CODEWITHSUNDEEP
csockets
testermaster
testermaster

Reputation: 1065

Read socket and memorization not working as expected

I've the following problem with this code:

#define success "success"
#define fail "fail"
char *verify = fail;
char b[1024];

int main(){
   ...
   ...connect to server code...
   ...
   read(sock,verify,1024);
   printf("%s",verify); //there's the problem, it always prints fail. If I change with this:
   read(sock,b,1024);
   printf("%s",b); //this works and prints the received string.
}

Can you help me to understand? The problem is in this part of this code or should I check elsewhere?

If I do an easy assign in the code like 'verify=success' it seems to works fine, reading from a socked isn't the same as an asisgnation?

Upvotes: 0

Views: 47

Answers (1)

SzG
SzG

Reputation: 12629

You try to read from the socket into the memory area pointed by verify, which is "fail". In C the expression "fail" allocates 5 bytes in read-only memory, fills them with {'f','a','i','l','\0'} in compile time and returns its address.

So you're trying to overwrite read-only memory. I'm wondering why you don't get a segmentation fault. Did you check the return value of read? It might be a kernel-side sanity check within read which prevents it from attempting to write into RO memory.

In the second read call, it's OK, because b points to the start of the array variable, which is writable.

You're actually confusing yourself with your own coding style: defining fail, which looks like a variable, does not make it a variable. The C preprocessor just replaces it and you end up with

char *verify = "fail";

Upvotes: 1

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