deprecated conversion from string to char *

Question

I want to get a filename from the command line or set a default name for that. So, previously I used

static char *fName;
if (argc == 2 ) {
    fName = argv[1];
}
else {
    fName = "default_file.txt";
}

But for the else, I get warning: conversion from a string literal to "char *" is deprecated. If I try something like this:

static char fName [30] = "default_file.txt";
if (argc == 2 ) {
    fName = argv[1]; //error
}

Now, I get error: expression must be a modifiable lvalue. So, how can I fix that?

Answer 1

warning: conversion from a string literal to "char *" is deprecated

This almost certainly means that you are using a C++ compiler. String literals (like "default_file.txt" in your example) was once of type char[] in C and C++ both. Writing to a string literal invokes undefined behavior. Because of this, C++ changed the type of string literals to const char[]. But in C, the type remains char[]. The warning is therefore an indication that you are using a C++ compiler for compiling C code, which is often not wise.

Note that it is however bad practice in C as well as C++, to have a non-const-qualified pointer to a string literal.

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Now, I get error: expression must be a modifiable lvalue.

Because you can't assign a value to an array in run-time using assignment operators. The C syntax does not allow this. You would have to use memcpy, strcpy or similar.

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Corrected code should be something like this:

static const char* fName = "default_file.txt";

if (argc == 2 ) {
   fName = argv[1];
}

This is fine for C and C++ both, though in C++ it is recommended practice to use std::string instead.