How can I print to standard error in C with 'printf'?

Question

In C, printing to standard output (stdout) is easy, with printf from stdio.h.

However, how can I print to standard error (stderr)? We can use fprintf to achieve it apparently, but its syntax seems strange. Maybe we can use printf to print to standard error?

What's so "strange" about it's syntax? It's print *where*, *how* and *what*. — Eugene Sh., Aug 17 '16 at 16:42
I am focusing on it. The only problem which is arising from the question, that you find the solution "strange". Otherwise there is no question. Use `fprintf`. — Eugene Sh., Aug 17 '16 at 16:46
@Eugene. I agree with you. I thought it was strange as I did not realize stderr is a FILE :) — wad, Aug 17 '16 at 16:49
@Schilive Thanks for letting me know - I deleted mine now as well. — Per Lundberg, Apr 08 '23 at 15:04

score 286 · Accepted Answer · edited Mar 31 '23 at 18:30

The syntax is almost the same as printf. With printf you give the string format and its contents ie:

printf("my %s has %d chars\n", "string format", 30);

With fprintf it is the same, except now you are also specifying the place to print to:

FILE *myFile;
...
fprintf(myFile, "my %s has %d chars\n", "string format", 30);

Or in your case:

fprintf(stderr, "my %s has %d chars\n", "string format", 30);

Paul R · Answer 2 · 2020-10-27T13:35:17.967

56

Some examples of formatted output to stdout and stderr:

printf("%s", "Hello world\n");              // "Hello world" on stdout (using printf)
fprintf(stdout, "%s", "Hello world\n");     // "Hello world" on stdout (using fprintf)
fprintf(stderr, "%s", "Stack overflow!\n"); // Error message on stderr (using fprintf)

edited Oct 27 '20 at 13:35

answered Aug 17 '16 at 16:48

Paul R

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score 21 · Answer 3 · answered Aug 18 '16 at 09:07

#include<stdio.h>

int main ( ) {
    printf( "hello " );
    fprintf( stderr, "HELP!" );
    printf( " world\n" );
    return 0;
}

$ ./a.exe
HELP!hello  world
$ ./a.exe 2> tmp1
hello  world
$ ./a.exe 1> tmp1
HELP!$

stderr is usually unbuffered and stdout usually is. This can lead to odd looking output like this, which suggests code is executing in the wrong order. It isn't, it's just that the stdout buffer has yet to be flushed. Redirected or piped streams would of course not see this interleave as they would normally only see the output of stdout only or stderr only.
Although initially both stdout and stderr come to the console, both are separate and can be individually redirected.

score 8 · Answer 4 · edited Jun 22 '23 at 15:08

8

If you don't want to modify current code and just for debug usage.

Add this macro:

#define printf(args...) fprintf(stderr, ##args)
// Under GCC

#define printf(args...) fprintf(stderr, __VA_ARGS__)
// Under MSVC

Change stderr to stdout if you want to roll back.

It's helpful for debug, but it's not a good practice.

edited Jun 22 '23 at 15:08

Peter Mortensen

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answered Mar 13 '17 at 11:53

lxiange

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You could also give it a separate name, perhaps `eprintf` or something. (:O This is actually GCC (3.2)'s example of a variadic macro: https://gcc.gnu.org/onlinedocs/gcc-3.2/cpp/Variadic-Macros.html) – Solomon Ucko Aug 15 '20 at 19:27

score 2 · Answer 5 · edited Jun 22 '23 at 15:04

2

You may know sprintf. It's basically the same thing with fprintf. The first argument is the destination (the file in the case of fprintf i.e. stderr), the second argument is the format string, and the rest are the arguments as usual.

I also recommend this printf (and family) reference.

edited Jun 22 '23 at 15:04

Peter Mortensen

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answered Aug 17 '16 at 16:43

Some programmer dude

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This is unrelated to the OP and is worded confusingly. sprintf will "print" to a string and cannot print to stderr. – David Ljung Madison Stellar Sep 22 '20 at 00:27

score -1 · Answer 6 · answered Sep 18 '22 at 15:40

-1

For some reason, no one has mentioned eprintf() yet, which is the same thing as fprintf(stderr, ...).

eprintf("ERROR: You have caused an error!!1!");

Quick tip: If you are using stderr for debugging, you can use the __LINE__ macro

#define printfError(str, ...) eprintf("ERROR at line " #__LINE__ ": " str, __VA_ARGS__)

printfError("Division by zero"); // "ERROR at line 2: Division by zero

answered Sep 18 '22 at 15:40

4

Nobody mentioned `eprintf` because that's not a standard C library function. – Adrian Mole Sep 22 '22 at 17:56
2

@AdrianMole Oh. – Sep 23 '22 at 18:50

score -7 · Answer 7 · edited Jun 22 '23 at 15:07

-7

To print your context, you can write code like this:

FILE *fp;
char *of;
sprintf(of, "%s%s", text1, text2);
fp = fopen(of, 'w');
fprintf(fp, "your print line");

edited Jun 22 '23 at 15:07

Peter Mortensen

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answered Mar 02 '17 at 03:08

winterli

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This doesn't add anything to the previous answers and doesn't really answer the question. – Fantastic Mr Fox Apr 30 '19 at 08:43
4

That sprintf() statement causes memory corruption! ...I highly advise you read up on how pointers, arrays, and strings work. – BlueChip Jun 13 '20 at 17:15

How can I print to standard error in C with 'printf'?

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