4','%s'",* protoName);
free(protoName);

Upvotes: 1

simonc
simonc

Reputation: 42165

If you just want to change which string literal protoName points to, you just need to change

*protoName = "HOPOPT";

to

protoName = "HOPOPT";

*protoName = attempts to write to the first character pointed to by protoName. This won't work in your case as protoName points to a string literal which cannot be modified.

You also need to change your sprintf call to

sprintf(all,"IPv4','%s'", protoName);

The %s format specifier signals that you'll be passing a pointer to a nul-terminated char array. *protoName gives you the character code of the first character pointed to by protoName; sprintf doesn't know this so would treat that character code as the address of the array to read from. You don't own this memory so the effects of reading from it would be undefined; a crash would be likely.

As an aside, if you had a writeable char array and wanted to change its contents, you'd need to use strcpy to copy a new array of chars into it.

Upvotes: 8

Rahul Tripathi
Rahul Tripathi

Reputation: 172418

You just need to do this:-

protoName = "HOPOPT";

instead of 

*protoName = "HOPOPT";

So change like :-

char* protoName = "";
  if(h->extended_hdr.parsed_pkt.l3_proto==0)
  protoName = "HOPOPT";    
  else if(h->extended_hdr.parsed_pkt.l3_proto==1)
  protoName = "ICMP";  
  else if(h->extended_hdr.parsed_pkt.l3_proto==2)
  protoName = "IGMP";  
  else if(h->extended_hdr.parsed_pkt.l3_proto==3)
  protoName = "IGMP";  
  else if(h->extended_hdr.parsed_pkt.l3_proto==4)
  protoName = "IGMP";

  char all[500];
    sprintf(all,"IPv4','%s'",* protoName);

Upvotes: 1

trojanfoe
trojanfoe

Reputation: 122391

If you are using constants, then simply reassign the pointer, not the contents:

const char* protoName = "";
if(h->extended_hdr.parsed_pkt.l3_proto==0)
    protoName = "HOPOPT";    
else if(h->extended_hdr.parsed_pkt.l3_proto==1)
    protoName = "ICMP";  
else if(h->extended_hdr.parsed_pkt.l3_proto==2)
    protoName = "IGMP";  
else if(h->extended_hdr.parsed_pkt.l3_proto==3)
    protoName = "IGMP";  
else if(h->extended_hdr.parsed_pkt.l3_proto==4)
    protoName = "IGMP";

Upvotes: 2

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