Mach-O里与方法有关的Load Command有两种类型，LC_MAIN 和 LC_FUNCTION_STARTS。

LC_MAIN

LC_MAIN是描述可执行文件的入口函数int main(int argc, char * argv[])的，它的定义如下：

struct entry_point_command {
    uint32_t  cmd;  /* LC_MAIN only used in MH_EXECUTE filetypes */
    uint32_t  cmdsize;  /* 24 */
    uint64_t  entryoff; /* file (__TEXT) offset of main() */
    uint64_t  stacksize;/* if not zero, initial stack size */
};

从定义上可以看到入口函数的地址计算公式，也即是

Entry Point = vm_addr(__TEXT) + entryOff + Slide

从dyld的源码里能看到对Entry Point的获取和调用

namespace dyldbootstrap {

uintptr_t start(const dyld3::MachOLoaded* appsMachHeader, int argc, const char* argv[],
                const dyld3::MachOLoaded* dyldsMachHeader, uintptr_t* startGlue) {
    //
    // Entry point for dyld.  The kernel loads dyld and jumps to __dyld_start which
    // sets up some registers and call this function.
    //
    // Returns address of main() in target program which __dyld_start jumps to
    //
    uintptr_t
    _main(const macho_header* mainExecutableMH, uintptr_t mainExecutableSlide, 
            int argc, const char* argv[], const char* envp[], const char* apple[], 
            uintptr_t* startGlue) {
        // find entry point for main executable
        result = (uintptr_t)sMainExecutable->getEntryFromLC_MAIN();
        return result;
    }   
}

}

Entry Point的调用在dyldStartup.s里，有兴趣的可以深入看下。

LC_FUNCTION_STARTS

它的数据结构如下：

#define LC_FUNCTION_STARTS 0x26 /* compressed table of function start addresses */
struct linkedit_data_command {
    uint32_t    cmd;        /*LC_FUNCTION_STARTS, etc*/
    uint32_t    cmdsize;    /* sizeof(struct linkedit_data_command) */
    uint32_t    dataoff;    /* file offset of data in __LINKEDIT segment */
    uint32_t    datasize;   /* file size of data in __LINKEDIT segment  */
};

从注释上看到Load Command里的数据是函数地址列表。

使用MachOView查看

template <typename A>
void DyldInfoPrinter<A>::printFunctionStartsInfo()
{
    if ( (fFunctionStartsInfo == NULL) || (fFunctionStartsInfo->datasize() == 0) ) {
        printf("no function starts info\n");
    }
    else {
        const uint8_t* infoStart = (uint8_t*)fHeader + fFunctionStartsInfo->dataoff();
        const uint8_t* infoEnd = &infoStart[fFunctionStartsInfo->datasize()];
        uint64_t address = fBaseAddress;
        for(const uint8_t* p = infoStart; (*p != 0) && (p < infoEnd); ) {
            uint64_t delta = 0;
            uint32_t shift = 0;
            bool more = true;
            do {
                uint8_t byte = *p++;
                delta |= ((byte & 0x7F) << shift);
                shift += 7;
                if ( byte < 0x80 ) {
                    address += delta;
                    printFunctionStartLine(address);
                    more = false;
                }
            } while (more);
        }
    }
}

方法的顺序在Linkmap里Symbols部分也能看到。

方法重排

链接器可以按照你指定的顺序排布方法，Xcode的设置如图：