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Full Discussion: question about popen in C
Top Forums Programming question about popen in C Post 302505224 by Corona688 on Wednesday 16th of March 2011 01:01:50 PM
Old 03-16-2011
Quote:
Originally Posted by omega666
also if i change a value of a file pointer array to 0, and then if it gets compared to a NULL value will it crash?
I repeat: Setting an array element doesn't cause a program to crash. (Unless you went past the end of the array). Invalid pointers can't do anything -- they just sit there. Even intentionally setting an invalid pointer won't cause a program to crash right then and there.

Your program crashes when it tries to dereference an invalid pointer.

In short:
Code:
int *x[2]; // Doesn't crash
x[0]=NULL; // doesn't crash.
x[0]=0; // same as NULL, but generates compiler warning.  NULL is better.
x[0]=0xdeadbeef; // Doesn't crash, but generates compiler warning.
printf("contents of pointer are %d\n", *(x[0])); // CRASH!


Quote:
also when testing when i changed all the fread/fwrite amount to 1000 for maximum bytes used, i get errors like
/bin/sh: hdü·ls: not found

but if i keep it to like 512 it works fine, why is this?
Post your complete code. I can't tell why from here but it sounds like you forgot to check how many bytes you actually read again. You can't assume you got 1000, you could easily have gotten less!

Furthermore, don't write 1000 bytes unless you actually have 1000 bytes to write. Only write the number of bytes you actually have...
Quote:
also can you explain again why the command "ls; ls" prints the files line by line for each filename and not all in a line?
There is a system call isatty() which you can use to check if an open file descriptor is a terminal or not.

When standard output is a terminal, ls checks to see how wide your terminal is and splits up its columns accordingly.

When standard output is not a terminal it knows nothing about your screen and just prints one column.

Try ls | cat, it will print one column for exactly the same reason.

Quote:
if i do fwrite, i get the output like

___ ___ ___ ___
___ ___ ___ ___
This is because you're letting ls print directly to the screen. It will know that standard output is a terminal by checking isatty(), and check how wide your screen is, and print columns accordingly.
 

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LEARN ABOUT CENTOS

netsnmp_trap_api

NETSNMP_TRAP_API(3)						     Net-SNMP						       NETSNMP_TRAP_API(3)

NAME

       send_easy_trap, send_trap_vars, send_v2trap - send TRAPs or INFORMs from a Net-SNMP MIB module

SYNOPSIS

       #include <net-snmp/agent/agent_trap.h>

       void send_easy_trap(int trap, int specific);

       void send_trap_vars(int trap, int specific, struct variable_list *vars);

       void send_v2trap(struct variable_list *vars);

DESCRIPTION

       These three routines may be used to send traps from a MIB module within the Net-SNMP agent (including an AgentX subagent).

       send_easy_trap()  sends	an SNMPv1 trap (or the SNMPv2 equivalent) to the list of configured trap destinations (or "sinks"), using the pro-
       vided values for the generic trap type, and specific trap value.

       send_trap_vars() is similar, but appends the supplied list of variable bindings to the traps that are sent.

       send_v2trap() uses the supplied list of variable bindings to form an SNMPv2 trap, which is sent to SNMPv2-capable sinks on  the	configured
       list.  An equivalent INFORM is sent to the configuredq list of inform sinks.  Sinks that can only handle SNMPv1 traps are skipped.

       The  various "send_trap()" calls allow you to specify traps in different formats.  And the various "trapsink" directives allow you to spec-
       ify destinations to receive different formats.  But *all* traps are sent to *all* destinations, regardless of how they were specified.
       I.e. it's
						___  trapsink
					       /
	   send_easy_trap ___  [  Trap	    ] ____  trap2sink
			   ___	[ Generator  ]
	   send_v2trap	  /	[	     ] ----- informsink
					       ____
						     trapsess

       *Not*
	    send_easy_trap  ------------------->  trapsink
	    send_v2trap     ------------------->  trap2sink
	    ????	    ------------------->  informsink
	    ????	    ------------------->  trapsess

WARNINGS

       These routines are used to send the traps immediately they are called.  Invoking them at the appropriate time is left  to  the  MIB  module
       programmer.

SEE ALSO

       snmpd.conf(5), snmptrapd(8)

V5.7.2								    13 Aug 2010 					       NETSNMP_TRAP_API(3)
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