Open Sound System
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ioctl(fd, SNDCTL_DSP_GETPLAYVOL, &level);
The above code fragment lacks all error checks for clarity. Real world applications must always check for the errors and handle them as described below. Also most OSS ioctl calls will return information in the argument variable and it's usually necessary to check it too.
level=(left)|(right << 8). The permitted ranges for the left and right volumes are 0 (muted) to 100 (max).
Some devices or OSS implementations may use only mono volumes. For this reason it's highly recommended that applications implement only mono control and set both channel values to the same level.
This ioctl call replaces the SOUND_MIXER_PCM method used with earlier OSS versions.
It's possible that the level changes "itself" because some mixer application has touched it. It might be a good practice to check the level occasionally and update the GUI when necessary.
Traditionally many audio applications have used the mixer ioctl calls to change the playbac volume. This must be avoided because in practice no program does this correctly. In addition the only correct way doesn't work with the freeware OSS clones anyway.
The return value from the OSS ioctl calls will be -1 if a fatal error occurred. Other values mean that the ioctl call was more or less successful. However in most cases the application must check the value returned in the argument to see what was the accepted value.
Please see the Possible error codes (errno) returned by OSS calls section for more info about the error codes returned by OSS.