How to save to a file in C?

darkangel29 · 12-22-2008, 07:22 PM

I want to save everything this function prints on screen to a file. I'm losing my mind, I have been trying but no luck so far. Can anybody help to accomplish this? Thanks.
Also this is in C.

Code:

static void print_ip_header(struct iphdr *ip){
	struct in_addr    saddr, daddr;
	char            sip[24], dip[24];

	saddr.s_addr = ip->saddr;
	strcpy(sip, inet_ntoa(saddr));
	daddr.s_addr = ip->daddr;
	strcpy(dip, inet_ntoa(daddr));
	printf("IP %s -> %s \n IP  ver.%d header-length.%d", sip, dip, ip->version, ip->ihl);
	printf("IP  ver.%d header-length.%d TOS.%04X total-len.%d TTL.%d \n",ip->tos, ntohs(ip->tot_len), ip->ttl);
	printf("id.%d offset.%d checksum.%d\n ", ntohs(ip->id), ntohs(ip->frag_off), ntohs(ip->check));
}

Sergei Steshenko · 12-22-2008, 07:29 PM

Ever heard of fprintf ?

What about 'tee' and redirection on command line:

Code:

your_program ARGS | tee > screen_output.txt

?

darkangel29 · 12-22-2008, 08:09 PM

Thanks!!!

theNbomr · 12-25-2008, 01:12 AM

I thought you wanted to do this in C...

Code:

static void print_ip_header(struct iphdr *ip){
	struct in_addr    saddr, daddr;
	char            sip[24], dip[24];
        FILE * logfile;
        logfile = fopen( "logfile.txt", "w" );
        if( NULL == logfile ){
               perror( "can't open logfile" );
               return;
        }
	saddr.s_addr = ip->saddr;
	strcpy(sip, inet_ntoa(saddr));
	daddr.s_addr = ip->daddr;
	strcpy(dip, inet_ntoa(daddr));
	printf("IP %s -> %s \n IP  ver.%d header-length.%d", sip, dip, ip->version, ip->ihl);
	printf("IP  ver.%d header-length.%d TOS.%04X total-len.%d TTL.%d \n",ip->tos, ntohs(ip->tot_len), ip->ttl);
	printf("id.%d offset.%d checksum.%d\n ", ntohs(ip->id), ntohs(ip->frag_off), ntohs(ip->check));

	fprintf(logfile, "IP %s -> %s \n IP  ver.%d header-length.%d", sip, dip, ip->version, ip->ihl);
	fprintf(logfile, "IP  ver.%d header-length.%d TOS.%04X total-len.%d TTL.%d \n",ip->tos, ntohs(ip->tot_len), ip->ttl);
	fprintf(logfile, "id.%d offset.%d checksum.%d\n ", ntohs(ip->id), ntohs(ip->frag_off), ntohs(ip->check));
        fclose( logfile );
}

--- rod.

Sergei Steshenko · 12-25-2008, 09:36 AM

Quote:

Originally Posted by theNbomr

I thought you wanted to do this in C...

Code:

static void print_ip_header(struct iphdr *ip){
	struct in_addr    saddr, daddr;
	char            sip[24], dip[24];
        FILE * logfile;
        logfile = fopen( "logfile.txt", "w" );
        if( NULL == logfile ){
               perror( "can't open logfile" );
               return;
        }
	saddr.s_addr = ip->saddr;
	strcpy(sip, inet_ntoa(saddr));
	daddr.s_addr = ip->daddr;
	strcpy(dip, inet_ntoa(daddr));
	printf("IP %s -> %s \n IP  ver.%d header-length.%d", sip, dip, ip->version, ip->ihl);
	printf("IP  ver.%d header-length.%d TOS.%04X total-len.%d TTL.%d \n",ip->tos, ntohs(ip->tot_len), ip->ttl);
	printf("id.%d offset.%d checksum.%d\n ", ntohs(ip->id), ntohs(ip->frag_off), ntohs(ip->check));

	fprintf(logfile, "IP %s -> %s \n IP  ver.%d header-length.%d", sip, dip, ip->version, ip->ihl);
	fprintf(logfile, "IP  ver.%d header-length.%d TOS.%04X total-len.%d TTL.%d \n",ip->tos, ntohs(ip->tot_len), ip->ttl);
	fprintf(logfile, "id.%d offset.%d checksum.%d\n ", ntohs(ip->id), ntohs(ip->frag_off), ntohs(ip->check));
        fclose( logfile );
}

--- rod.

Do you really think to do copy-paste of each output string is a good idea ?

What about the concept of "single point of change" ?

What about defining, say, a macro for each string to be printed - nowadays compilers know well to allocate memory only once.

What about writing a function (maybe one already exists) which would wrap 'fprintf' and would have a pointer to an array of open file descriptors and would write the same stuff to all of them ? Or maybe a varargs macro.