创建另一个文件比什么都麻烦。而是创建一个目录并检查创建结果。 Unix 手册规定只有一个任务可以成功创建目录，如果目录已经存在，另一个任务将失败，包括两个任务同时尝试的情况。操作系统本身会处理此问题，因此您不必这样做。
如果不是因为可能的陈旧锁，这就是您所要做的。然而，事情发生了，程序中止并且并不总是删除它们的锁。所以实现可以稍微复杂一点。
在一个脚本中，我经常使用下面的代码。它会自动处理陈旧的锁。您可以在 C 中实现相同的功能。查看手册页：
man -s 2 mkdir
EXECUTION_CONTROL_FILE：是名称 PATH 和目录名称，类似于 /usr/tmp/myAppName
second_of_now：以秒为单位返回当前时间（包括在下面）
LOCK_MAX_TIME：一个锁在被认为是陈旧之前可以存在多长时间
sleep 5：总是假设锁会做一些简短而甜蜜的事情。如果没有，也许你的睡眠周期应该更长。
LockFile() {
L_DIR=${EXECUTION_CONTROL_FILE}.lock
L_DIR2=${EXECUTION_CONTROL_FILE}.lock2
(
L_STATUS=1
L_FILE_COUNT=2
L_COUNT=10
while [ $L_STATUS != 0 ]; do
mkdir $L_DIR 2>/dev/null
L_STATUS=$?
if [ $L_STATUS = 0 ]; then
# Create the timetime stamp file
second_of_now >$L_DIR/timestamp
else
# The directory exists, check how long it has been there
L_NOW=`second_of_now`
L_THEN=`cat $L_DIR/timestamp 2>/dev/null`
# The file does not exist, how many times did this happen?
if [ "$L_THEN" = "" ]; then
if [ $L_FILE_COUNT != 0 ]; then
L_THEN=$L_NOW
L_FILE_COUNT=`expr $L_FILE_COUNT - 1`
else
L_THEN=0
fi
fi
if [ `expr $L_NOW - $L_THEN` -gt $LOCK_MAX_TIME ]; then
# We will try 10 times to unlock, but the 10th time
# we will force the unlock.
UnlockFile $L_COUNT
L_COUNT=`expr $L_COUNT - 1`
else
L_COUNT=10 # Reset this back in case it has gone down
sleep 5
fi
fi
done
)
L_STATUS=$?
return $L_STATUS
}
####
#### Remove access lock
####
UnlockFile() {
U_DIR=${EXECUTION_CONTROL_FILE}.lock
U_DIR2=${EXECUTION_CONTROL_FILE}.lock2
(
# This 'cd' fixes an issue with UNIX which sometimes report this error:
# rm: cannot determine if this is an ancestor of the current working directory
cd `dirname "${EXECUTION_CONTROL_FILE}"`
mkdir $U_DIR2 2>/dev/null
U_STATUS=$?
if [ $U_STATUS != 0 ]; then
if [ "$1" != "0" ]; then
return
fi
fi
trap "rm -rf $U_DIR2" 0
# The directory exists, check how long it has been there
# in case it has just been added again
U_NOW=`second_of_now`
U_THEN=`cat $U_DIR/timestamp 2>/dev/null`
# The file does not exist then we assume it is obsolete
if [ "$U_THEN" = "" ]; then
U_THEN=0
fi
if [ `expr $U_NOW - $U_THEN` -gt $LOCK_MAX_TIME -o "$1" = "mine" ]; then
# Remove lock directory as it is still too old
rm -rf $U_DIR
fi
# Remove this short lock directory
rm -rf $U_DIR2
)
U_STATUS=$?
return $U_STATUS
}
####
second_of_now() {
second_of_day `date "+%y%m%d%H%M%S"`
}
####
#### Return which second of the date/time this is. The parameters must
#### be in the form "yymmddHHMMSS", no centuries for the year and
#### years before 2000 are not supported.
second_of_day() {
year=`printf "$1\n"|cut -c1-2`
year=`expr $year + 0`
month=`printf "$1\n"|cut -c3-4`
day=`printf "$1\n"|cut -c5-6`
day=`expr $day - 1`
hour=`printf "$1\n"|cut -c7-8`
min=`printf "$1\n"|cut -c9-10`
sec=`printf "$1\n"|cut -c11-12`
sec=`expr $min \* 60 + $sec`
sec=`expr $hour \* 3600 + $sec`
sec=`expr $day \* 86400 + $sec`
if [ `expr 20$year % 4` = 0 ]; then
bisex=29
else
bisex=28
fi
mm=1
while [ $mm -lt $month ]; do
case $mm in
4|6|9|11) days=30 ;;
2) days=$bisex ;;
*) days=31 ;;
esac
sec=`expr $days \* 86400 + $sec`
mm=`expr $mm + 1`
done
year=`expr $year + 2000`
while [ $year -gt 2000 ]; do
year=`expr $year - 1`
if [ `expr $year % 4` = 0 ]; then
sec=`expr 31622400 + $sec`
else
sec=`expr 31536000 + $sec`
fi
done
printf "$sec\n"
}
这样使用：
# Make sure that 2 operations don't happen at the same time
LockFile
# Make sure we get rid of our lock if we exit unexpectedly
trap "UnlockFile mine" 0
.
. Do what you have to do
.
# We need to remove the lock
UnlockFile mine