c - Why gcc does not give a warning or error when invalid mode is used in fopen()?

I am practicing some practice questions in FILE IO in C. Below is one of the programs.

#include
#include

int main()
{
    char fname[]="poem.txt";

    FILE *fp;
    char ch;
    fp = fopen ( fname, "tr");
    if (fp == NULL)
    {
    printf("Unable to open file...\n");
    exit(1);
    }
    while((ch =fgetc(fp)) != EOF)
    {

            printf("%c",ch);
    }
    printf("\n");

    return 0;
}

As you can see in the statement

  fp = fopen ( fname, "tr");

The mode "tr" is not a valid mode (as I understand). I was expecting gcc to give an error (or a warning) while compiling the above program. However, gcc does not give any error (or warning) while compiling it.

However, as expected, when i run the program it exits printing "Unable to open file..." which means fopen() returned NULL , because there was error while opening file.

-bash-4.1$ ./a.out
Unable to open file...
-bash-4.1$

(The file poem.txt exists so this is because of the invalid mode given to fopen(). I checked by changing the mode to "r" and it works fine displaying the content of "poem.txt")

-bash-4.1$ ./a.out
THis is a poem.

-bash-4.1$

I was expecting gcc to give an error (or warning) message for the invalid mode.

Why gcc does not give any error (or warning) for this ?

Answer

This is Undefined Behavior:

Per Annex J.2 "Undefined Behavior", it is UDB if:

—The string pointed to by the mode argument in a call to the fopen function does not exactly match one of the specified character sequences (7.19.5.3).

Although Annex J is informative, looking at §7.19.5.3:

/3 The argument mode points to a string. If the string is one of the following, the file is open in the indicated mode. Otherwise, the behavior is undefined.

Basically, the compiler can blow you off here - a standard library function name (and behavior) can be used outside of the inclusion of a standard header (for example, non-standard extensions, completely user-defined behavior, etc.). The Standard specifies what a conforming library implementation shall include and how it shall behave, but does not require you to use that standard library (or define behavior for a specific implementation explicitly specified as UDB territory: at this point, if your parameter types match it's a legal function call).

A really good lint utility might help you here.