ref: 7742661aebb96a26f1cb34a68bcf631065913806
dir: /sys/man/2/read/
.TH READ 2 .SH NAME read, readn, write, pread, pwrite \- read or write file .SH SYNOPSIS .B #include <u.h> .br .B #include <libc.h> .PP .B long read(int fd, void *buf, long nbytes) .PP .B long readn(int fd, void *buf, long nbytes) .PP .B long write(int fd, void *buf, long nbytes) .PP .B long pread(int fd, void *buf, long nbytes, vlong offset) .PP .B long pwrite(int fd, void *buf, long nbytes, vlong offset) .SH DESCRIPTION .I Read reads .I nbytes bytes of data from the offset in the file associated with .I fd into memory at .IR buf . The offset is advanced by the number of bytes read. It is not guaranteed that all .I nbytes bytes will be read; for example if the file refers to the console, at most one line will be returned. In any event the number of bytes read is returned. A return value of 0 is conventionally interpreted as end of file. .PP .I Readn is just like read, but does successive .I read calls until .I nbytes have been read, or a read system call returns a non-positive count. .PP .I Write writes .I nbytes bytes of data starting at .I buf to the file associated with .I fd at the file offset. The offset is advanced by the number of bytes written. The number of characters actually written is returned. It should be regarded as an error if this is not the same as requested. .PP .I Pread and .I Pwrite are equivalent to a .IR seek (2) to .I offset followed by a .I read or .IR write . By combining the operations in a single atomic call, they more closely match the 9P protocol (see .IR intro (5)) and, more important, permit multiprocess programs to execute multiple concurrent read and write operations on the same file descriptor without interference. .SH SOURCE .B /sys/src/libc/9syscall .br .B /sys/src/libc/port/readn.c .SH SEE ALSO .IR intro (2), .IR dirread (2), .IR dup (2), .IR open (2), .IR pipe (2), .IR readv (2) .SH DIAGNOSTICS These functions set .IR errstr .