execv() and const-ness

Question

I often use the execv() function in C++ but if some of the arguments are in C++ strings. It annoys me that I cannot do this:

const char *args[4];

args[0] = "/usr/bin/whatever";
args[1] = filename.c_str();
args[2] = someparameter.c_str();
args[3] = 0;

execv(args[0], args);

This doesn't compile because execv() takes char *const argv[] which is not compatible with const char * so I have to copy my std::strings to character arrays using strdup() which is a pain.

Does anyone know the reason for this ?

Answer 1

const is a C++ thing - execv has taken char * arguments since before C++ existed.

You can use const_cast instead of copying, because execv doesn't actually modify its arguments. You might consider writing a wrapper to save yourself the typing.

Actually, a bigger problem with your code is that you declared an array of characters instead of an array of strings.

Try: const char* args[4];

Answer 2

The Open Group Base Specifications explains why this is: for compatibility with existing C code. Neither the pointers nor the string contents themselves are intended to be changed, though. Thus, in this case, you can get away with const_cast-ing the result of c_str().

Quote:

The statement about argv[] and envp[] being constants is included to make explicit to future writers of language bindings that these objects are completely constant. Due to a limitation of the ISO C standard, it is not possible to state that idea in standard C. Specifying two levels of const- qualification for the argv[] and envp[] parameters for the exec functions may seem to be the natural choice, given that these functions do not modify either the array of pointers or the characters to which the function points, but this would disallow existing correct code. Instead, only the array of pointers is noted as constant.

The table and text after that is even more insightful. However, Stack Overflow doesn't allow tables to be inserted, so the quote above should be enough context for you to search for the right place in the linked document.