I am suppose to read the number of thread requested by the client. So if someone run my program like this:
./test 2 I have to read the number of thread equal to 2. I try getchar() but it's not working. Any idea?
3 Answers
Here is a minimal example with complete, correct error checking and diagnostic messages. Note that setting errno to 0 is necessary for distinguishing range errors from valid strtoul() outputs, this is an annoying quirk of the function.
#include <stdlib.h> #include <stdio.h> #include <errno.h> int main(int argc, char *argv[]) { if (argc != 2) { fputs("usage: test NTHREAD\n", stderr); exit(1); } char *e; errno = 0; unsigned long nthread = strtoul(argv[1], &e, 0); if (!*argv[1] || *e) { fputs("error: invalid NTHREAD\n", stderr); exit(1); } if (nthread == (unsigned long) -1 && errno == ERANGE) { fputs("error: NTHREAD out of range\n", stderr); exit(1); } // Your code goes here } 
7 Comments
paxdiablo
Won't errno be zero to start with?
Dietrich Epp
@paxdiablo: I put
errno = 0 in order to allow the block of code to be copied and pasted into sections where the precondition errno == 0 is not guaranteed. It also means that people reading the code do not have to examine surrounding code to verify the precondition.paxdiablo
Fair point, that makes sense.
neilr8133
Perhaps I misunderstand, but I don't think this is valid C (although it might be OK for a C++ sample) because C requires all variable declarations to occur before any code (so the declarations of
*e and nthread would cause errors)...Dietrich Epp
@neilr8133: You are referring to an obsolete version of the C standard. The C standard has allowed such declarations since 1999, and widespread compiler support for the feature is older than that.
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This has nothing to do with threads per se.
The first argument to your program, if given, will be found in argv[1] but as a string, so you need to turn it into an integer with something like atoi or strtol.
Comments
int main(int argc, int **argv)
Using arguments of main, you should know the first argument argv[0] is the name of current executing file, and the following arguments are the parameters sent to your program.
In your case, you must read argv[1].
Always check argc to count the entered arguments.
argv[0]always contains the program name invoked by the command. Thus, the first real argument is stored inargv[1]. But you have to check if such an argument exist. To do so, just readargc, it tells you the number of elements pointed byargv. In your case, it should equal 2.