pcap_open_offline(3PCAP) man page

pcap_open_offline(3PCAP) man page

Updated: 6 September 2026 • View in plain text • Return to Main Contents

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

SYNOPSIS

#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);

DESCRIPTION

pcap_open_offline() and pcap_open_offline_with_tstamp_precision() are called to open a ``savefile'' for reading.

fname specifies 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 can have the pcapng file format, although 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; if PCAP_TSTAMP_PRECISION_MICRO is specified, packet time stamps will be supplied in seconds and microseconds, and if PCAP_TSTAMP_PRECISION_NANO is specified, 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.

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(3PCAP) 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.

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 a null pointer on failure. If a null pointer is returned, errbuf is filled in with an appropriate error message.

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.

SEE ALSO

pcap(3PCAP), pcap-savefile(5)

COLOPHON

This HTML man page was generated at 15:15:21 GMT, September 13, 2026 from a source man page in “The Tcpdump Group” git repositories using man2html and other tools.