lm flag is needed to link the math library?
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Because of ridiculous historical practice that nobody is willing to fix. Consolidating all of the functions required by C and POSIX into a single library file would not only avoid this question getting asked over and over, but would also save a significant amount of time and memory when dynamic linking, since each .so file linked requires the filesystem operations to locate and find it, and a few pages for its static variables, relocations, etc.
An implementation where all functions are in one library and the -lm , -lpthread , -lrt , etc. options are all no-ops (or link to empty .a files) is perfectly POSIX conformant and certainly preferable.
Note: I'm talking about POSIX because C itself does not specify anything about how the compiler is invoked. Thus you can just treat gcc -std=c99 -lm as the implementation-specific way the compiler must be invoked for conformant behavior.
Because time() and some other functions are builtin defined in the C library ( libc ) itself and GCC always links to libc unless you use the -ffreestanding compile option. However math functions live in libm which is not implicitly linked by gcc.
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