fgets without the line return

Question

What do you think of the following function to offer an alternative to fgets, looking for insights about portability, safety, readability and performance:

#include <stdio.h>
#include <errno.h>
int sc_gets(char *buf, int n)
{
 int count = 0;
 char c;
 if (__glibc_unlikely(n <= 0))
 return -1;
 while (--n && (c = fgetc(stdin)) != '\n')
 buf[count++] = c;
 buf[count] = '0円';
 return (count != 0 || errno != EAGAIN) ? count : -1;
}
#define BUFF_SIZE 10
int main (void) {
 char buff[BUFF_SIZE];
 sc_gets(buff, sizeof(buff)); // how to use
 printf ("%s\n", buff);
 return 0;
}

NB: I have already read fgets() alternative which was interesting but my code is different.

---

Second version thanks to the comments and vnp's insight:

#include <stdio.h>
#include <errno.h>
long sc_gets(char *buf, long n)
{
 long count = 0;
 char c;
 while (--n && (c = fgetc(stdin)) != '\n') {
 if (c == EOF)
 break;
 buf[count++] = c;
 }
 buf[count] = '0円';
 return (count != 0 || errno != EAGAIN) ? count : -1;
}
#define BUFF_SIZE 1024
int main (void) {
 char buff[BUFF_SIZE];
 sc_gets(buff, sizeof(buff));
 printf ("%s\n", buff);
 return 0;
}

• Loki Astari: If you are going to use implementation specific API __glibc_unlikely() it is probably best to wrap them in a macro to provide alternatives for implementations that don't have them.
• Antonin GAVREL: I already checked about them, there is no need to replace them as "the platforms that don't support them just define them to expand to empty strings. " stackoverflow.com/questions/109710/…
• Loki Astari: I think you misunderstand that. This is systems that know about likely/unlikely but don't implement it will expand to white space. Implementations that don't understand it don't automatically do something special to support it. I could be wrong (does the language standard say something special)?
• anastaciu: It looks good to me, but @MartinYork is right, portability is not guaranteed, I ran it in a Windows environment both with gcc version 9.2.0 (tdm64-1) POSIX adapted version, and with MSVC and the compilation fails for both with undefined reference to __glibc_unlikely' and unresolved external symbol __glibc_unlikely respectively.
• Antti Haapala: long is not size_t. long on wibdows has exact same representation as int.

Answer 1

• The function does not handle end-of-file correctly. stdin is just a stream, and could be closed like any other stream. For example, type Ctrl-D. Or

   echo -n a | ./a.out
   a????????
  

  Keep in mind that reading beyond end-of-file doesn't set errno.

• There is no reason to special-case count == 0.

  An IO error doesn't break the loop. If it happened, all the subsequent invocations of fgets would return EOF, and upon the loop termination the count will be n.

• Pass the buffer size as size_t. This is what sizeof yields, and the buffers could be large enough for their size to overflow an integer.

• Antonin GAVREL: Thanks a lot for your insight, please have a look at version 2 :)