C + + calls library functions undefined reference to__jquery
Source: Internet
Author: User
C + + calls library functions, if the header file is not properly defined, there may be clearly a function in the obj file exists, but the link failed, the following error occurred:
Undefined reference to ' xxx '
The problem occurs because the C library function is compiled into an obj file and differs from C + + for the processing of the function symbols. Because C + + functions support overloading, the processing of function symbols is more complex, and c is often not decorated.
For example, there are functions:
* DOFUNC.C * *
#include <stdio.h>
int Dofunc ()
{
printf ("dofunc\n");
}
After using GCC to compile into obj
Gcc-c DOFUNC.C
#生成 DOFUNC.O
SYMBOL TABLE:
[0] (sec-2) (fl 0x00) (Ty 0) (SCL 103) (NX 1) 0x00000000 DOFUNC.C
File
[2] (sec 1) (fl 0x00) (Ty 20) (SCL 2) (NX 1) 0x00000000
__Z6DOFUNCV AUX tagndx 0 ttlsiz 0x0 Lnnos 0 Next 0
[4] (sec 1) (fl 0x00) (Ty 0) (SCL 3) (NX 1) 0x00000000. Text
AUX Scnlen 0x14 Nreloc 2 Nlnno 0
[6] (sec 2) (fl 0x00) (Ty 0) (SCL 3) (NX 1) 0x00000000. Data
AUX Scnlen 0x0 Nreloc 0 Nlnno 0
[8] (sec 3) (fl 0x00) (Ty 0) (SCL 3) (NX 1) 0x00000000. BSS
AUX Scnlen 0x0 Nreloc 0 Nlnno 0
[10] (sec 4) (fl 0x00) (Ty 0) (SCL 3) (NX 1) 0x00000000. rdata
AUX Scnlen 0x8 nreloc 0 Nlnno 0
[12] (sec 0) (fl 0x00) (Ty 20) (SCL 2) (NX 0) 0x00000000 _printf
g++ the compiled function symbol name is more bizarre:
__Z6DOFUNCV
Visible C and C + + in the processing function name is very different.
If there is a C + + program to use DOFUNC.O, the following program's function declaration is wrong
Main_dev.cpp
int Dofunc ();
int main (int argc, char* args[])
{
Dofunc ();
System ("pause");
}
g++-o main_dev main_dev.cpp DOFUNC.O
main_dev.cpp:undefined reference to ' Dofunc () '
Collect2:ld returned 1 exit status
The reason is that the function name of the Dofunc function should be
__Z6DOFUNCV, DOFUNC.O file inside is _dofunc, so can't find.
If there is Dofunc source code, the solution is very simple, will dofunc.c use C + + to compile.
If unfortunately dofunc function in someone else's library, and this library is written in C and GCC compiled, the source code is not visible, then how to do it.
Thanks to the designers of C + + and compilers already anticipating this problem, it provides a common solution: use extern "C" to modify the external function declaration of the old C library.
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