linux下c程序如何包含头文件
刚学linux下的c编程，写了一个简单的程序，如下：
#include <stdio.h>
#include <math.h>
int main()
{
int i;
for(i=0;i<10;i++)
{
printf("%f\n",sin(i));
}
}
编译时如下：
# cc first.c
出现如下提示：
first.o(.text+0x30):In function 'main';
: undefined reference to 'sin'
collect2: Id returned 1 exit status
不知道这是为什么，他提示说没有包含数学库文件，但是我程序里
已经有#include <math.h>，为何不行呢？
如果编译时用 #cc first.c -lm 就可以．
如果包含的文件都得在编译行里面加进去，那代码行里面的include有什么用呢？
请各位高手指点一下，忙了几天也没弄明白．
[解决办法]
lm是共享库.类似于windows的静态库或者动态链接库
[解决办法]
楼上说的对，编多线程时要-lpthread
[解决办法]
#include <math.h>
math.h有sin函数的声明. sin函数的代码在libm.so/libm.a里. 链接时需要sin函数的代码所以要加-lm
一个不恰当的比喻. math.h就象你们公司的程序员名单. libm.a/libm.so就像公司的程序员们.
在做软件开发计划的时候看名单就可以了,真正开发软件的时候就需要程序员了.
[解决办法]
你这个过程实际上包含了编译和链接两个过程
第一个过程是
cc -c first.c -o first.o
这个是编译过程，include头文件在这里起作用，声明math库中的常量和函数
第二个过程是连接
cc first.o -o first -lm
这里将编译出的目标文件first.o和math函数库做链接，生成可执行文件，-lm是告诉编译器与哪个库进行连接
[解决办法]
C标准库的函数,类型和宏标准头文件里声明:
<assert.h> <float.h> <math.h> <stdarg.h> <stdlib.h>
<ctype.h> <limits.h> <setjmp.h> <stddef.h> <string.h>
<errno.h> <locale.h> <signal.h> <stdio.h> <time.h>
使用math.h里的函数需要-lm, 其他的函数需要-lc.
用gcc编译和链接时,到了链接阶段gcc会加上-lc参数取调用ld(链接编辑程序).
可以传--verbose参数给ld查看链接的详细步骤.
gcc -Wl,--verbose -o hello hello.c
GNU ld version 2.15 [FreeBSD] 2004-05-23
Supported emulations:
elf_i386_fbsd
elf_x86_64_fbsd
using internal linker script:
==================================================
/* Script for -z combreloc: combine and sort reloc sections */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
"elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)
SEARCH_DIR("/lib"); SEARCH_DIR("/usr/lib");
/* Do we need any of these for elf?
__DYNAMIC = 0; */
SECTIONS
{
/* Read-only sections, merged into text segment: */
PROVIDE (__executable_start = 0x400000); . = 0x400000 + SIZEOF_HEADERS;
.interp : { *(.interp) }
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*(.rela.ctors)
*(.rela.dtors)
*(.rela.got)
*(.rela.bss .rela.bss.* .rela.gnu.linkonce.b.*)
}
.rel.plt : { *(.rel.plt) }
.rela.plt : { *(.rela.plt) }
.init :
{
KEEP (*(.init))
} =0x90909090
.plt : { *(.plt) }
.text :
{
*(.text .stub .text.* .gnu.linkonce.t.*)
/* .gnu.warning sections are handled specially by elf32.em. */
*(.gnu.warning)
} =0x90909090
.fini :
{
KEEP (*(.fini))
} =0x90909090
PROVIDE (__etext = .);
PROVIDE (_etext = .);
PROVIDE (etext = .);
.rodata : { *(.rodata .rodata.* .gnu.linkonce.r.*) }
.rodata1 : { *(.rodata1) }
.eh_frame_hdr : { *(.eh_frame_hdr) }
/* Adjust the address for the data segment. We want to adjust up to
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. = ALIGN (0x100000) - ((0x100000 - .) & (0x100000 - 1)); . = DATA_SEGMENT_ALIGN (0x100000, 0x1000);
/* Ensure the __preinit_array_start label is properly aligned. We
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. = ALIGN(64 / 8);
PROVIDE (__preinit_array_start = .);
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.init_array : { *(.init_array) }
PROVIDE (__init_array_end = .);
PROVIDE (__fini_array_start = .);
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PROVIDE (__fini_array_end = .);
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SORT(CONSTRUCTORS)
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.eh_frame : { KEEP (*(.eh_frame)) }
.gcc_except_table : { *(.gcc_except_table) }
.dynamic : { *(.dynamic) }
.ctors :
{
/* gcc uses crtbegin.o to find the start of
the constructors, so we make sure it is
first. Because this is a wildcard, it
doesn't matter if the user does not
actually link against crtbegin.o; the
linker won't look for a file to match a
wildcard. The wildcard also means that it
doesn't matter which directory crtbegin.o
is in. */
KEEP (*crtbegin*.o(.ctors))
/* We don't want to include the .ctor section from
from the crtend.o file until after the sorted ctors.
The .ctor section from the crtend file contains the
end of ctors marker and it must be last */
KEEP (*(EXCLUDE_FILE (*crtend*.o ) .ctors))
KEEP (*(SORT(.ctors.*)))
KEEP (*(.ctors))
}
.dtors :
{
KEEP (*crtbegin*.o(.dtors))
KEEP (*(EXCLUDE_FILE (*crtend*.o ) .dtors))
KEEP (*(SORT(.dtors.*)))
KEEP (*(.dtors))
}
.jcr : { KEEP (*(.jcr)) }
.got : { *(.got.plt) *(.got) }
_edata = .;
PROVIDE (edata = .);
__bss_start = .;
.bss :
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*(.dynbss)
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*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections. */
. = ALIGN(64 / 8);
}
. = ALIGN(64 / 8);
_end = .;
PROVIDE (end = .);
. = DATA_SEGMENT_END (.);
/* Stabs debugging sections. */
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==================================================
attempt to open /usr/lib/crt1.o succeeded
/usr/lib/crt1.o
attempt to open /usr/lib/crti.o succeeded
/usr/lib/crti.o
attempt to open /usr/lib/crtbegin.o succeeded
/usr/lib/crtbegin.o
attempt to open /var/tmp//ccqW1UTa.o succeeded
/var/tmp//ccqW1UTa.o
attempt to open /usr/lib/libgcc.so failed
attempt to open /usr/lib/libgcc.a succeeded
attempt to open /usr/lib/libc.so succeeded
-lc (/usr/lib/libc.so)
attempt to open /usr/lib/libgcc.so failed
attempt to open /usr/lib/libgcc.a succeeded
attempt to open /usr/lib/crtend.o succeeded
/usr/lib/crtend.o
attempt to open /usr/lib/crtn.o succeeded
/usr/lib/crtn.o
[解决办法]
用man可以比较方便查看函数需要什么头文件和库比如:
NAME
printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
vsprintf, vsnprintf, vasprintf -- formatted output conversion
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdio.h>
================
NAME
sin, sinf -- sine functions
LIBRARY
Math Library (libm, -lm)
SYNOPSIS
#include <math.h>
double
sin(double x);
float
sinf(float x);
DESCRIPTION
