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| <h4 class="subsection">12.12.6 Other Output Conversions</h4> |
| |
| <p>This section describes miscellaneous conversions for <code>printf</code>. |
| |
| <p>The ‘<samp><span class="samp">%c</span></samp>’ conversion prints a single character. In case there is no |
| ‘<samp><span class="samp">l</span></samp>’ modifier the <code>int</code> argument is first converted to an |
| <code>unsigned char</code>. Then, if used in a wide stream function, the |
| character is converted into the corresponding wide character. The |
| ‘<samp><span class="samp">-</span></samp>’ flag can be used to specify left-justification in the field, |
| but no other flags are defined, and no precision or type modifier can be |
| given. For example: |
| |
| <pre class="smallexample"> printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o'); |
| </pre> |
| <p class="noindent">prints ‘<samp><span class="samp">hello</span></samp>’. |
| |
| <p>If there is a ‘<samp><span class="samp">l</span></samp>’ modifier present the argument is expected to be |
| of type <code>wint_t</code>. If used in a multibyte function the wide |
| character is converted into a multibyte character before being added to |
| the output. In this case more than one output byte can be produced. |
| |
| <p>The ‘<samp><span class="samp">%s</span></samp>’ conversion prints a string. If no ‘<samp><span class="samp">l</span></samp>’ modifier is |
| present the corresponding argument must be of type <code>char *</code> (or |
| <code>const char *</code>). If used in a wide stream function the string is |
| first converted in a wide character string. A precision can be |
| specified to indicate the maximum number of characters to write; |
| otherwise characters in the string up to but not including the |
| terminating null character are written to the output stream. The |
| ‘<samp><span class="samp">-</span></samp>’ flag can be used to specify left-justification in the field, |
| but no other flags or type modifiers are defined for this conversion. |
| For example: |
| |
| <pre class="smallexample"> printf ("%3s%-6s", "no", "where"); |
| </pre> |
| <p class="noindent">prints ‘<samp><span class="samp"> nowhere </span></samp>’. |
| |
| <p>If there is a ‘<samp><span class="samp">l</span></samp>’ modifier present the argument is expected to be of type <code>wchar_t</code> (or <code>const wchar_t *</code>). |
| |
| <p>If you accidentally pass a null pointer as the argument for a ‘<samp><span class="samp">%s</span></samp>’ |
| conversion, the GNU library prints it as ‘<samp><span class="samp">(null)</span></samp>’. We think this |
| is more useful than crashing. But it's not good practice to pass a null |
| argument intentionally. |
| |
| <p>The ‘<samp><span class="samp">%m</span></samp>’ conversion prints the string corresponding to the error |
| code in <code>errno</code>. See <a href="Error-Messages.html#Error-Messages">Error Messages</a>. Thus: |
| |
| <pre class="smallexample"> fprintf (stderr, "can't open `%s': %m\n", filename); |
| </pre> |
| <p class="noindent">is equivalent to: |
| |
| <pre class="smallexample"> fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno)); |
| </pre> |
| <p class="noindent">The ‘<samp><span class="samp">%m</span></samp>’ conversion is a GNU C library extension. |
| |
| <p>The ‘<samp><span class="samp">%p</span></samp>’ conversion prints a pointer value. The corresponding |
| argument must be of type <code>void *</code>. In practice, you can use any |
| type of pointer. |
| |
| <p>In the GNU system, non-null pointers are printed as unsigned integers, |
| as if a ‘<samp><span class="samp">%#x</span></samp>’ conversion were used. Null pointers print as |
| ‘<samp><span class="samp">(nil)</span></samp>’. (Pointers might print differently in other systems.) |
| |
| <p>For example: |
| |
| <pre class="smallexample"> printf ("%p", "testing"); |
| </pre> |
| <p class="noindent">prints ‘<samp><span class="samp">0x</span></samp>’ followed by a hexadecimal number—the address of the |
| string constant <code>"testing"</code>. It does not print the word |
| ‘<samp><span class="samp">testing</span></samp>’. |
| |
| <p>You can supply the ‘<samp><span class="samp">-</span></samp>’ flag with the ‘<samp><span class="samp">%p</span></samp>’ conversion to |
| specify left-justification, but no other flags, precision, or type |
| modifiers are defined. |
| |
| <p>The ‘<samp><span class="samp">%n</span></samp>’ conversion is unlike any of the other output conversions. |
| It uses an argument which must be a pointer to an <code>int</code>, but |
| instead of printing anything it stores the number of characters printed |
| so far by this call at that location. The ‘<samp><span class="samp">h</span></samp>’ and ‘<samp><span class="samp">l</span></samp>’ type |
| modifiers are permitted to specify that the argument is of type |
| <code>short int *</code> or <code>long int *</code> instead of <code>int *</code>, but no |
| flags, field width, or precision are permitted. |
| |
| <p>For example, |
| |
| <pre class="smallexample"> int nchar; |
| printf ("%d %s%n\n", 3, "bears", &nchar); |
| </pre> |
| <p class="noindent">prints: |
| |
| <pre class="smallexample"> 3 bears |
| </pre> |
| <p class="noindent">and sets <code>nchar</code> to <code>7</code>, because ‘<samp><span class="samp">3 bears</span></samp>’ is seven |
| characters. |
| |
| <p>The ‘<samp><span class="samp">%%</span></samp>’ conversion prints a literal ‘<samp><span class="samp">%</span></samp>’ character. This |
| conversion doesn't use an argument, and no flags, field width, |
| precision, or type modifiers are permitted. |
| |
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