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-rw-r--r--Documentation/00-INDEX2
-rw-r--r--Documentation/printk-formats.txt35
2 files changed, 37 insertions, 0 deletions
diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
index edef85ce119..50f99eab0e1 100644
--- a/Documentation/00-INDEX
+++ b/Documentation/00-INDEX
@@ -290,6 +290,8 @@ powerpc/
- directory with info on using Linux with the PowerPC.
preempt-locking.txt
- info on locking under a preemptive kernel.
+printk-formats.txt
+ - how to get printk format specifiers right
prio_tree.txt
- info on radix-priority-search-tree use for indexing vmas.
ramdisk.txt
diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
new file mode 100644
index 00000000000..1b5a5ddbc3e
--- /dev/null
+++ b/Documentation/printk-formats.txt
@@ -0,0 +1,35 @@
+If variable is of Type, use printk format specifier:
+---------------------------------------------------------
+ int %d or %x
+ unsigned int %u or %x
+ long %ld or %lx
+ unsigned long %lu or %lx
+ long long %lld or %llx
+ unsigned long long %llu or %llx
+ size_t %zu or %zx
+ ssize_t %zd or %zx
+
+Raw pointer value SHOULD be printed with %p.
+
+u64 SHOULD be printed with %llu/%llx, (unsigned long long):
+
+ printk("%llu", (unsigned long long)u64_var);
+
+s64 SHOULD be printed with %lld/%llx, (long long):
+
+ printk("%lld", (long long)s64_var);
+
+If <type> is dependent on a config option for its size (e.g., sector_t,
+blkcnt_t, phys_addr_t, resource_size_t) or is architecture-dependent
+for its size (e.g., tcflag_t), use a format specifier of its largest
+possible type and explicitly cast to it. Example:
+
+ printk("test: sector number/total blocks: %llu/%llu\n",
+ (unsigned long long)sector, (unsigned long long)blockcount);
+
+Reminder: sizeof() result is of type size_t.
+
+Thank you for your cooperation and attention.
+
+
+By Randy Dunlap <rdunlap@xenotime.net>