A few days ago, I saw a thread about /dev/shm and have learned that the device /dev/shm holds no less than 1/2 of the current available memory by default. In my /etc/fstab, tmpfs is set as follows:
tmpfs /dev/shm tmpfs defaults 0 0
Today, I copied gcc-4.6.0.tar.gz to /dev/shm and compiled it. But after some time, I got an error that there was no left space on the device. However, I found there was about 1.5GB memory available before compiling and during compiling, I didn't launch new tasks. I don't know why this happens. Doesn't 0.75GB memory support the compiling of gcc4.6.0?
I think after coping some software into /dev/shm, the software only has the volume of 1/2 of the current available memory by default and cann't be swapped between the hard disk and the memory. Does it right?
Thanks in advance for help.

Minor correction: by default /dev/shm uses no more than one-half the RAM. The defaults can be modified. Using up to one-half does not mean half is reserved.

The special /dev/shm file system and tmpfs will use swap but I believe that applies only to shared memory segments and not to traditional files.

Occasionally some packages will not compile because of the lack of sufficient temporary storage space. I think I have run into that problem only once, maybe twice. That problem is not limited to using /dev/shm as $TMP/$TEMP/$TMPDIR. The default for those environment variables is /tmp. If /tmp is located on a separate partition, a common practice for some people, that problem could happen there too if the partition size is too small.

You did not mention the amount of RAM you have installed. I have 4 GB of RAM installed. The latest version of GCC might be one such package but I never have tried to build that package and can't say for sure. Possible simple fix: Try changing the build script to a build location that has ample storage space.

If you return to that thread you'll notice a disagreement about using /dev/shm in such a manner. Decide for yourself what is best for you.

My RAM volume is 2GB.
Thank you very much! I have learned a lot from your and other people's posts. I didn't have the knowledge that some packages are compiled in assigned place.

Typical SlackBuild build scripts will have an assignment for the $TMP environment variable like this:

TMP=${TMP:-/tmp/SBo}

or

TMP=${TMP:-/tmp}

That location is where the build will take place.