fprintf: print literal hex string

Question

Suppose I have a hex string that I have calculated from a set of bytes, in a particular format that suits me:

std::string s("#00ffe1");

And I less than sign it to std::cout

std::cout << s; //prints: #00ffe1

Although I like the way cout works, for my purposes it is easier to use fprintf, as this is outputting a formatted string that is just easier with fprintf.

I go to write the same string from fprintf:

fprintf(stdout,"foo=%s",s); // outputs: G* // (i.e., nonsense)

How do I output this string using fprintf?

@RemyLebeau nothing against it, just becomes a lot when formatting within brackets, braces, and many values — Chris
– Chris, Commented Jan 31, 2019 at 22:35

Govind Parmar · Accepted Answer · 2019-01-31 22:25:21Z

std::string is a class, not a "string" as the term applies in C (where fprintf comes from). the %s format specifier expects a pointer to a null-terminated array of char []. Use the std::string method c_str() to return the null-terminated-string, and pass that to fprintf:

fprintf(..., s.c_str());

Remy Lebeau · Accepted Answer · 2019-01-31 22:26:02Z

4

You'll have to convert the std::string to a null-terminated const char* using its c_str() member function:

fprintf(stdout,"foo=%s",s.c_str());

Note that this is still C++, even if you use fprintf(). C does not contain a data type std::string.

edited Jan 31, 2019 at 22:26

Remy Lebeau

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answered Jan 31, 2019 at 22:25

Stephan Lechner

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3 Comments

Govind Parmar Over a year ago

@bordeo Not really, since a variadic function (one with ... as a parameter) can't really do type-checking at compile-time

Remy Lebeau Over a year ago

Actually, several modern compilers DO now offer compile-time validation of printf-style function parameters, where passing a std::string to %s would be a mismatch that fails validation.

Chris Over a year ago

@GovindParmar in the past, I think I got some heads up--a segfault

Remy Lebeau · Accepted Answer · 2019-01-31 23:03:32Z

1

fprintf expects a C-style string, char*. std::string has a c_str() method that returns just that:

fprintf(stdout, "foo=%s", s.c_str());

edited Jan 31, 2019 at 23:03

Remy Lebeau

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answered Jan 31, 2019 at 22:29

Cristian Bidea

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