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In C, function arguments are passed by value rather than by reference. While a function may change the values passed in, these changed values are discarded once the function returns. For this reason, many programmers assume a function will not change its arguments, and declaring the functions function's parameters as const is unnecessary.

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Unlike passed-by-value arguments and pointers, pointed-to values are a concern. A function may modify a value referenced by a pointer argument, leading to a side effect which persists even after the function exits. Modification of the pointed-to value is not diagnosed by the complier, which assumes this was the intended behavior.

void foo(int *x) {
  if (x != NULL) {
    *x = 3; /* visible outside function */
  }
  /* ... */
}

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This noncompliant code example , defines a fictional version of the standard strcat() function called strcat_nc(). This function differs from strcat() in that the second argument is not const-qualified.

char *strcat_nc(char *s1, char *s2);

char *str1 = "str1";
const char *str2 = "str2";
char str3[9] = "str3";
const char str4[9] = "str4";

strcat_nc(str3, str2);	/* Compiler warns that str2 is const */
strcat_nc(str1, str3);  /* Attempts to overwrite string literal! */
strcat_nc(str4, str3);  /* Compiler warns that str4 is const */

The function would behave behaves the same as strcat(), but the compiler generates warnings in incorrect locations , and fails to generate them in correct locations.

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char *strcat(char *s1, const char *s2); 

char *str1 = "str1";
const char *str2 = "str2";
char str3[9] = "str3";
const char str4[9] = "str4";

strcat(str3, str2); 

/* Args reversed to prevent overwriting string literal */ 
strcat(str3, str1);  
strcat(str4, str3);  /* Compiler warns that str4 is const */

The const-\qualification of the second argument s2 eliminates the spurious warning in the initial invocation , but maintains the valid warning on the final invocation in which a const-qualified object is passed as the first argument (which can change). Finally, the middle strcat() invocation is now valid, as str3 is a valid destination string and may be safely modified.

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