Appending to char array with sprintf

Question

So it is tempting to do this:

char buf[1024] = "this is"; std::sprintf(buf, "%s test", buf);

But it is undefined behaviour. I suppose it can be solved via temporary:

char buf[1024] = "this is"; std::sprintf(buf, "%s test", std::string(buf).c_str());

What are the downsides of this approach?

Alternatives: strcpy (in this case); sprintf(buf + strlen(buf), " test");. You should also consider using the safer versions (sprintf_s). — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Dec 22, 2019 at 21:42
This is C++, you should be using a std::string instead of a char[] to begin with: string buf = "this is"; buf += " test"; — Remy Lebeau
– Remy Lebeau, Commented Dec 22, 2019 at 21:48
@RemyLebeau true, but for various reasons it has to be as presented in the example — Killzone Kid
– Killzone Kid, Commented Dec 22, 2019 at 21:50
@1201ProgramAlarm sprintf_s is not any safer than sprintf, as well as not being widely supported. snprintf would be a better option — M.M
– M.M, Commented Dec 22, 2019 at 22:29
@M.M I meant snprintf but the wrong function name came to mind. Sigh. — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Dec 23, 2019 at 2:59

KamilCuk · Accepted Answer · 2019-12-22 21:57:27Z

6

To append a string to a char array use strcat:

char buf[1024] = "this is"; std::strcat(buf, " test");

answered Dec 22, 2019 at 21:50

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2 Comments

Why do you cast " test" to char *?

or memmove (buf + strlen(buf), " test", sizeof " test");