---
name: "getprotoent_r"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/getprotoent_r"
---

# getprotoent\_r(3)

get protocol entry (reentrant)

## NAME

getprotoent\_r, getprotobyname\_r, getprotobynumber\_r - get protocol entry (reentrant)

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <netdb.h>
```

**int getprotoent\_r(**size\_t size;                   **struct protoent \*restrict** *result\_buf***,**                   **char** *buf***\[restrict** *size***\], size\_t** *size***,**                   **struct protoent \*\*restrict** *result***);** **int getprotobyname\_r(**size\_t size;                   **const char \*restrict** *name***,**                   **struct protoent \*restrict** *result\_buf***,**                   **char** *buf***\[restrict** *size***\], size\_t** *size***,**                   **struct protoent \*\*restrict** *result***);** **int getprotobynumber\_r(**size\_t size;                   **int** *proto***,**                   **struct protoent \*restrict** *result\_buf***,**                   **char** *buf***\[restrict** *size***\], size\_t** *size***,**                   **struct protoent \*\*restrict** *result***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**getprotoent\_r**(), **getprotobyname\_r**(), **getprotobynumber\_r**():

```
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION

The **getprotoent\_r**(), **getprotobyname\_r**(), and **getprotobynumber\_r**() functions are the reentrant equivalents of, respectively, [**getprotoent**(3)](/getprotoent/3.md), [**getprotobyname**(3)](/getprotobyname/3.md), and [**getprotobynumber**(3)](/getprotobynumber/3.md). They differ in the way that the *protoent* structure is returned, and in the function calling signature and return value. This manual page describes just the differences from the nonreentrant functions.

Instead of returning a pointer to a statically allocated *protoent* structure as the function result, these functions copy the structure into the location pointed to by *result\_buf*.

The *buf* array is used to store the string fields pointed to by the returned *protoent* structure. (The nonreentrant functions allocate these strings in static storage.) The size of this array is specified in *size*. If *buf* is too small, the call fails with the error **ERANGE**, and the caller must try again with a larger buffer. (A buffer of size 1024 bytes should be sufficient for most applications.)

If the function call successfully obtains a protocol record, then *\*result* is set pointing to *result\_buf*; otherwise, *\*result* is set to NULL.

## RETURN VALUE

On success, these functions return 0. On error, they return one of the positive error numbers listed in ERRORS.

On error, record not found (**getprotobyname\_r**(), **getprotobynumber\_r**()), or end of input (**getprotoent\_r**()) *result* is set to NULL.

## ERRORS

- **ENOENT**

  (**getprotoent\_r**()) No more records in database.

- **ERANGE**

  *buf* is too small. Try again with a larger buffer (and increased *size*).

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **getprotoent\_r**(), **getprotobyname\_r**(), **getprotobynumber\_r**() | Thread safety | MT-Safe locale |

## VERSIONS

Functions with similar names exist on some other systems, though typically with different calling signatures.

## STANDARDS

GNU.

## EXAMPLES

The program below uses **getprotobyname\_r**() to retrieve the protocol record for the protocol named in its first command-line argument. If a second (integer) command-line argument is supplied, it is used as the initial value for *size*; if **getprotobyname\_r**() fails with the error **ERANGE**, the program retries with larger buffer sizes. The following shell session shows a couple of sample runs:

```
$ ./a.out tcp 1
ERANGE! Retrying with larger buffer
getprotobyname_r() returned: 0 (success)  (size=78)
p_name=tcp; p_proto=6; aliases=TCP
$ ./a.out xxx 1
ERANGE! Retrying with larger buffer
getprotobyname_r() returned: 0 (success)  (size=100)
Call failed/record not found
```

### Program source

```
#define _GNU_SOURCE
#include <ctype.h>
#include <errno.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_BUF 10000
int
main(int argc, char *argv[])
{
    int size, erange_cnt, s;
    struct protoent result_buf;
    struct protoent *result;
    char buf[MAX_BUF];
    if (argc < 2) {
        printf("Usage: %s proto-name [size]\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    size = 1024;
    if (argc > 2)
        size = atoi(argv[2]);
    if (size > MAX_BUF) {
        printf("Exceeded buffer limit (%d)\n", MAX_BUF);
        exit(EXIT_FAILURE);
    }
    erange_cnt = 0;
    do {
        s = getprotobyname_r(argv[1], &result_buf,
                             buf, size, &result);
        if (s == ERANGE) {
            if (erange_cnt == 0)
                printf("ERANGE! Retrying with larger buffer\n");
            erange_cnt++;
            /* Increment a byte at a time so we can see exactly
               what size buffer was required.  */
            size++;
            if (size > MAX_BUF) {
                printf("Exceeded buffer limit (%d)\n", MAX_BUF);
                exit(EXIT_FAILURE);
            }
        }
    } while (s == ERANGE);
    printf("getprotobyname_r() returned: %s  (size=%d)\n",
           (s == 0) ? "0 (success)" : (s == ENOENT) ? "ENOENT" :
           strerror(s), size);
    if (s != 0 || result == NULL) {
        printf("Call failed/record not found\n");
        exit(EXIT_FAILURE);
    }
    printf("p_name=%s; p_proto=%d; aliases=",
           result_buf.p_name, result_buf.p_proto);
    for (char **p = result_buf.p_aliases; *p != NULL; p++)
        printf("%s ", *p);
    printf("\n");
    exit(EXIT_SUCCESS);
}
```

## SEE ALSO

[**getprotoent**(3)](/getprotoent/3.md), [**protocols**(5)](/protocols/5.md)