PCAP_OPEN_OFFLINE(3)

🦋 pcap_open_offline & friends 🦋

okay so bestie, this is the PCAP_OPEN_OFFLINE(3) page from the Library Functions Manual, and it's literally all about opening a saved capture file for reading. no cap, it's giving “let me read that packet tea” energy. 💅

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✨ NAME ✨

pcap_open_offline, pcap_open_offline_with_tstamp_precision, pcap_fopen_offline, pcap_fopen_offline_with_tstamp_precision — open a saved capture file for reading 🫶

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🎀 SYNOPSIS 🎀

include this header, and have your errbuf ready, bestie:

#include <pcap/pcap.h>

char errbuf[PCAP_ERRBUF_SIZE];

pcap_t *pcap_open_offline(const char *fname, char *errbuf);
pcap_t *pcap_open_offline_with_tstamp_precision(const char *fname,
    u_int precision, char *errbuf);
pcap_t *pcap_fopen_offline(FILE *fp, char *errbuf);
pcap_t *pcap_fopen_offline_with_tstamp_precision(FILE *fp,
    u_int precision, char *errbuf);

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🌸 DESCRIPTION 🌸

pcap_open_offline() and pcap_open_offline_with_tstamp_precision() are your girlies for opening a savefile for reading. iconic behavior honestly.

fname is the name of the file to open. The file can have the pcap file format as described in pcap-savefile(5) — which is the file format used by, among other programs, tcpdump(1) and tcpslice(1) — or it can have the pcapng file format. But heads up: not all pcapng files can be read. 🙈 The name - is a synonym for stdin.

pcap_open_offline_with_tstamp_precision() takes an additional precision argument specifying the time stamp precision desired:

• PCAP_TSTAMP_PRECISION_MICRO — packet time stamps will be supplied in seconds and microseconds.
• PCAP_TSTAMP_PRECISION_NANO — packet time stamps will be supplied in seconds and nanoseconds.

If the time stamps in the file do not have the same precision as the requested precision, they will be scaled up or down as necessary before being supplied. periodt, that's important. 💯

Alternatively, you may call pcap_fopen_offline() or pcap_fopen_offline_with_tstamp_precision() to read dumped data from an existing open stream fp. This stream will be closed by a subsequent call to pcap_close(3) unless it is stdin. pcap_fopen_offline_with_tstamp_precision() takes an additional precision argument as described above. Note that on Windows, that stream should be opened in binary mode. 🪟

errbuf is a buffer large enough to hold at least PCAP_ERRBUF_SIZE chars. yes bestie, make it that big. 🫶

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🔥 RETURN VALUE 🔥

pcap_open_offline(), pcap_open_offline_with_tstamp_precision(), pcap_fopen_offline(), and pcap_fopen_offline_with_tstamp_precision() return a pcap_t * on success and NULL on failure. If NULL is returned, errbuf is filled in with an appropriate error message. no mystery, no cap. 💅

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🕰️ BACKWARD COMPATIBILITY 🕰️

pcap_open_offline_with_tstamp_precision() and pcap_fopen_offline_with_tstamp_precision() became available in libpcap release 1.5.1. In previous releases, time stamps from a savefile are always given in seconds and microseconds. so if you're time-traveling through libpcap history, that's the vibe. 🕰️✨

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👀 SEE ALSO 👀

• pcap(3)
• pcap-savefile(5)

go read these next, bestie, they're part of the lore. 📚

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okay that's the whole page! 💖 this was PCAP_OPEN_OFFLINE(3) from the Library Functions Manual, dated 12 October 2024. mwah, go read some packets 🫶💅