Linkage error when compiling C file and Assembly file - c

Question

I have .s (x86 assembly at&t syntax) file, .h (header) file with struct defenition and functions decleration which are implemented in the assembly file, and main.c file with function call (from the .s file).
when trying to compile it all together, i get the following error:
main.o: In function `main':
/home/user/workspace/Assembly/main.c:7: undefined reference to `pstrlen'
collect2: error: ld returned 1 exit status
make: *** [a.out] Error 1
pstring.h:
typedef struct {
char len;
char str[255];
} Pstring;
char pstrlen(Pstring* pstr);
main.c:
#include <stdio.h>
#include "pstring.h"
int main() {
Pstring a;
a.len = 4;
printf("Length: %d", pstrlen(&a));
return 0;
}
pstring.s:
.file "pstring.s"
.section .rodata
invalid_input: .string "invalid input!\n"
.text
.type pstrlen, #function
pstrlen:
pushl %ebp
movl %ebp, %esp
movl 8(%ebp), %eax # assign given pstring ptr address to eax
movzbl (%eax), %ecx # set ecx with the value of the first byte (length) of the address at eax
movl %ecx, %eax # set return value as the value at ecx
ret
.type pstrcpy, #function
makefile:
a.out: main.o pstring.o
gcc -m32 -g -o a.out main.o pstring.o
main.o: main.c pstring.h
gcc -m32 -g -c -o main.o main.c
pstring.o: pstring.s
gcc -m32 -g -c -o pstring.o pstring.s
clean:
rm -f *.o a.out
Thank you.

Answer 1

I resolved the problem by declaring pstrlen as global as follows:
.text
.globl pstrlen
.type pstrlen, #function
pstrlen:
pushl %ebp
movl %ebp, %esp
movl 8(%ebp), %eax # assign given pstring ptr address to eax
movzbl (%eax), %ecx # set ecx with the value of the first byte (length) of the address at eax
movl %ecx, %eax # set return value as the value at ecx
ret