The __svc_indirect keyword passes an operation code to the SVC handler in r12.
__svc_indirect is a function qualifier. It affects the type of a function.
__svc_indirect(int svc_num ) return-type function-name(int real_num [, argument-list]);
Is the immediate value used in the SVC instruction.
It is an expression evaluating to an integer in the range:
0 to 224–1 (a 24-bit value) in an ARM instruction.
0-255 (an 8-bit value) in a 16-bit Thumb instruction.
Is the value passed in r12 to the handler to determine the function to perform.
To use the indirect mechanism, your system handlers must make use of the r12 value to select the required operation.
You can use this feature to implement indirect SVCs.
int __svc_indirect(0) ioctl(int svcino, int fn, void *argp);
ioctl(IOCTL+4, RESET, NULL);
compiles to SVC #0 with IOCTL+4 in r12.
When an ARM architecture variant or ARM architecture-based processor that does not support an SVC instruction is specified on the command line using the --cpu option, the compiler generates an error.