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I have the below part of code in which I noticed that if I change the 0 to 1 the result is the same. I get the STACKprint(); with "on" as the second argument, nothing with anything else and if there is no argument I get a segmentation fault. I guess for the segmentation fault I need to check if the argument is NULL but I am not sure how to do that with the second parameter and it really bugs me out why (strcmp (argv[2],"on") == 1) has no effect. Is it not supposed to take a TRUE value? 

 if (strcmp (argv[2],"on") == 0) { STACKprint(); printf("\n"); }
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    To prevent the segmentation fault, you need to check argc. Commented Jul 28, 2015 at 16:19
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    You would use argc (a count) to determine if argv[2] exists. Commented Jul 28, 2015 at 16:20
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    @aschepler Sorry for stupid question but what is STACKprint() ? Commented Jul 28, 2015 at 16:20
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    Changing 0 to 1 should change the outcome. Something else is wrong. Could you edit your post to include the entire main() function, with the 1 in place, and show the output, too? Commented Jul 28, 2015 at 16:23

3 Answers 3

Reset to default
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To avoid the segfault, check the value of argc to discover whether argv[2] exists. If argc < 3, argv[2] wasn't supplied.

strcmp() doesn't return true/false; it returns a value either less than, equal to, or greater than zero depending on the relative values of its arguments.

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First of all, to check if there is an argument, you should use the argc variable of the main(int argc, char** argv), which indicates the length of your argv array.

if (argc < 3) { printf("missing argument\n"); exit(-1); }

As for strcmp, the man page only states that it returns 0 if the two strings in argument are equal, else non-zero... but not necessarily 1. In fact it is implementation dependent. The way to use it to check for string equality is therefore :

if (0 == strcmp(argv[2], "on")) { // do something } else { // do something else }
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You mean argc<3, since he's using argv[2], right?
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If your program is something like this

#include <stdio.h> int main (int argc, char**argv) { if (argc >= 3 && strcmp (argv[2],"on") == 1){ // STACKprint(); printf("\n"); } }

and you try to run it with myexe 1 on, It will never go into the if block and if you change the 1 to 0, it will go.

Something else is wrong.

It will be nice if you can post your code and the way you are calling it.

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Sure you want strcmp (argv[2],"on") == 1 instead of strcmp (argv[2],"on") == 0 or strcmp (argv[2],"on") != 0?
Good point @chux. This is not going to work as expected (programiz.com/c-programming/library-function/string.h/strcmp0

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