Linux getpwnam（）库依赖项

Question

I had a program whose system call getpwnam() was failing at runtime. To debug this, I decided to run getpwnam() in isolation with this code (it came from a forum):

#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
struct passwd *pw;

if (argc != 2) {
    printf("usage: %s username\n", argv[0]);
    exit(0);
}
pw = getpwnam(argv[1]);
if (pw == NULL)
    printf("getpwnam failed\n");
else
    printf("home dir = %s\n", pw->pw_dir);
    exit(0);
}

Oddly it seems to depend on libnss_compat-2.3.5.so being present:

With libnss_compat:

./pwnam root home dir = /root

Without libnss_compat:

./pwnam root getpwnam failed

So my question is; why is getpwnam() dependent on libnss_compat*.so ? I found out with the nm -D command that libc-2.3.5.so is the lib which provides getpwnam() .

readelf -d shows me that libc in turn only depends on ld.so.1 . That in turn depends on nothing. So why on earth is libnss_compat having an impact?

Thanks for your help everyone!!

Answer 1

NSS is the Name Service Switch, a library that can look up user information in various sources (traditional password files, Network Information Service , LDAP). getpwnam may be defined in libc , but that will load the actual NSS library at runtime. Looking inside libc , I find

$ strings /lib/x86_64-linux-gnu/libc.so.6 | grep libnss
libnss_
libnss_
libnss_%s.so.%d.%d

The last line is clearly a format string for snprintf that is used to construct the name of the actual implementation library to load using dlopen . The implementation is determined using /etc/nsswitch.conf .

EDIT I found the place in the Glibc sources where the library is loaded . It's not (no longer?) using snprintf , but the principle is still the same.