printf() function causes segfault - c

Question

This code when compiled allows me to connect to a server using a correct IP address as the program parameter. So far, the connection is fine and data is sent to the server, but when I attempt to use the recv() function to try to collect data in non-blocking mode, I receive a segmentation fault. It always appears after the "receiving data..." line.
As you can see, I reserved 5000 bytes of stack memory for the operation and I'm testing with trying to read only one byte without success. Does recv() not work with stack memory?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
extern errno;
struct sockaddr_in a;
struct timeval tv;
fd_set wready;
long conntoserver(const char* ip,const char* data){
memset((char*)&a,0,sizeof(a));
if (inet_aton(ip,a.sin_addr.s_addr) == 0){printf("Can't set address\n");return -1;}
long s=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);if (s < 0){printf("Can't make socket\n");return -1;}
tv.tv_sec=1;tv.tv_usec=0;FD_ZERO(&wready);FD_SET(s,&wready);
//start non-blocking mode
if (fcntl(s,F_SETFL,O_NONBLOCK) < 0){printf("Can't make socket non-blocking\n");close(s);return -1;}
a.sin_family=AF_INET;a.sin_port=htons(80);long ret=connect(s,(struct sockaddr*)&a,sizeof(a));
if (ret < 0 && errno != EINPROGRESS){printf("Can't connect to IP. %s\n",strerror(errno));close(s);return -1;}
if (errno == EINPROGRESS){
printf("connection in progress...\n");
ret=select(s+1,NULL,&wready,NULL,&tv);
if (ret < 0){printf("select() error. %s\n",strerror(errno));close(s);return -1;}
if (ret==0){printf("select() timeout. %s\n",strerror(errno));close(s);return -1;}
}
unsigned long sz=strlen(data);
ssize_t ns=send(s,data,sz,0);
if (ns==sz){return s;}else{return -1;}
}
int main(int argc,char* argv[]){
if (argc < 2){printf("Format: %s < IP to connect to >\n",argv[0]);return -1;}
long s=conntoserver(argv[1],"GET / HTTP/1.1\nHost: example.com\n\n");
if (s < 0){printf("Error making server request. %s\n",strerror(errno));close(s);return -1;}
printf("receiving data...\n");
char b[5001];
ssize_t br=recv(s,b,1,0);
printf("br=%s\n",br);
close(s);
return 0;
}

Answer 1

printf("br=%s\n",br);
Should be:
printf("br=%ld\n",br);
BTW -- I had to fix a number of problems to make this code compile. You should always pay attention to compiler warnings.

Answer 2

In addition to the error pointed out by #keithmo, you have an error here:
if (inet_aton(ip,a.sin_addr.s_addr) == 0){printf("Can't set address\n");return -1;}
The second argument to inet_aton() is expected to be a pointer to a struct in_addr but you are passing a value of type uint32_t. In fact, that value is initialized to zero, so the result is likely (but not certain) to be equivalent to passing a NULL pointer. Either way, this produces undefined behavior, and I'd surprised if that didn't manifest as a segfault. Even if the program runs past that, the call as you wrote it cannot produce the (apparently) desired effect of initializing the address in variable a.
What you want instead is
if (inet_aton(ip, &a.sin_addr) == 0) {
printf("Can't set address\n");
return -1;
}
And for goodness's sake, do make free with the spaces and newlines. They greatly improve code legibility. If you're short, I can lend you some.