C starter question

[Wolfgang Draxinger]

You need to supply a FILE* to ferror().

Sorry, that were my mistakes, and I hereby take responsibility for them. I
didn't test that code yet, yes lazy and a little bit careless. Anyway, I
did that now, here's there working and checked code, complete with testing
entry point. Compile with

cc -DTEST -o calluname calluname.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

#define BLOCK_LENGTH 100
char * get_operating_system() {
FILE *fil;
char *output = 0;
size_t output_length = 0;
size_t output_read = 0;

fil = popen("uname -snm", "r");
if( !fil ) {
perror("error calling uname: ");
return 0;
}

while( !feof(fil) ) {
if( output_read + BLOCK_LENGTH + 1 > output_length ) {
char * o_new =
realloc(output,
output_length += BLOCK_LENGTH);
if( !o_new ) {
perror("error allocating memory: ");
break;
}
output = o_new;
}
output_read += fread(output +
output_read,
BLOCK_LENGTH,
1,
fil);
if( ferror(fil) ) {
fprintf(stderr, "error reading from uname
stdout\n");
break;
}
}
if( output &&
output_length > 0 &&
output_read > 0 ) {
memset(output + output_read, 0,
output_length - output_read);
}

pclose(fil);

return output;
}

#ifdef TEST
int main(int argc, char *argv[])
{
char *OS = get_operating_system();
if( !OS )
return EXIT_FAILURE;
puts(OS);
free(OS);
return EXIT_SUCCESS;
}
#endif

 

[Wolfgang Draxinger]

Thanks for all your reply.
As for WD, fortran do provide a way to get output from stdout, but no
std. conforming way for call system info like nisdomain or uname!

How about doing syscalls? If fork, pipe, dup2 and exec are avaliable, you
can reimplement the functionality of popen with those.

Wolfgang

 

[Keith Thompson]

How about doing syscalls? If fork, pipe, dup2 and exec are avaliable, you
can reimplement the functionality of popen with those.

If the system provides fork, pipe, dup2, and exec, it will probably
provide popen as well.

 

[Kaz Kylheku]

If the system provides fork, pipe, dup2, and exec, it will probably
provide popen as well.

I see something about Fortran above there. It's conceivable that POSIX bindings
for Fortran might have fork, pipe, dup and exec, but omit popen. Why? Because
popen is tied to FILE *, which is very C specific. You want a function like
popen, but one that integrates with the way Fortran natively does I/O.

Of course, if we are talking about C POSIX bindings, you clearly don't have
a complete POSIX implementation if popen is missing.

 

[rudra]

leave it...my initial problem is solved!