MMAP(2) BSD Programmer's Manual MMAP(2)
mmap - map files or devices into memory
#include <sys/types.h> #include <sys/mman.h> void * mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
The mmap function causes the pages starting at addr and continuing for at most len bytes to be mapped from the object described by fd, starting at byte offset offset. If offset or len is not a multiple of the pagesize, the mapped region may extend past the specified range. If addr is non-zero, it is used as a hint to the system. (As a conveni- ence to the system, the actual address of the region may differ from the address supplied.) If addr is zero, an address will be selected by the system. The actual starting address of the region is returned. A success- ful mmap deletes any previous mapping in the allocated address range. The protections (region accessibility) are specified in the prot argument by OR'ing the following values: PROT_EXEC Pages may be executed. PROT_READ Pages may be read. PROT_WRITE Pages may be written. PROT_NONE No permissions. The flags parameter specifies the type of the mapped object, mapping op- tions, and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. Shar- ing, mapping type, and options are specified in the flags argument by OR'ing the following values: MAP_ANON Map anonymous memory not associated with any specific file. The file descriptor used for creating MAP_ANON must currently be -1 indicating no name is associated with the region. MAP_FILE Mapped from a regular file or character-special device memory. (This is the default mapping type, and need not be specified.) MAP_FIXED Do not permit the system to select a different address than the one specified. If the specified address cannot be used, mmap will fail. If MAP_FIXED is specified, addr must be a multiple of the pagesize. Use of this option is discouraged. MAP_HASSEMAPHORE Notify the kernel that the region may contain semaphores and that special handling may be necessary. MAP_INHERIT Permit regions to be inherited across exec(3) system calls. MAP_PRIVATE Modifications are private. MAP_SHARED Modifications are shared. MAP_COPY Modifications are private and, unlike MAP_PRIVATE, modifica- tions made by others are not visible. This option is depre- cated, shouldn't be used and behaves just like MAP_PRIVATE in the current implementation. The close(2) function does not unmap pages; see munmap(2) for further in- formation. The current design does not allow a process to specify the location of swap space. In the future we may define an additional mapping type, MAP_SWAP, in which the file descriptor argument specifies a file or dev- ice to which swapping should be done.
Upon successful completion, mmap returns a pointer to the mapped region. Otherwise, a value of MAP_FAILED is returned and errno is set to indicate the error. The symbol MAP_FAILED is defined in the header <sys/mman.h>. No successful return from mmap() will return the value MAP_FAILED.
mmap() will fail if: [EACCES] The flag PROT_READ was specified as part of the prot param- eter and fd was not open for reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the flags and prot parameters and fd was not open for writing. [EBADF] fd is not a valid open file descriptor. [EINVAL] MAP_FIXED was specified and the addr parameter was not page aligned. fd did not reference a regular or character spe- cial file. [ENOMEM] MAP_FIXED was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available.
madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munmap(2), getpagesize(3)
Due to a limitation of the current vm system (see uvm(9)), mapping descriptors PROT_WRITE without also specifying PROT_READ is useless (results in a segmentation fault when first accessing the mapping). This means that such descriptors must be opened with O_RDWR, which requires both read and write permissions on the underlying object. MirOS BSD #10-current June 4, 1993 1
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