cprintfglibcerrnoformat-specifiers

What's the meaning of the %m formatting specifier?

The output for this code printed out ‘Success’. 

printf("%m\n");
Solution

  • m conversion specifier is not C but is a GNU extension to printf:

    From GNU documentation:

    http://www.gnu.org/software/libc/manual/html_node/Other-Output-Conversions.html

    The ‘%m’ conversion prints the string corresponding to the error code in errno. See Error Messages. Thus:

    fprintf (stderr, "can't open `%s': %m\n", filename);

    is equivalent to:

    fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));

    The ‘%m’ conversion is a GNU C Library extension.

    So:

    printf("%m\n", d);

    is equivalent to

    printf("%s\n", strerror (errno), d);

    which is equivalent to

    printf("%s\n", strerror (errno));

    Note that %mdoes not require an argument. Here printf("%m\n", d) and printf("%s\n", strerror (errno), d) have more arguments than required: with printf if there are extra trailing arguments, they are just evaluated and ignored.