strcmp failing to compare strings properly

Question

I'm having trouble using strcmp in C.

I'm trying to compare a program's arguments using strcmp but even though the strings are the same it doesn't work. Here is the portion of code.

while(strcmp(argv[i], "-e") != 0)

So for i = 11 if I print the value of argv[i] I get

printf("String %s i %d", argv[i],i);

>> String -e i 11

But the while keeps on going. Any ideas why this is happening?

Code:

while(strcmp(argv[i], "-e") != 0 || i != argc)
{
    printf("String %s i %d", argv[i],i);
    if(!isdigit((unsigned char)*argv[i]) && strcmp(argv[i], "-t") != 0)
    {
        archivo = fopen(argv[i] , "r");
        TOT_IMG = TOT_IMG + 1;
        for(t=0;t<NUM_FUNC_TRAZO;t++)
        {
            for(d=0;d<NUM_FUNC_DIAMETRICA;d++)
            {
                for(c=0;c<NUM_FUNC_CIRCO;c++)
                {
                    if (fscanf(archivo, "%s",el) != EOF)
                    {
                        par->vector_circo[t][d][c] = strtod(el,NULL);
                        par->clase = clase;
                    }
                    else
                    {
                        break;
                    }
                }
            }
        }
        par_temp = par;
        par->siguiente = (parametros_lista) malloc(sizeof(parametros_elem));
        par = par->siguiente;
        par->anterior = par_temp;
    }
    else
    {
        if(strcmp(argv[i], "-t") != 0)
        {
            clase = atoi(argv[i]);
            CLASES = CLASES + 1;
        }
    }
    i = i + 1;
}

Paste the full code rather than two excerpts with no explanation of how they're connected. — David Schwartz
– David Schwartz, Commented Mar 22, 2013 at 21:55
Nope I'm following the value of i with the debugger and it's fine. I'll post the rest of the code. — Atirag
– Atirag, Commented Mar 22, 2013 at 21:56

Nik Bougalis · Accepted Answer · 2013-03-22 22:14:12Z

2

Let's look at this:

while(strcmp(argv[i], "-e") != 0 || i != argc)

OK, so let's assume strcmp correctly returns 0 when argv[i] is "e". We'll assume this because it's exceedingly unlikely that there's a bug in your library implementation of strcmp.

What happens if strcmp returns 0? Well, things don't just stop, your code checks whether i != argc is true. Is it? My psychic debugging skills tell me that you should look into that second part of the while.

You may also want to note that it's possible that your code could, potentially, access argv[argc], which is NULL. You may get lucky if strcmp is lenient when the input is NULL, but it's a bug that you should fix.

edited Mar 22, 2013 at 22:14

answered Mar 22, 2013 at 22:06

Nik Bougalis

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4 Comments

D.Shawley Over a year ago

I would add that while(i < argc && strcmp(argv[i], "-e") != 0) is probably a better choice here.

Atirag Over a year ago

hmmm ok, but I'm using "||" on the while statement so the first part of the while should act independently from the second part shouldn't it? I'm going to delete the second part to check it

Nik Bougalis Over a year ago

@Omri: Cool, you learn something new every day. Thanks, I'll edit accordingly.

Nik Bougalis Over a year ago

@Atirag think of how || operates - if the left hand side is false, it checks the right, if the left hand side is true, it doesn't check the right hand side.

Community · Accepted Answer · 2017-05-23 12:12:40Z

1

I'd rather recommend you to use getopt (3). This is widely used approach to parameters parsing conforming with POSIX.

Also there was another question related to achieving getopt.h interface on windows: getopt.h: Compiling UNIX C-Code in Windows. What's important it is answered (Xgetopt) so portability should be not a case.

edited May 23, 2017 at 12:12

CommunityBot

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answered Mar 22, 2013 at 21:56

Roman Nikitchenko

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5 Comments

cli_hlt Over a year ago

getopt is NOT a part of the C standard. Its POSIX.

user529758 Over a year ago

@cli_hlt POSIX defines the "POSIX C standard library".

cli_hlt Over a year ago

IEEE POSIX has nothing to do with the ANSI C standard. Google will explain it to you. @Roman you should clarify your answer there.

Roman Nikitchenko Over a year ago

@cli_hit where did you found word 'ANSI' even in original post (OK, now I at all deleted word 'standard' to not feed trolls)? Yes, it's POSIX, isn't it standard? Especially for you: en.wikipedia.org/wiki/C_POSIX_library

cli_hlt Over a year ago

@RomanNikitchenko POSIX is a standard, it defines POSIX compliant operating systems. POSIX compliant operating systems(!) require a POSIX C library. *nix is such an OS, for example. OTOH, Windows is not. ANSI C is the standard of the C programming language. ANSI C conformant programs run on whatever system that has an ANSI C conformant compiler. ANSI C has also a standard library, which together with the language description defines the programming language C. It is very important not to mix these things up.

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