3.3.2. Conditional Statements

Format:

if (condition)
  statement1
else
  statement2

The statement1 is executed if the condition expression is non-zero. The statement2 is executed if the condition expression is zero. The else clause (else statement2) is optional. Both statement1 and statement2 can be statement blocks.

global countread, countnonread
probe kernel.function("vfs_read"),kernel.function("vfs_write")
{
  if (probefunc()=="vfs_read") 
    countread ++ 
  else 
    countnonread ++
}
probe timer.s(5) { exit() }
probe end 
{
  printf("VFS reads total %d\n VFS writes total %d\n", countread, countnonread)
}

Example 3.9, “ifelse.stp” is a script that counts how many virtual file system reads (vfs_read) and writes (vfs_write) the system performs within a 5-second span. When run, the script increments the value of the variable countread by 1 if the name of the function it probed matches vfs_read (as noted by the condition if (probefunc()=="vfs_read")); otherwise, it increments countnonread (else {countnonread ++}).

Format:

while (condition)
  statement

So long as condition is non-zero the block of statements in statement are executed. The statement is often a statement block and it must change a value so condition will eventually be zero.

Format:

for (initialization; conditional; increment) statement

The for loop is simply shorthand for a while loop. The following is the equivalent while loop:

initialization
while (conditional) {
   statement
   increment
}

Greater than or equal to

Less than or equal to

Is not equal to